Search

In this study, we obtained the first leading mode and principal component 1 (PC1) of North Pacific sea surface temperature (SST). The PC1‐related SST anomalies, located relatively north/south, are referred to as North/South PC1 events. Model outputs, observations, reanalysis datasets and sensitivity experiments show that during the North PC1 events, an enhanced Aleutian low occurs and is located relatively north, favoring anomalous southerlies over western North America. The anomalous southerlies induce a strong warming anomaly over North America via warm advection of the anomalous southerlies and temperature advection of the climatological westerlies over North America. However, the Aleutian low anomaly and corresponding southerly anomaly associated with South PC1 events shift southward, favoring weakened effects of South PC1 events on atmospheric circulations and surface temperatures over North America. The meridional position of PC1 events deserves to be considered in the studies of the PC1 and its related climate changes. Plain Language Summary: Understanding of surface temperature variations is important since it largely affects human and society. This paper categorizes the North Pacific sea surface temperature anomaly events, which are associated with the first leading mode of North Pacific sea surface temperature, into North and South events according to their meridional positions. Using model outputs, observations, reanalysis datasets and sensitivity experiments, we found that the surface temperature anomalies over North America during the North events are quite different from those during the South events. During the North events, an enhanced Aleutian low occurs over high‐latitude North Pacific. The enhanced Aleutian low tends to induce anomalous southerlies over western North America. The anomalous southerlies favor warming anomalies over North America via the warm advection of anomalous southerlies and temperature advection of climatological westerly. However, the Aleutian low anomaly and meridional wind anomaly during the South events shift southward, leading to weakened effects of the South events on atmospheric circulations and surface temperatures over North America. Key Points: North American warming anomaly occurs during North Pacific sea surface temperature anomaly events located northThe warming anomaly is related to an enhanced Aleutian low anomaly located north and climatological westerlyDuring the events located south, North American surface temperature responses are weak due to a southward‐shift Aleutian low anomaly [ABSTRACT FROM AUTHOR]

The lack of evidence for large‐scale glacial landscapes on Mars has led to the belief that ancient glaciations had to be frozen to the ground. Here we propose that the fingerprints of Martian wet‐based glaciation should be the remnants of the ice sheet drainage system instead of landforms generally associated with terrestrial ice sheets. We use the terrestrial glacial hydrology framework to interrogate how the Martian surface gravity affects glacial hydrology, ice sliding, and glacial erosion. Taking as reference the ancient southern circumpolar ice sheet that deposited the Dorsa Argentea formation, we compare the theoretical behavior of identical ice sheets on Mars and Earth and show that, whereas on Earth glacial drainage is predominantly inefficient, enhancing ice sliding and erosion, on Mars the lower gravity favors the formation of efficient subglacial drainage. The apparent lack of large‐scale glacial fingerprints on Mars, such as drumlins or lineations, is to be expected. Plain Language Summary: Water accumulates under ice masses, including glaciers and ice sheets, lubricating the base of the ice and accelerating ice motion. On Earth, this glacial motion has produced scoured landscapes in northern Europe and North America. Mars lacks such large‐scale glacial erosion even in areas with other signs of widespread glaciation. This paper uses the existing framework describing the physical interactions of water and ice, and how they affect ice motion, to show that a lack of landforms recording glacial erosion is expected even if glaciation were widespread on Mars. Key Points: Glacial hydrology feedback dynamics can explain the lack of glacial sliding on the Martian geological recordSubglacial water drainage develops faster, and is more resilient under lower Martian gravityThe fingerprints of Martian wet‐based glaciation are predicted to be channels and eskers [ABSTRACT FROM AUTHOR]

This paper compares surface signatures of the zonally symmetric and asymmetric modes of stratospheric variability, which describe the strength of the polar vortex and a planetary wave‐1 pattern, respectively. Unlike a weak polar vortex followed by negative Arctic Oscillation–like anomalies, strong stratospheric wave activity features a polar vortex displacement with a deep planetary wave‐1 structure, resulting in positive North Atlantic Oscillation–like North American cooling in about 10 days. Moreover, the linkage between the stratosphere and surface is examined in two reanalyzes and four models of different configurations, which show more robust North American cooling following the displacement of the polar vortex due to strong stratospheric wave activity than the zonally symmetric weakening of the polar vortex. This suggests strong stratospheric wave activity acts as a better predictor for cold spells in the northern U.S. and Canada compared with a weak polar vortex. Plain Language Summary: A weak polar vortex in the winter stratosphere is likely associated with cold spells at the surface, yet there are large uncertainties in predicting surface cold events based on stratospheric signals. This study introduces a new method to separate stratospheric variability into the leading modes of zonally symmetric and asymmetric circulations. The zonally symmetric mode describes the strength of the polar vortex. A weak polar vortex is followed by cooling over northern Eurasia and midlatitude North America. In contrast, the zonally asymmetric mode characterizes a displacement of the polar vortex or a planetary wave‐1 pattern, with the positive phase featuring a high anomaly over North America and a low anomaly over Eurasia. This stratospheric wave pattern extends down to the surface, resulting in North American cooling and Eurasian warming in about 10 days. While both stratospheric modes are associated with surface cooling over North America, the linkage is more robust for the displacement than the zonally symmetric weakening of the polar vortex, based on analyses of several reanalyzes and models. This indicates that strong stratospheric wave activity serves as a better predictor for cold air outbreaks in the northern U.S. and Canada than the commonly used weak polar vortex events. Key Points: Distinct North American cooling signatures follow zonally symmetric and asymmetric modes of the stratosphereStrong stratospheric wave activity precedes positive North Atlantic Oscillation like North American coolingThe linkage between the stratospheric wave activity and the surface is robust in reanalyzes and models [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) observations in the North Atlantic since 1870 reveal a region of enhanced warming off the northeastern coast of North America, and a region of cooling to the south of Greenland. It has been hypothesized that these adjacent SST trends are a result of long‐term changes in the buoyancy‐driven ocean circulation—a slowdown of the Atlantic Meridional Overturning Circulation. The impacts of these historical SST trends on the atmosphere are estimated using idealized atmospheric general circulation model experiments in which the global atmosphere is exposed to modern climatological forcing minus the aforementioned regional SST trends. The local response includes a negative North Atlantic Oscillation tendency and southward shift of the wind forcing for the subtropical gyre. Due to planetary wave propagation, the regional SST trends also induce a northward shift of the intertropical convergence zone over the Indian Ocean. Implications for climate feedbacks and projections are discussed. Plain Language Summary: The ocean surface is warming over most of the planet, with a couple of notable exceptions. One is over the North Atlantic Ocean, just south of Greenland, where the temperature of the ocean surface has actually been cooling by about 1°C since 1870. Nearby, extending off the northeastern coastline of North America is a region where the ocean surface has been warming much faster than the global ocean on average—by about 1.5°C since 1870. This side‐by‐side pair of accelerated warming and cooling trends is likely due to a slowdown of the ocean's overturning circulation, which carries large amounts of heat energy from the tropics toward the poles. This paper reveals the effects of those unusual ocean temperature trends in the North Atlantic on the atmospheric circulation using a computer model of the global atmosphere. The model simulations indicate that these ocean temperature trends are causing shifts in the jet stream, changing the way the winds propel the upper ocean currents, and even have impacts quite far away by moving the tropical rain belt northward in the Indian Ocean. Key Points: The impact of historical sea surface temperature trends in the North Atlantic since 1870 is simulated with a global atmospheric modelAdjacent warming and cooling trends in the North Atlantic Ocean induce a negative NAO‐like response and southward‐shifted gyre forcingPropagation of planetary waves away from the North Atlantic induces a northward‐shifted intertropical convergence zone in the Indian Ocean [ABSTRACT FROM AUTHOR]

The crustal evolution of the southernmost ∼2000–1800 Ma Trans‐Hudson orogen (THO) is enigmatic due to burial by Phanerozoic sediments. We provide new insights through petrochronologic analysis of a paragneiss drill core sample. Detrital zircon age peaks at 2625, 2340, and 1880 Ma and Hf isotopes suggest Paleoproterozoic arc development proximal to Archean source(s). Phase equilibria modeling and ternary feldspar thermometry suggest peak conditions of ≥1 GPa, ≥900°C, the first recognition of extreme, ultra‐high temperature metamorphism in the THO. The largely isobaric P‐T path, rapid heating rate, and ∼20 Myr duration (1872–1850 Ma) of peak conditions suggest that this metamorphism occurred in a back‐arc tectonic setting. The sample records post‐peak (1850–1815 Ma) mid‐crustal residence, slow cooling, and exhumation. Further retrogression occurred during Proterozoic regional exhumation (1630–1470 Ma) and Phanerozoic (360–220 Ma) reheating and/or fluid influx. Evidence for Paleoproterozoic arc(s) supports geophysical data for Archean cratonic and Paleoproterozoic arc crust in this region. Plain Language Summary: The Trans‐Hudson orogeny was a Himalayan‐scale collisional mountain building event approximately 1.8 billion years ago that formed the core of North America. The southern portion of the Trans‐Hudson orogen (THO) is poorly understood because it is almost entirely buried by younger rocks. It is unknown whether the southern THO is underlain by Archean or Proterozoic crust. The timing, conditions, and setting of high‐grade metamorphism in this region are also poorly known. We carried out petrologic, geochemical, and geochronologic analyses on a metasedimentary rock sample recovered from drill core. Detrital zircon and tracer isotopic data show derivation from both Paleoproterozoic and Archean sources. This suggests that Paleoproterozoic magmatic arc(s) developed on or near Archean crust. The rock sample experienced extreme temperatures (≥900°C, ≥1 GPa) and partial melting between approximately 1870 and 1850 million years ago perhaps in a back‐arc tectonic setting. This marks the first recognition of extreme, ultra‐high temperature metamorphism in the THO. Later retrograde metamorphism occurred in the Mesoproterozoic and Phanerozoic. These constraints provide new understanding of the enigmatic southern THO and highlight the ability of a combined petrologic, geochemical, and geochronologic study to extract a comprehensive record of crustal evolution from a single sample. Key Points: Paleoproterozoic arc(s) buried beneath North Dakota developed proximal to, or on, Archean craton(s)Metamorphic temperatures of >900°C are the first recognition of ultra‐high temperature (UHT) metamorphism in the Trans‐Hudson orogenBack‐arc setting for UHT metamorphism indicated by isobaric P‐T path and short timescales of heating and peak conditions [ABSTRACT FROM AUTHOR]

ENSO's atmospheric teleconnections drive anomalous North Pacific sea surface temperatures through changes in surface heat fluxes ("the atmospheric bridge"). Previous research focusing on the bridge as a seasonal phenomenon did not consider how ENSO‐related changes in synoptic variability might also impact surface turbulent heat fluxes (STHF). In this study, we find that while well over half of ENSO's impact on STHF occurs on low‐frequency (>8 days) time scales, up to 20% of its impact arises on high‐frequency (<8 days) time scales, through changes in the covariance between surface wind speed and air‐sea enthalpy difference that typically warms the ocean south of the storm track. During El Niño, the North Pacific storm track and its attendant sea surface warming shift southward, reducing warming of the central North Pacific ocean and thereby enhancing the bridge signal there. Additionally, changes in the bulk formula coefficients between ENSO phases drive STHF differences (5%–10%). Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) has global impacts through teleconnections, which can influence the underlying ocean via the "atmospheric bridge." The atmospheric bridge results in upward surface heat flux and colder sea surface temperature (SST) over the central North Pacific and downward surface heat flux and warmer SST along the coast of North America during El Niño winters. While previous research has studied the influence of longer timescale features on this bridge‐related surface heat flux pattern, the corresponding impact of shorter timescale variability remains unexplored. We find that while longer timescales (>8 days) dominate, shorter timescale influence on the fluxes is non‐negligible. Shorter timescale variability warms the ocean south of the track of North Pacific storms. This warming contribution shifts with the storm location between the two ENSO phases. While the resulting difference between ENSO phases is small, it is mostly aligned with the total surface turbulent heat flux (STHF) difference between El Niño and La Niña. Also, the bulk formula coefficients used to compute the fluxes vary, leading to 5%–10% of the STHF difference between ENSO phases. In summary, the contribution of shorter timescale variability and coefficient changes to the North Pacific bridge‐related STHF is small but non‐negligible. Key Points: While seasonal variability dominates ENSO‐related surface turbulent heat fluxes over the North Pacific, synoptic effects are non‐negligibleENSO‐related storm track shift affects the high‐frequency contribution to the turbulent heat fluxesDifferences in the bulk formula coefficient between two ENSO phases have a small but non‐negligible influence on the surface heat fluxes [ABSTRACT FROM AUTHOR]

In boreal winter, the El Niño/Southern Oscillation (ENSO)‐induced Pacific‐North American (PNA) teleconnection pattern is farther westward during La Niña relative to that in El Niño, causing discernible distinct climate implications. However, there has been a lack of consensus regarding the underlying mechanism driving this asymmetric structure. This study highlights the contribution of nonlinear kinetic energy advection (nKA) to this asymmetry. The zonally symmetric responses to ENSO, specifically the anomalies in zonal mean zonal flow, generate opposing nKA patterns by advecting anomalous eddy kinetic energy in the North Pacific, which leads to the shift of the PNA teleconnection pattern. In addition to nKA, transient eddy activities responded to changes of baroclinicity help maintain the asymmetry through a feedback effect. These findings underscore the importance of considering extratropical factors, such as nonlinear energy processes and synoptic‐scale transient eddies, in understanding the mechanism responsible for the asymmetric structure of the PNA teleconnection pattern. Plain Language Summary: The El Niño/Southern Oscillation (ENSO)‐induced atmospheric anomalies over the North Pacific and North America (PNA region) are more westward during La Niña relative to that during El Niño in boreal winter. The disparity of atmospheric responses causes discernible distinct climate implications, and limits the seasonal prediction. The mechanism underlying this asymmetry, however, remains in disagreement. Some believe that it is due to variations in the location of tropical convective anomalies. Whereas others show that the climatological zonal flow could anchor the disturbance. Utilizing a more comprehensive energy diagnostic framework, we find that the nonlinear component, in particular, the nonlinear kinetic energy advection and the feedback effect of synoptic‐scale disturbances contribute to the asymmetric atmospheric responses. The study emphasizes the importance of nonlinear processes and multi‐scale interactions in the asymmetric structure of ENSO‐induced atmospheric anomalies in the PNA region, which are essential for understanding ENSO teleconnections, comprehending model biases, and improving seasonal predictions. Key Points: The El Niño/Southern Oscillation‐induced Pacific‐North American (PNA) teleconnection pattern is farther westward during La Niña relative to that in El NiñoThe nonlinear energy advection redistributes the kinetic energy, hence contributing to the asymmetric PNA teleconnectionsDifferent synoptic‐scale transient eddy activities help to maintain and amplify the asymmetry through a feedback effect [ABSTRACT FROM AUTHOR]

SNOwpack TELemetry observations and model simulations both demonstrate that the fractional contribution of October through December (early season) snowfall to peak snow accumulation in North America increases with latitude due to both colder temperatures and Pacific storm tracks focusing further north earlier in the season. Early season snowfall also makes up greater than 60 percent of peak accumulation in interior low‐precipitation locations leeward of mountains, particularly those that are subject to strong, warm winds and midwinter snow loss. Early season snow observations show promise in predicting peak snow water equivalent in locations where large‐scale ocean‐atmosphere patterns similarly influence fall and winter conditions, and in northern maritime locations where winter temperatures are warm enough that rain on snow and midwinter melt occur. Because climate change is likely to increase the extent of midwinter melt, the latter relationship is expected to become important over more locations in the future. Plain Language Summary: Across North America, snow accumulates from October until the following spring. Early season snow, defined as the snow that accumulates by the end of December, is a greater fraction of total snow accumulation at higher latitudes and at colder locations. Early season snow accumulation can be used to predict peak snow accumulation. Predictions are more skillful at locations where fall precipitation is correlated with winter precipitation because more snow at the end of December indicates that weather patterns are favorable for greater than average snowfall to continue. Predictions also have skill at warmer northern locations where more snow on the ground early season increases the likelihood that mixed rain‐snow events add water to the snowpack rather than melting the snow. Predictions at warm southern locations do not have skill because winter and spring snowfall is highly variable, and frequently little to no snow has accumulated by the end of December. Key Points: Fall snow contributes more to peak snow at colder and more northerly locationsFall snow can be used to predict peak snow where large scale ocean and atmosphere patterns influence annual snowfall variationsFall snow regulates cold content, which in warm regions may tip a snowpack between lasting or ablating midwinter, influencing peak snow [ABSTRACT FROM AUTHOR]

In recent decades, severe cold winters and springs have frequently occurred over mid‐latitude North America, despite the anthropogenic global warming trend. In this study, we reveal a possible mechanism by investigating the teleconnection between tropical oceans and North America. Through observational analysis and numerical experiments, we reveal that an anomalous tropical Atlantic warming can trigger a cold spring over central‐western mid‐latitude North America. The tropical Atlantic warming intensifies regional deep atmospheric convection and generates a stationary Rossby wave train propagating poleward, forming an anomalous low pressure center over the mid‐latitude North Atlantic. This low‐level circulation adjustment further intensifies the cold advection and increases the cloud cover over central‐western North America, cooling the surface through cloud radiative feedback. The mechanisms revealed in this study may contribute to the improvement of predictability of cold springs over North America, and have broad implications for agriculture production, power supply, and public health. Plain Language Summary: North America has experienced a series of cold springs in recent decades, even under the background of global warming. These cold springs have large impacts on transportation, agriculture, power supply, and public health. In this study, we find that anomalous cold springs over mid‐latitude North America are tightly associated with the tropical Atlantic variability. In particular, an increase in the tropical Atlantic temperature of 1°C may induce up to 4°C decrease over central‐western North America. Further investigation based on observational analysis and numerical model experiments reveals the pathway of this linkage. The tropical Atlantic warming triggers planetary waves, forming an anticlockwise circulation anomaly over the mid‐latitude North Atlantic. This circulation adjustment transports cold air from the higher‐latitudes to central North America. It meanwhile transports more moisture from the North Atlantic to central‐western North America and increases cloud cover, which reflects the solar radiation and further cools the land surface. While previous studies focus on the impacts of Arctic amplification and tropical Pacific variabilities on cold winters/springs over North America, this work highlights the importance of the tropical Atlantic in triggering North American cold springs. Key Points: Sea surface temperature warming over the tropical Atlantic can induce a cold spring over mid‐latitude North AmericaTropical Atlantic warming can trigger a cyclonic circulation anomaly over the mid‐latitude North Atlantic through Rossby wave dynamicsAtmospheric circulation adjustment further drives North American cooling through cold advection and cloud radiative feedback [ABSTRACT FROM AUTHOR]

Tropical atmospheric convection generated by the El Niño–Southern Oscillation (ENSO) plays a crucial role in affecting the western Pacific pattern (WP) in the boreal winter by triggering an atmospheric teleconnection. Here we show from analysis of observations and model simulations that the Atlantic Multidecadal Oscillation (AMO) asymmetrically modulates the relationship between ENSO and WP. We find a significant modulatory effect of AMO on the relationship between wintertime El Niño and WP. A robust El Niño−WP relation can be attributed to the negative AMO phase (−AMO), yet a weak relationship during the positive AMO phase (+AMO). In contrast, the relationship between La Niña and WP is independent of AMO modulation. Furthermore, during the −AMO period, stronger El Niño amplitudes lead to stronger atmospheric convection anomalies over the tropical western North Pacific, which excites stronger atmospheric teleconnection and thus has a more significant effect on WP than during the +AMO period. Plain Language Summary: The western Pacific pattern (WP) is one of the most crucial teleconnection patterns in boreal winter over the Northern Hemisphere, which exerts substantial impacts on weather and climate in Eurasia and North America. El Niño–Southern Oscillation (ENSO) is the most prominent air‐sea coupling system in the tropics, considered to exert great impacts on the WP via triggering atmospheric teleconnection. In this study, we revealed that the Atlantic Multidecadal Oscillation (AMO) has a significant asymmetrical modulation on the relation between the ENSO and WP. In particular, a significant El Niño‒WP connection only occurs in the negative AMO phase (‒AMO). Physical mechanisms of the modulation effects of AMO on the ENSO–WP relationship are further analyzed. The results suggest that stronger El Niño amplitude during the −AMO phase leads to larger atmospheric convection anomalies in the equatorial central Pacific and tropical western North Pacific compared to those during the +AMO phase. As such, stronger atmospheric convection anomalies during El Niño events have a greater impact on WP events during the −AMO phase; however, for La Niña events, there is no significant impact on WP events during the −AMO phase. Results obtained in this study may help to improve our understanding of the WP variability. Key Points: The connection between El Niño–Southern Oscillation (ENSO) and the western Pacific pattern (WP) varies markedly in different Atlantic Multidecadal Oscillation (AMO) phasesA robust connection between ENSO and the WP in the boreal winter can only be observed during the negative AMO phaseThe AMO influences the ENSO‐WP relationship via modulating the ENSO amplitude and the associated change in atmospheric convection [ABSTRACT FROM AUTHOR]

Subsidence after a subduction zone earthquake can cause major changes in estuarine bathymetry. Here, we quantify the impacts of earthquake‐induced subsidence on hydrodynamics and habitat distributions in a major system, the lower Columbia River Estuary, using a hydrodynamic and habitat model. Model results indicate that coseismic subsidence increases tidal range, with the smallest changes at the coast and a maximum increase of ∼10% in a region of topographic convergence. All modeled scenarios reduce intertidal habitat by 24%–25% and shifts ∼93% of estuarine wetlands to lower‐elevation habitat bands. Incorporating dynamic effects of tidal change from subsidence yields higher estimates of remaining habitat by multiples of 0–3.7, dependent on the habitat type. The persistent tidal change and chronic habitat disturbance after an earthquake poses strong challenges for estuarine management and wetland restoration planning, particularly when coupled with future sea‐level rise effects. Plain Language Summary: The land in many estuaries along the Pacific Rim has been repeatedly changed by major earthquakes. Previous earthquakes, such as the CE 1700 rupture along the 1,000‐km Cascadia Subduction Zone of western North America, produced as much as 2 m of land‐surface lowering. This study employs a hydrodynamic model to show that the relative sea‐level rise resulting from such lowering would increase tidal range and affect nearly all existing wetland habitat in the lower Columbia River Estuary. In our primary scenario, approximately ∼93% of current wetland habitat converts to lower‐elevation types, with changes largely due to land‐surface lowering but also influenced by tidal changes. Thus, the chronic, long‐term disturbance to habitat conditions after an earthquake poses a similar long‐term risk to estuarine ecology as extreme global sea‐level rise scenarios. Understanding such earthquake effects may assist the development of more resilient habitat restoration strategies. Key Points: Earthquake‐induced subsidence results in the movement of ∼93% of intertidal habitat to lower habitat zonesPost earthquake, tidal range increases by up to 0.25 m in channels; capturing such changes requires a dynamic modelBoth subsidence and changes in tidal range alter habitats, especially sandflats and low marshes [ABSTRACT FROM AUTHOR]

Our understanding of Earth's paleogeography relies heavily on paleomagnetic apparent polar wander paths (APWPs), which represent the time‐dependent position of Earth's spin axis relative to a given block of lithosphere. However, conventional approaches to APWP construction have significant limitations. First, the paleomagnetic record contains substantial noise that is not integrated into APWPs. Second, parametric assumptions are adopted to represent spatial and temporal uncertainties even where the underlying data do not conform to the assumed distributions. The consequences of these limitations remain largely unknown. Here, we address these challenges with a bottom‐up Monte Carlo uncertainty propagation scheme that operates on site‐level paleomagnetic data. To demonstrate our methodology, we present an extensive compilation of site‐level Cenozoic paleomagnetic data from North America, which we use to generate a high‐resolution APWP. Our results demonstrate that even in the presence of substantial noise, polar wandering can be assessed with unprecedented temporal and spatial resolution. Plain Language Summary: Records of Earth's ancient magnetic field preserved in rocks provide valuable information for understanding past tectonic plate motions. These "paleomagnetic" records are collected from individual rock samples and subsequently grouped to develop global‐scale paths called apparent polar wander (APW) paths. However, the standard methods for analyzing and grouping paleomagnetic data are limited in the way they propagate and quantify uncertainties, and the consequences of these limitations are not known. In this study, we address these limitations through the introduction of a new methodological approach, which we use to study a large data set of paleomagnetic data from North America for the past 60 million years. We demonstrate that through our new methodology it is possible to generate APW paths with unprecedented spatial and temporal resolution, which may offer new insights into Earth's deep time evolution. Key Points: We present an extensive compilation of Cenozoic paleomagnetic and geochronologic data from North America at the site levelA bottom‐up Monte Carlo uncertainty propagation scheme enables the propagation of errors from site level data into apparent polar wander pathsThese advances allowed us to describe Cenozoic North American polar wander at the unprecedented temporal resolution of 1 Myr [ABSTRACT FROM AUTHOR]

Winter cold spells over North America have been correlated with European wind extremes, but the physical mechanisms behind such "pan‐Atlantic" compound extremes have not been clarified yet. In this study, we propose that pan–Atlantic cold and windy extremes occur following two possible dynamical pathways. The first one involves the propagation of a Rossby wave train from the Pacific Ocean, associated with windstorms over north‐western Europe in the 5–10 days after the cold spell peak. The second is associated with a high‐latitude anticyclone over the North Atlantic and an equatorward‐shifted jet, leading to windstorms over south‐western Europe already in the days preceding the cold spell peak. European windstorms are thus systematically tied to North American cold spells according to the different flow configuration. The analysis underscores that apparently similar surface extremes might be driven by different processes, and conflating them together could lead to misleading conclusions. Plain Language Summary: Previous research noticed that cold spells over North America and windstorms over Europe tend to occur within a few days of each other. This connection is supported by the fact that winds usually blow from west to east over the North Atlantic, so that cyclones forming off the coast of North America move toward western Europe. However, the chain of processes behind this connection remained not fully clarified. Here we explain the complex relationship between the occurrence of North American cold spells and European windstorms. While previous work tried to identify a single physical mechanism, we suggest that two separate pathways can connect the two types of extremes. The first pathway involves the propagation of a train of cyclones and anticyclones from the North Pacific to the North Atlantic, leading to a cold spell over central North America and, a few days later, to windstorms over north‐western Europe. The second pathway, on the other hand, involves an anomalous anticyclone over the North Atlantic, which acts to induce cold spells over North America and windstorms over south‐western Europe roughly at the same time: this still leads to a correlation between the two extremes, but without a clear causality direction. Key Points: North American cold extremes and European windy extremes can be connected physically by two distinct dynamical pathwaysThe first pathway involves Rossby wave propagation from the North Pacific and the cold spell preceding the European windstormThe second pathway features both extremes occurring roughly at the same time thanks to an upper‐level anticyclone west of Greenland [ABSTRACT FROM AUTHOR]

The El Niño‐Southern Oscillation (ENSO) influences climate variability across the globe. ENSO is highly predictable on seasonal timescales and therefore its teleconnections are a source of extratropical forecast skill. To fully harness this predictability, teleconnections must be represented accurately in seasonal forecasts. We find that a multimodel ensemble from five seasonal forecast systems can successfully capture the spatial structure of the late winter (JFM) El Niño teleconnection to the North Atlantic via North America, but the simulated amplitude is half of that observed. We find that weak amplitude teleconnections exist in all five models throughout the troposphere, and that the La Niña teleconnection is also weak. We find evidence that tropical forcing of the El Niño teleconnection is not underestimated and instead, deficiencies are likely to emerge in the extratropics. We investigate the impact of underestimated teleconnection strength on North Atlantic winter predictability, including its relevance to the signal‐to‐noise paradox. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) describes the cycle of warmer and cooler sea surface temperatures in the tropical Pacific Ocean, which influences climate around the globe. The high heat capacity of the ocean means that ENSO changes relatively slowly and so the ENSO phase—known as El Niño or La Niña—can be predicted with high accuracy several months ahead. Far‐flung influences—known as teleconnections—of ENSO can provide predictability away from the tropics in seasonal forecasts if they are accurately modeled. In this work, the late winter (January–March) ENSO teleconnection to the North Atlantic, traveling via the North Pacific and North America, is investigated in five forecast models. We find that in each model, the pattern of the teleconnection is accurately captured, but the strength of the modeled teleconnection is half of that in the real world. We find that the strength of processes in the tropics which cause the El Niño teleconnection—including changes in sea surface temperatures and rainfall—are not underestimated by models, meaning that the problem arises further along the pathway to the extratropics. This error likely contributes to the currently unresolved "signal to noise paradox" in climate forecasts. Key Points: Seasonal forecasts severely underestimate the response to El Niño‐Southern Oscillation (ENSO) in the extratropical North PacificThe underestimated model response exists throughout the troposphere and is present for both El Niño and La NiñaTropical processes which generate the El Niño teleconnection are well predicted and so the error does not appear to originate in the tropics [ABSTRACT FROM AUTHOR]

Separating the climate response to external forcing from internal climate variability is a key challenge. While most previous studies have focused on surface responses, here we examine zonal‐mean patterns of North Pacific subsurface temperature responses. In particular, the changes since 1950 driven by anthropogenic aerosol emissions are found by using a pattern recognition method. Based on the single‐forcing large‐ensemble simulations from two models, we show that aerosol forcing caused a nonmonotonic temporal response and a characteristic zonal‐mean pattern within North Pacific, which is distinct from the pattern associated with internal variability. The aerosol‐forced pattern with the nonmonotonic temporal feature shows a substantial temperature change in subpolar regions and a reversed change on the southern flank of the subtropical gyre. A similar characteristic pattern and nonmonotonic time evolution are extracted from the subsurface observations, which likely reflect the subsurface responses to the aerosol forcing, although differences exist with the simulated responses. Plain Language Summary: Pacific climate has long been of interest as considerable evidence has emerged of a decades‐long change. As the background noise, such as Interdecadal Pacific Oscillation and El Niño‐Southern Oscillation, is very strong in the Pacific, it remains a major challenge to detect and understand the changes due to human activities. In this study, we take advantage of a pattern recognition method to investigate the North Pacific subsurface temperature response to aerosol emissions, a suspension of fine particles in air, due to industrial development. The results are primarily based on two sets of model simulations with different phases of background noise and identical external forcing, which stems from anthropogenic aerosol emissions only. We find that the aerosol emissions from North America and Europe cause a clear nonmonotonic time evolution (increased during 1950–1985 and then decreased afterward) and a characteristic zonal mean pattern of the subsurface ocean temperature within the North Pacific. The extracted temporal and spatial features from ocean observations are broadly consistent with those from models, although there are some differences. Key Points: A pattern recognition method is applied to extract aerosol‐induced subsurface temperature pattern from internal variability in North PacificBased on two large‐ensemble simulations, aerosols govern a nonmonotonic feature in temperature change with a characteristic patternThis spatiotemporal feature can be obtained from observations since 1950, although there are some differences with the modeled results [ABSTRACT FROM AUTHOR]

This work uses reanalysis and NOAA Geophysical Fluid Dynamics Laboratory's Coupled Model Intercomparison Project 6 model, CM4, to investigate the colocation of heat extremes and atmospheric blocking in the current climate and an end of 21st century, extreme‐emissions projection. In the present day, the colocation of heat events and blocking is greatest for the strongest heat events. Block‐heat extreme colocation is found to be less prevalent over ocean than land, exhibiting regional variation throughout the Northern Hemisphere. Over North America, colocation is greatest near the northwestern and northeastern coasts, minimizing near the center; over Eurasia, colocation is most prevalent in northern regions. In an RCP 8.5 projection, the historical 90th percentile temperature decreases to 0–70th percentile, depending on the region. This is primarily driven by mean state warming. Blocking is found to decrease along with the colocation of blocking and heat extremes, suggesting that in some regions, the mechanisms driving heat extremes will change in future climates. Plain Language Summary: This work investigates extreme heat events and atmospheric blocks (persistent high‐pressure systems), which are known to be strongly linked. The analysis herein shows that the strongest extreme heat events coincide with atmospheric blocks, however, the likelihood of the events being colocated varies from region to region. In a high emissions warming scenario, temperatures exceeding the warmest 10% in the current climate, occur 30%–100% of the time depending on the region. This is mostly driven by an increase in average temperature everywhere—rather than an increase in the variability of the temperature. At the same time, the model predicts that blocking, and the colocation of extremes heat events with blocking, decreases, suggesting the physical drivers of heat extremes will change in a warmer climate. Key Points: Blocking is more colocated with heat extremes over land than ocean. Stronger heat extremes are more colocated with blockingEnd of 20th century 90th percentile temperatures become 0–70th percentile by the end of the 21st century in a high emissions scenarioLand heat extreme duration is unchanged in RCP 8.5, but colocated blocking decreases, suggesting changes in dynamics driving heat extremes [ABSTRACT FROM AUTHOR]

Climate model hindcasts are analyzed to reveal the impacts of the North Atlantic Oscillation (NAO) on the North Atlantic subpolar ocean, which exhibits variability on seasonal to decadal timescales. The ocean response to a single winter NAO event is separated into fast and slow responses. The fast response persists over winter–spring seasons, during which wind stress and heat flux anomalies associated with the NAO rapidly modify ocean temperatures via changes in Ekman transport and ocean‐atmosphere heat exchanges. The slow response persists for 3–4 years, during which overturning and gyre circulations redistribute opposing‐signed surface temperature anomalies created by the NAO. This redistribution modifies east‐west temperature contrasts altering the meridional heat transport associated with gyres and changing the strength of the overturning circulation. Hence, the fast and slow responses lead to opposing‐signed subpolar temperature anomalies in time from the competing effects of local forcing and horizontal heat convergence. Plain Language Summary: The North Atlantic Oscillation (NAO) index is one of the most common metrics to characterize atmospheric regimes in the northern hemisphere, revealing how different patterns of atmospheric winds and air‐sea fluxes over Europe and North America affect the ocean state. In the present research, coupled climate model simulations are utilized to study the impacts of particular atmospheric events on the subpolar North Atlantic Ocean. Using NAO indices during the winter season, sets of multiple simulations having similar types of atmospheric events are created to carry out a composite analysis. The analyses reveal that the impact of atmospheric events on the oceans can last up to 4–5 years (or even longer in some cases). The fast response is controlled by the effect of the local winds and air‐sea heat fluxes, while the slow response is related to the role of the meridional overturning circulation and ocean gyres in transporting heat anomalies from the subtropics to the subpolar ocean. The combination of these fast and slow responses leads to the sign reversal of the subpolar surface temperature anomalies in time. Key Points: Subpolar ocean changes due to the North Atlantic Oscillation (NAO) are separated into competing fast, seasonal and slow, inter‐annual responsesFast response is related to the anomalous Ekman transport and air‐sea heat exchange associated with the NAOSlow response is due to the redistribution of subtropical temperature anomalies via overturning and gyre circulations [ABSTRACT FROM AUTHOR]

Observations from the RAPID array at 26.5°N suggest a weakening in the Atlantic meridional overturning circulation between 2004 and 2020. The shortness of this time series raises the question of whether this weakening should be thought of as natural variability or as related to climate change. To estimate natural variability, preindustrial control runs of the Coupled Model Intercomparison Project Phase 6 are used and the observed weakening is found to fall within the distribution of naturally occurring trends. These trends result from model variability on periods longer than the observational window, and variation between models at these frequencies is especially pronounced. Finally, adding noise to inflate model variability also inflates model trends; however, the effect on trends is sensitive only to the low frequency portion of the noise spectrum. Plain Language Summary: The Atlantic meridional overturning circulation (AMOC) is a major ocean circulation system that transports heat northward in the Atlantic basin thus contributing to the regulation of the climate system. Fluctuations in the AMOC have been linked with changes in Arctic sea ice and to variations in the climate of Europe and North America. The longest direct observational record of the AMOC spans only 15.8 years and has shown a weakening of the circulation. Whether this weakening is due to natural oscillations in the climate system or is a response to anthropogenic greenhouse gases in the atmosphere is unclear. To assess whether the ocean circulation produces spontaneously weakening signals of such intensity, long simulations of climate models run under a preindustrial scenario are used to estimate the natural variability of the AMOC. These models suggest that the current weakening of the AMOC is consistent with natural variability. An accurate representation of slow timescale fluctuations in the AMOC is what matters most to the detection of an anthropogenically driven AMOC trend. This variability, however, differs markedly across models. Key Points: The 2004–2020 observed Atlantic meridional overturning circulation (AMOC) trend is not significantly relative to Coupled Model Intercomparison Project Phase 6 (CMIP6) model natural variabilityCMIP6 models largely disagree on AMOC variability, including at decadal and longer periodsIt is periods longer than the observational window that set AMOC trend detectability thresholds [ABSTRACT FROM AUTHOR]

The consecutive occurrence of meteorological dry and wet extremes has been receiving increasing attention due to potentially larger social and environmental impacts than single extremes. However, the changing characteristics of transitions between dry and wet periods remain poorly understood. Here we investigate the dynamic evolution of dry‐to‐wet transitions in response to climate warming, using the Standardized Precipitation Evapotranspiration Index as well as observations and general circulation models for the periods of 1954–2014 and 2040–2100. We find that approximately three‐fifths of global land area are projected to experience an accelerated dry‐to‐wet transition. Southern Asia has been experiencing the most severe dry‐to‐wet transitions and is projected to suffer more frequent, more intense, and accelerated dry‐to‐wet transitions. The increasing potential evapotranspiration (PET) variability plays an important role in accelerating the dry‐to‐wet transition in Southern Asia, while precipitation and PET variabilities contribute to the intensification of dry‐to‐wet transitions in Southern North America. Plain Language Summary: The consecutive occurrence of droughts and floods has widespread impacts on agriculture, the ecosystem, and the environment. Here we examine the changes in frequency, intensity, and elapsed time of transitions between dry and wet periods under a warming climate, based on observations and an ensemble of 10 Coupled Model Intercomparison Project Phase 6 climate simulations. We find that approximately 59% of global land area are expected to experience a shorter transition time between dry and wet periods. Southern Asia suffers the most severe dry‐to‐wet transition. Furthermore, our findings reveal that enhanced potential evapotranspiration (PET) variability contributes to the accelerated transition between dry and wet periods over Southern Asia, and the acceleration of dry‐to‐wet transitions in Southern North America is associated with an increase in precipitation and PET variabilities as climate warms. Key Points: Approximately three‐fifths (59%) of global land area are projected to experience accelerated transition between dry and wet periodsSouthern Asia suffers the most from severe transition between dry and wet periodsEnhanced precipitation and potential evapotranspiration variabilities contribute to the accelerated dry‐to‐wet transitions [ABSTRACT FROM AUTHOR]

This study investigates the seasonality of near‐surface wind speeds associated with extratropical cyclones (ETCs) over northeastern North America using a global reanalysis data set during 1979–2020. As opposed to most studies that emphasize winter storms, ETCs during the fall exhibit significantly stronger 10‐m winds over this region due to the slightly stronger continental cyclones and significantly weaker low‐level stability during that time of the year. Also, ETCs favor inland lakes and Hudson Bay during the low‐ice‐content fall season, leading to lower surface roughness. Combining these results, we derive simple linear regressions to predict the 10‐m wind speed given three variables: 850‐hPa wind speed, low‐level Richardson number, and surface roughness length. This formula captures the observed seasonality and serves as a valuable tool for cyclone near‐surface wind risk assessment. Plain Language Summary: Extratropical cyclones can bring powerful winds that can cause severe damage to infrastructure. We find that cyclones with severe winds are the most frequent in the fall season over continental northeastern North America. Three reasons are found responsible: stronger continental cyclones, weaker low‐level atmospheric stability, and the lower surface roughness over lakes and Hudson Bay, where cyclones frequently occur in fall. A simple formula that can effectively assess the near‐surface wind speeds associated with cyclones is derived based on these results. Key Points: Extratropical‐cyclone‐associated 10‐m wind speeds are the strongest in the fall season over northeastern North AmericaBesides stronger continental cyclones and 850‐hPa winds, weaker low‐level stability in fall favors stronger 10‐m wind speeds in this regionLinear regression using 850‐hPa wind, Richardson number, and surface roughness well predicts cyclones' 10‐m wind speeds and seasonality [ABSTRACT FROM AUTHOR]

A warmer climate impacts streamflows and these changes need to be quantified to assess future risk, vulnerability, and to implement efficient adaptation measures. The climate simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5), which have been the basis of most such assessments over the past decade, are being gradually superseded by the more recent Coupled Model Intercomparison Project Phase 6 (CMIP6). Our study portrays the added value of the CMIP6 ensemble over CMIP5 in a first North America wide comparison using 3,107 catchments. Results show a reduced spread of the CMIP6 ensemble compared to the CMIP5 ensemble for temperature and precipitation projections. In terms of flow indicators, the CMIP6 driven hydrological projections result in a smaller spread of future mean and high flow values, except for mountainous areas. Overall, we assess that the CMIP6 ensemble provides a narrower band of uncertainty of future climate projections, bringing more confidence for hydrological impact studies. Plain Language Summary: Greenhouse gas emissions are causing the climate to warm significantly, which in turn impacts flows in rivers worldwide. To adapt to these changes, it is essential to quantify them and assess future risk and vulnerability. Climate models are the primary tools used to achieve this. The main data set that provides scientists with state‐of‐the‐art climate model simulations is known as the Coupled Model Intercomparison Project (CMIP). The fifth phase of that project (CMIP5) has been used over the past decade in multiple hydrological studies to assess the impacts of climate change on streamflow. The more recent sixth phase (CMIP6) has started to generate projections, which brings the following question: is it necessary to update the hydrological impact studies performed using CMIP5 with the new CMIP6 models? To answer this question, a comparison between CMIP5 and CMIP6 using 3,107 catchments over North America was conducted. Results show that there is less spread in temperature and precipitation projections for CMIP6. This translates into a smaller spread of future mean, high and low flow values, except for mountainous areas. Overall, we assess that using the CMIP6 data set would provide a more concerted range of future climate projections, leading to more confident hydrological impact studies. Key Points: A comparison of hydrological impacts using Coupled Model Intercomparison Project version 5 (CMIP5) and Coupled Model Intercomparison Project Phase 6 (CMIP6) ensembles is performed over 3,107 catchments in North AmericaThe CMIP6 ensembles provide a narrower band of uncertainty for hydrological indicators in the futureIt is recommended to update hydrological impact studies performed using CMIP5 with the CMIP6 ensemble [ABSTRACT FROM AUTHOR]

We report 20 10Be exposure ages from glacial erratics and bedrock on the west coast of British Columbia, Canada, which add to existing chronologies of Cordilleran Ice Sheet retreat along ∼600 km of coastal North America. Collectively, these data show the western ice limit reached the present coast by 18–16 ka then retreat slowed for ∼4,000 years until 14–13 ka. We attribute initial retreat to destabilization and grounding line retreat resulting from rising sea level and/or ocean warming in the northern Pacific. Subsequent stability of the ice sheet at the present coastal margin was likely due to the transition from marine to terrestrial margins despite increasing temperatures that may have driven ice sheet thinning. Our findings demonstrate the importance of understanding both climatic and nonclimatic drivers of ice sheet change through time. Plain Language Summary: The Cordilleran Ice Sheet once covered western North America and was similar in size and characteristics to the modern Greenland Ice Sheet. It is therefore a good analog for how the Greenland Ice Sheet may respond to changing climate and underlying topography in the future. We show the western margin of the Cordilleran Ice Sheet retreated rapidly from around 18,000 years ago but then stabilized for several thousand years close to the present coastline. Ice may still have been lost during this period of relative stability, but through vertical thinning rather than lateral retreat, demonstrating the importance of understanding ice sheets in three‐dimensions. Our reconstruction shows that atmospheric and oceanic temperatures and underlying topography can all control ice sheet stability. We also show that hundreds of kilometers of coastline were free of ice prior to an important period of early human migration into the Americas. Key Points: Consistent, western ice sheet margin retreat at ∼18–16 ka driven by destabilization due to sea level rise and/or ocean warmingRetreat stabilized during ∼17–13 ka after reaching present coastIce sheet margin may have thinned before ∼13 ka, consistent with interior but without substantial marginal retreat [ABSTRACT FROM AUTHOR]

In the early summer (late June–early July) of 2021, a prolonged and extremely high temperature event was observed in the Northwest United States and Southwest Canada. In this study, a connection was found between the subseasonal variations of the East Asian monsoon system and the anomalous heat dome over western North America (WNA). Around 2–3 pentads prior to the period of maximum heat, a northward‐propagating rain belt and abnormally intensified jet stream in East Asia excited a Rossby wave train, causing the high‐pressure anomaly over WNA that was responsible for the occurrence of this heat wave in 2021. Multivariate singular value decomposition analysis results using historical data and a series of numerical experiments further supported the finding of this East Asia–western North America teleconnection. The results provide precursor signals for better monitoring and predicting heat waves over WNA. Plain Language Summary: A persistent and extremely high temperature event was observed in several states in the Northwest United States and Southwest Canada in summer 2021. This severe heat wave resulted in the hottest early‐summer season in western North America (WNA) since 1979 and huge social and economic losses. Although the heat dome (high‐pressure anomaly) over WNA was the factor responsible for this mega heat wave, the cause of this high‐pressure anomaly was unclear. The present study reveals the importance of an atmospheric wave train emanating from East Asia toward North America in inducing the heat dome over WNA. The generation of the wave train is attributable to a northward‐propagating rain belt and abnormally enhanced jet stream over East Asia from 10 to 15 days before the peak of this heat wave. Thus, monitoring the East Asian summer monsoon and associated wave train activity may help in predicting heat waves in WNA, and therefore in knowing when to put measures in place to mitigate against their effects. Key Points: An extreme heat wave occurred in western North America (WNA) in the early summer of 2021, causing huge socioeconomic losses in several big citiesThe subseasonal activity of East Asian summer monsoon contributed to the occurrence of the heat wave via excitation of a Rossby wave trainThe East Asia‐North America teleconnection can provide precursor signals for forecasting heat waves in WNA [ABSTRACT FROM AUTHOR]

Previous studies have reported that enhanced eddy moisture convergence causes future increase of winter precipitation in the eastern North America in a warmer climate. This study investigates the mechanisms of the change in moisture convergence. Using ensemble simulations from climate models and composite analysis of moisture budget, we show that in the southeast United States, two thirds of the enhanced moisture convergence is caused by local thermodynamics due to increase of moisture, while one third is caused by stronger eddy wind convergence. In the north, it is primarily caused by the increase of moisture gradient associated with the spatial distribution of both the climatological temperature and the large amplitudes of the eddy temperature in the region of the storm tracks. Results help to better understand the physical mechanism of more winter precipitation in the eastern North America in a warmer climate. Plain Language Summary: It has been reported that winter precipitation will increase in the eastern North America under global warming, which is caused by the enhanced eddy moisture convergence. We perform composite analysis of moisture budget using large ensemble simulations of climate models, and find out that the enhanced eddy moisture convergence in the southeast United States can be explained by the combination of climatological eddy wind convergence and increase of moisture content. The intensity of eddy wind convergence will enhance and contribute about one third of the eddy moisture convergence increase in the southeast United States. In the north of eastern North America, the climatological eddy wind brings more moisture into the region due to enhanced moisture gradient. The contrast of different mechanisms in the south and north can be explained by the climatology and future change of the spatial pattern of water vapor and temperature. These results help us to better understand the physical mechanisms that are responsible for the future precipitation increase during winter in the eastern North America under global warming. Key Points: Winter precipitation increases with warming due to enhanced eddy moisture convergence in the eastern North AmericaLocal thermodynamics associated with increased moisture and stronger eddies induce more moisture convergence in the southeast United StatesEnhanced moisture gradient in regions of large eddy magnitudes causes more moisture convergence in the north of eastern North America [ABSTRACT FROM AUTHOR]

We present the first application of a fast super resolution convolutional neural network (FSRCNN) based approach for downscaling earth system model (ESM) simulations. Unlike other SR approaches, FSRCNN uses the same input feature dimensions as the low resolution input. This allows it to have smaller convolution layers, avoiding over‐smoothing, and reducing computational costs. We adapt the FSRCNN to improve reconstruction on ESM data, we term the FSRCNN‐ESM. We use high‐resolution (∼0.25°) monthly averaged model output of five surface variables over North America from the US Department of Energy's Energy Exascale Earth System Model's control simulation. These high‐resolution and corresponding coarsened low‐resolution (∼1°) pairs of images are used to train the FSRCNN‐ESM and evaluate its use as a downscaling approach. We find that FSRCNN‐ESM outperforms FSRCNN and other super‐resolution methods in reconstructing high resolution images producing finer spatial scale features with better accuracy for surface temperature, surface radiative fluxes, and precipitation. Plain Language Summary: High resolution global climate data is computationally expensive to run but crucial for assessing climate change effects at local and regional scales. Here, we adapt a new deep learning technique, called fast super‐resolution convolutional neural network, to remap climate data from low resolution to high resolution grids. This approach is faster and more accurate for statistical downscaling climate data compared to other prevalent methods. Key Points: We present a fast super resolution convolutional neural network (FSRCNN) based approach for downscaling gridded earth system model (ESM) dataFSRCNN‐ESM's reconstruction of high resolution spatial patterns improves upon both traditional and machine learning downscaling methodsThe FSRCNN is computationally less expensive to train than other machine learning downscaling methods [ABSTRACT FROM AUTHOR]

Unaweep Canyon (Uncompahgre Plateau, Colorado) represents an enigmatic landscape with a complex evolution. Interpretations for its origin have ranged from ancestral fluvial erosion in the late Cenozoic to glacial erosion in the Paleozoic, or some combination thereof, with significant implications for global climatic and large‐scale tectonic reconstructions. To address the conflicting interpretations, we acquired a high‐resolution seismic reflection profile to investigate the depth, structure, and sedimentary infill in the canyon. The data set is further complemented with an electrical resistivity survey. Integrated with other geophysical and geological data, the results show an overdeepened Precambrian basement with transverse U shape and support the hypothesis of a pre‐Quaternary glacial origin. Our data constitute the first detailed image of a buried pre‐Quaternary glacial valley in North America; if substantiated with core studies, these results have far‐reaching implications for our understanding of global ice houses as well as the tectonic conditions, enabling preservation of such systems. Plain Language Summary: The shape of valleys depends on the processes that created them. Broad valleys with the transverse shape of a "U" are distinct signs of glaciers, which carved the valley in the bedrock. Since old valleys are filled up with sediments, we need geophysical tools to image their structure in the buried subsurface. Using such tools, we have found a buried U‐shaped valley in western Colorado. Since we know from other observations that there were no glaciers in this specific location in the last ice age, the valley must have been created in a previous ice age. The most likely time period is the late Paleozoic, ca. 300 million years ago. Due to plate tectonics, the landmass of Colorado was located at the equator during that time. This implies that there was ice at the equator at this time, which further means there were globally much lower temperatures than we think. Our result is the first example in North America of a glacial mountain valley carved by an ancient glacier. Key Points: We present the first high‐resolution seismic image of a buried paleovalley shaped by alpine glaciation in Earth's pre‐Quaternary recordOur data support the hypothesis of late Paleozoic glaciation at latitudes and elevations lower than suggested by current paleoclimate models [ABSTRACT FROM AUTHOR]

Plate motion changes provide powerful constrains on plate tectonic forces. North America is ideal to explore these forces. We use recently published high‐temporal resolution plate reconstructions of North America, which reveal a velocity slow‐down at ∼17 Ma, roughly coeval with the eruption of the Yellowstone plume, together with a stratigraphic analysis of hiatus surfaces across the continent, which provides proxy information for paleotopography. Using a simple Couette/Poiseuille flow models we estimate asthenosphere flow beneath North America and its impact on Neogene plate motion. We find that North America's Neogene plate motion change can be explained by Poiseuille flow in the asthenosphere generated upon the arrival of the Yellowstone plume and that the flow length‐scale matches the extent of hiatus surfaces. While plume driven upper mantle flow constitutes a geodynamically viable model to explain North America's Neogene plate motion change it provides an intrinsic link between vertical and horizontal plate motions. Plain Language Summary: Plate motion changes are increasingly well documented in the geologic record. However, the underlying forces that initiate these plate motion changes remain poorly understood. Over the past years a pressure driven, so‐called Poiseuille flow, model for upper mantle flux in the asthenosphere has gained increasing geodynamic attention–for a number of fluid dynamic arguments. This elegantly simple model makes a powerful testable prediction: Plate motion changes should coincide with regional scale mantle convection induced elevation changes. The latter are best inferred for continents from stratigraphic evidence. In this study we analyse newly available constrains on the Neogene kinematic plate history of North America together with a novel stratigraphic frame‐work based on the principle of hiatus analysis. The stratigraphic frame work provides proxy information for paleo elevation. Combining these newly available constrains we show for the first time that: (a) Poiseuille flow induced by the arrival of the Yellowstone hotspot provides a compelling explanation for the observed coeval velocity and topographic change of North America, and (b) the Yellowstone plume push force is sufficient to initiate a plate motion change of North America under considerations of a quantitative geodynamic force balance analysis. Key Points: We analyse the Neogene kinematic plate history of North America together with a stratigraphic frame‐work base on the principle of hiatusPoiseuille flow model links changes in dynamic topography and plate motion changes via a geodynamically plausible modelYellowstone plume provides a dynamically viable mechanism to drive Neogene North America platemotions [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) exert significant socioeconomic impacts in western North America, where 30% of the annual precipitation is determined by ARs that occur in less than 15% of wintertime. ARs are thus beneficial to water supply but can produce extreme precipitation hazards when making landfall. While most prevailing research has focused on the subseasonal (≤5 weeks) prediction of ARs, only limited efforts have been made for AR forecasts on multiseasonal timescales (≥3 months) that are crucial for water resource management and disaster preparedness. Through the analysis of reanalysis data and retrospective predictions from a new seasonal‐to‐decadal forecast system, this research shows the existing potential of multiseasonal AR frequency forecasts with predictive skills 9 months in advance. Additional analysis explores the dominant predictability sources and challenges for multiseasonal AR prediction. Plain Language Summary: Atmospheric rivers (ARs), narrow corridors of intense moisture transport and heavy precipitation, are an important water resource but also a cause of flooding‐related disasters for western North America. Consequently, predictions of AR frequency several seasons in advance potentially would be of great value, but such operational forecasts are currently lacking due to the challenges in simulating such intense, small‐scale weather phenomena and their predictability sources on seasonal timescales. In this study, we examine the forecast skill of AR frequency on seasonal‐to‐multiseasonal timescales (≥3 months) in a new generation seasonal‐to‐decadal prediction system developed at the Geophysical Fluid Dynamics Laboratory. We find that AR frequency can be skillfully forecast at least 9 months in advance over certain regions of the west coast of North America, such as California and Alaska, while the forecasts are only reliable for the first season in other regions. This regional variability can be further explained by the large‐scale climate variability pattern that is responsible for much of the skill, which is strongly modulated by slowly varying sea surface temperature (SST) variations. A prototype probabilistic seasonal AR forecast product is proposed. Key Points: Atmospheric river frequency can be skillfully forecast at least 9 months in advanceThe skills are significant over certain regions of western North America such as Alaska and CaliforniaEl Nino‐Southern Oscillation and Interdecadal Pacific Oscillation are important predictability sources [ABSTRACT FROM AUTHOR]

We use splitting in core‐refracted teleseismic shear waves (SKS, PKS, and similar) to investigate anisotropic properties of the upper mantle beneath the Superior craton in eastern North America and the Yilgarn craton in Western Australia. At four sites in each craton, we assemble extensive data sets that emphasize directional coverage, and use three different measurement methods to develop mutually consistent constraints on the nature of splitting and on the likely anisotropic properties that cause it. In both cratons, we see evidence of clear directional variation in both delays and fast polarization directions, as well as lateral differences between sites. Relatively small (0.3–0.8 s) amounts of splitting imply weak anisotropy within 150–220 km thick mantle lithosphere. Anisotropy in the asthenosphere likely contributes to splitting in North America where fast directions align with absolute plate motion, but not in Western Australia where fast polarizations and plate motion are nearly orthogonal. Plain Language Summary: Cratons, areas of the continents that did not deform for a billion years or more, are the only record of the geological events from the first half of Earth's existence. Large‐scale deformation of the rigid lithosphere of cratons is expected to imprint a systematic fabric within its rocks. This fabric makes the speed of seismic waves dependent on direction (anisotropic) and causes birefringence (splitting) in horizontally polarized (shear) waves from distant earthquakes. Surprisingly low level of birefringence is seen in many cratons, suggesting weak anisotropy and by inference—a poorly developed systematic fabric within them. Alternatively, competing effects of fossil anisotropy in the lithosphere and the anisotropy due to present‐day deformation beneath it can result in cancellation of birefringence. We measured splitting in large data sets from the Yilgarn craton of Western Australia and the Superior craton in eastern North America. We confirm the small amount of splitting in both areas and rule out mutually canceling effects of anisotropy within and beneath the lithosphere. This suggests weak anisotropy in the lithosphere of both cratons. Directions of tectonic plate motion suggest that anisotropy due to deformation beneath the lithosphere affects our measurements in North America but not in Australia. Key Points: Three consistent methods are used to measure splitting in core‐refracted shear waves in Yilgarn and Superior cratonsSmall amount of splitting in both regions is found indicating weak anisotropy in both cratons' lithosphereAnisotropy in the asthenosphere likely contributes to splitting in the Superior but not in the Yilgarn [ABSTRACT FROM AUTHOR]

Limited observational evidence indicates that ionospheric changes caused by Arctic sudden stratospheric warmings (SSWs) occur at middle latitudes in the Southern Hemisphere. However, it is not known if a similar interhemispheric linkage is produced by Antarctic SSWs. Here we examine thermospheric and ionospheric anomalies observed in September 2019 at middle latitudes in the Northern Hemisphere. We report persistent (at least 30 days) and strong (up to 80%–100%) positive anomalies in the daytime total electron content (TEC) and increases in the thermospheric O/N2 ratio in the western region of North America. However, central and eastern regions of North America experience moderate suppression of TEC reaching 20%–40% of the baseline. Different positive and negative anomalies are observed over the European sector. We hypothesize that regional differences in the TEC response could be related to modulation of thermospheric winds during SSWs, changes in thermospheric composition, and differences in declination angle. Plain Language Summary: Sudden stratospheric warmings (SSWs) are large‐scale meteorological disturbances that have been associated with profound anomalies in the Earth atmosphere, from troposphere all the way to the upper thermosphere and ionosphere. During the last decade, numerous studies shown that Arctic SSWs cause large anomalies in the low‐latitude ionosphere, and few studies pointed out especially large disturbances at middle latitudes in the southern hemisphere ionosphere. However, it is not known if similar mid‐latitude ionospheric anomalies on the other side of the globe are produced by Antarctic SSWs, mostly because Antarctic SSWs occur less often than Arctic ones. In this study we analyze ionospheric and thermospheric observations in September 2019, when a very strong SSW developed over Antarctica. We found both positive and negative disturbances in the thermospheric O/N2 ratio and total electron content over North America and over Europe. Surprisingly, these disturbances are limited to narrow (20–40°) longitude ranges, differ between North America and Europe, and persist for a long time. We discuss the reasons why these anomalies occur only at specific longitudes and suggest that differences in magnetic declination angle play an important role as they affect how SSW‐modified thermospheric wind influences ionospheric electron density. Key Points: Large regional ionospheric anomalies at northern hemisphere middle latitudes are linked to the 2019 Antarctic sudden stratospheric warmingThe ionospheric response has a quasi‐semidiurnal behavior and is most likely driven by more than one mechanismWe emphasize the potential role of thermospheric zonal wind and magnetic declination in the formation of these disturbances [ABSTRACT FROM AUTHOR]

Catastrophic meltwater drainage from glacial Lake Agassiz has been hypothesized as a trigger for large‐scale ocean circulation change initiating the Younger Dryas cold reversal. Here we quantify the flood discharge that formed the northwestern outlet of Lake Agassiz using a one‐dimensional step‐backwater model and a zero‐dimension gradual‐incision model. Applying these two independent models, we estimate a peak discharge range of 1.8–2.5 × 106 m3 s−1 and a flood volume of ∼21,000 km3. Such a discharge can only be derived from Lake Agassiz rather than one of the two smaller regional glacial lakes: Churchill or Meadow. When coupled with existing ice margin chronologies, these results demonstrate that the northwestern outlet of Lake Agassiz provides a viable link for catastrophic meltwater to drain to the Arctic Ocean over a 6–9 month period during the Younger Dryas, though it is unclear whether this was near its beginning. Plain Language Summary: The Younger Dryas was a short‐lived interval of cooling that interrupted warming during the last deglaciation. The cause of this rapid change in climate is debated. One suggestion is the drainage of meltwater from glacial Lake Agassiz, a large ice‐dammed lake in central North America, into the surrounding oceans may have affected ocean circulation, contributing to this climatic event. Here we model the discharge of water from the northwestern outlet of this lake. We estimate the discharge and demonstrate the connection between Lake Agassiz and the Arctic Ocean during the Younger Dryas. Key Points: Peak discharge modeled for the northwestern outlet of glacial Lake AgassizMagnitude of drainage estimated at 1.8–2.5 × 106 m3 s−1 (1.8–2.5 Sv)Northwestern outlet of glacial Lake Agassiz provides a viable link for catastrophic meltwater to the Arctic Ocean during the Younger Dryas [ABSTRACT FROM AUTHOR]

The Madden‐Julian oscillation (MJO) modulates atmospheric river (AR) activity along the West Coast of North America. The forecast skill of ARs based solely on the MJO phase is limited, motivating the identification of other predictive factors. Here we show that low‐frequency variability of the Pacific/North American (PNA) pattern is mostly independent of the MJO, and the monthly PNA pattern influences the MJO‐AR relationship much more robustly than other large‐scale patterns such as the El Niño Southern Oscillation and the quasi‐biennial oscillation. During PNA+ conditions, when the Pacific jet exit region is located further east than average, baroclinic wave packets extend downstream toward North America. The clear contrast in wave packets' zonal scale results in partitioning active and inactive AR phases. The PNA influence is most significant over the Pacific Northwest region of North America, where AR landfall is most frequent, with implied forecasting skill beyond 20 days. Plain Language Summary: The West Coast of North America often experiences heavy rainfall due to narrow bands of intense water vapor transport, called atmospheric rivers (ARs). Previous research showed slow oscillations in the tropical atmosphere modulate AR activity, indicating the potential of AR forecasts in the sub‐seasonal time scale (3–5 weeks). However, the relationship is influenced by the structure and the configuration of the extratropical state. Here we show that monthly values of the Pacific/North American index of the extratropical Pacific atmosphere clearly classifies the essential relationship. The results suggest that the strength of the Aleutian low and the Pacific jet stream are the key drivers of the relationship, by guiding storms toward the West Coast of North America. We demonstrate that AR activity over the Pacific Northwest region in particular may be skillfully forecast for more than 20 days. Key Points: Low‐frequency variations in the Pacific/North American (PNA) pattern play a major role in modulating Madden‐Julian oscillation (MJO) teleconnections to atmospheric river (AR) activity over the Northeast PacificAleutian low intensity and Pacific jet extension/retraction roughly partition active and inactive AR phasesAR composites suggest forecasting skill potentially up to 3 weeks by using both MJO and monthly PNA phases [ABSTRACT FROM AUTHOR]

Obtaining an efficient prediction of the Pacific Decadal Oscillation (PDO) phase transition is a worldwide challenge. Here, we employed the climate network analysis to uncover early warning signals prior to a PDO phase transition. This way an examination of cooperative behavior in the PDO region revealed an enhanced signal that propagated from the western Pacific to the northwest coast of North America. The detection of this signal corresponds very well to the time when the upper ocean heat content in the off‐equatorial northwestern tropical Pacific reaches a threshold, in which case a PDO phase transition may be expected with the arising of the next El Ni n∼o/La Nin∼ a event. The objectively detected early warning signal successfully forewarned all the six PDO phase transitions from the 1890–2000, and also underpinned the possible PDO phase transition around 2015, which may be triggered by the strong El Nin∼o event in 2015–2016. Plain Language Summary: The Pacific Decadal Oscillation (PDO), as one of the main internal decadal climate variabilities, has been shown to have substantial impacts on both the climate system and other parts of Earth system. Due to its complexity, however, it is still challenging to predict its phase changes. Here, a new technique from climate network analysis was employed to capture the potential precursors of the phase transition. By calculating the linkages between the warm and cold regions of the PDO spatial pattern, a sudden strengthening of negative connections between the two regions was detected a few years before the phase transition. This early warning signal arises from the cooperative behavior of the considered regions, that is, many more grid points from the two regions are becoming negatively linked at the time of alarming. This time corresponds well to the moment when the upper ocean heat content in the off‐equatorial northwestern tropical Pacific reaches a threshold, in which case a PDO phase transition may be expected with the arising of the next El Nin∼o/La Nin∼a event. Since the early warning signal is objectively detected from the climate network analysis, it may be used in operations for improved predictions of the PDO. Key Points: An early warning signal for the Pacific Decadal Oscillation (PDO) phase transition is detectedThe signal arises mainly from the cooperative behaviors of the grid points in the PDO regionA new strategy for the prediction of the PDO phase transition is proposed [ABSTRACT FROM AUTHOR]

Weeks 3–6 averaged winter North Atlantic Oscillation (NAO) predictive skill in a state‐of‐the‐art coupled climate prediction system is attributed to two principle sources: upper and lower boundary conditions linked to the stratosphere and El Niño‐Southern Oscillation (ENSO), respectively. A 20‐member ensemble of 45‐day reforecasts over 1999–2015 is utilized, together with uninitialized simulations with the atmospheric component of the prediction system forced with observed radiative forcing and lower boundary conditions. NAO forecast skill for lead times out to 6 weeks is higher following extreme stratospheric polar vortex conditions (weak and strong vortex events) compared to neutral states. Enhanced weeks 3–6 NAO predictive skill for weak vortex events results primarily from stratospheric downward coupling to the troposphere, while enhanced skill for strong vortex events can be partly attributed to lower boundary forcing related to the ENSO phenomenon. Implications for forecast system development and improvement are discussed. Plain Language Summary: Winter climate over Europe and eastern North America is significantly affected by variability of the North Atlantic Oscillation (NAO). In this study, we quantify the NAO predictive skill for lead times of 3–6 weeks and attribute it to two main sources: the stratosphere and the El Niño‐Southern Oscillation (ENSO) phenomenon. This is done by contrasting ensembles of 45‐day reforecasts over 1999–2015 with the corresponding uninitialized atmosphere model simulations forced with observed radiative forcing, sea‐surface temperature, and sea ice conditions. We find that the model is able to better predict the NAO up to 3 weeks following extreme weak or strong states of the stratospheric polar vortex compared to stratospheric neutral vortex states. Enhanced weeks 3–6 NAO predictive skill for weak vortex events results primarily from stratospheric coupling to the troposphere, while enhanced skill for strong vortex events can be attributed in part to lower boundary forcing related to ENSO. These results have implications for forecast model development and improvement. Key Points: Weeks 3–6 NAO predictive skill is attributed to stratospheric polar vortex conditions and ocean lower boundary forcingEnhanced NAO skill following weak vortex events results primarily from stratospheric coupling to the troposphereEnhanced NAO skill following strong vortex events can be partly attributed to ENSO [ABSTRACT FROM AUTHOR]

The Younger Dryas (YD, 12.9–11.7 ka) is the most recent, near‐global interval of abrupt climate change with rates similar to modern global warming. Understanding the causes and biodiversity effects of YD climate changes requires determining the spatial fingerprints of past temperature changes. Here we build pollen‐based and branched glycerol dialkyl glycerol tetraether‐based temperature reconstructions in eastern North America (ENA) to better understand deglacial temperature evolution. YD cooling was pronounced in the northeastern United States and muted in the north central United States. Florida sites warmed during the YD, while other southeastern sites maintained a relatively stable climate. This fingerprint is consistent with an intensified subtropical high during the YD and demonstrates that interhemispheric responses were more complex spatially in ENA than predicted by the bipolar seesaw model. Reduced‐amplitude or antiphased millennial‐scale temperature variability in the southeastern United States may support regional hotspots of biodiversity and endemism. Plain Language Summary: The Younger Dryas, circa 12,900 to 11,700 years ago, is a hemispheric abrupt climate change event that occurred at rates similar to those projected by the 21st century. Its cause has been linked to a reduction in northward oceanic heat transport in the Atlantic that led to Northern Hemispheric cooling and Southern Hemispheric warming. Here we present detailed reconstructions of Younger Dryas temperature variations in eastern North America that suggest a more complex spatial fingerprint than predicted by the standard global model. New England, Maritime Canada, and the Great Lakes Region all cooled, like Greenland and elsewhere in the Northern Hemisphere. However, regions south of Virginia experienced little temperature change and Florida warmed slightly. Possible mechanisms include atmospheric processes that enhanced advection from the subtropics and oceanic processes that transported heat northward from the equatorial Atlantic. These reconstructions also help explain the hotspot of biodiversity and endemic species in the southeastern United States, by showing that this region was buffered from past abrupt millennial‐scale climate reversals. Key Points: The spatial fingerprint of Younger Dryas (YD) temperature changes is reconstructed in eastern North America from brGDGTs and fossil pollenReconstructions demonstrate higher YD temperatures in Florida, no change south of 40°N, and cooling north of 40°NThese patterns are consistent with intensified subtropical highs during the YD and help explain high regional biodiversity [ABSTRACT FROM AUTHOR]

Based on the statistical analyses using ERA data sets, this study reveals a significant linkage between the tropospheric Western Hemisphere (WH) circulation pattern and the stratospheric EOF1 mode (North America [NA] pattern) of the wintertime daily height field at 50 hPa, which exhibits the displacement of the polar vortex to NA. The results of composite analysis indicate that the WH pattern leads the NA pattern at about 5 days. This time‐lagged linkage between the two patterns is further confirmed by the results of time‐lagged singular value decomposition (SVD) analyses. Through the linear interference with climatological waves, the WH pattern contributes to the enhanced (suppressed) wave‐2 (wave‐1) activities propagating into the stratosphere. The characteristics of planetary waves, with opposite sign of anomalous wave‐1 and wave‐2 activities, lead to the formation of the stratospheric NA pattern. We suggest the need for caution in interpreting the stratospheric influences on cold air outbreaks over NA. Key Points: There is an obvious time‐lagged linkage between the WH circulation pattern and the EOF1 mode of NH 50‐hPa daily height anomaliesThe WH pattern can enhance (suppress) the planetary wave‐2 (wave‐1) fluxes into the stratosphere through the linear interferenceThe anomalous wave‐1 and wave‐2 fluxes into the stratosphere with opposite sign are both important for the polar vortex morphology [ABSTRACT FROM AUTHOR]

We quantify GIA prediction uncertainties of 250 1D and 3D glacial isostatic adjustment (GIA) models through comparisons with deglacial relative sea‐level (RSL) data from North America and rate of vertical land motion (U̇) and gravity rate of change (Ġ) from GNSS and GRACE data, respectively. Spatially, the size of the RSL uncertainties varies across North America with the largest from Hudson Bay and near previous ice margins along the northern Atlantic and Pacific coasts, which suggests 3D viscosity structure in the lower mantle and laterally varying lithospheric thickness. Temporally, RSL uncertainties decrease from the Last Glacial Maximum to present except for west of Hudson Bay and the northeastern Pacific coast. The uncertainties of both these regions increase from 30 to 45 m between 15 and 11 ka BP, which may be due to the rapid decrease of surface loading at that time. Present‐day U̇ and Ġ uncertainties are largest in southwestern Hudson Bay with magnitudes of 2.4 mm/year and 0.4 μGal/year, mainly due to the 3D viscosity structure in the lower mantle. Key Points: We predict the GIA signal with uncertainties of RSL and present‐day U̇ and Ġ in North America that are associated with 3D structuresGIA‐predicted RSL uncertainties are largest around Hudson Bay and near former ice margins along northern Atlantic and Pacific coastsThe maximum uncertainties of present‐day U̇ and Ġ are centered in southwestern Hudson Bay and near former ice margins [ABSTRACT FROM AUTHOR]

How future tropical cyclone (TC) activity could change under global warming is enormously important to society, which has been widely assessed using state‐of‐the‐art climate models. However, these models were predominantly based on projection of an El Niño‐like warming pattern. Recent studies suggested that a La Niña‐like warming pattern is also possible. Here we compare the responses of TC track density (TCTD) over the western North Pacific and North Atlantic to the two distinct global warming patterns. We find that the La Niña‐like warming pattern reduces western North Pacific TCTD except in the South China Sea and along China coast and increases NA TCTD, while the El Niño‐like warming pattern generally reduces TCTD in both basins. This is due to different responses of large‐scale dynamic/thermodynamic conditions to the distinct zonal sea surface temperature gradients associated with the two warming patterns. These results help better understand potential future change in TC tracks. Key Points: The La Niña‐like warming reduces (increases) TC track density in the main WNP (NA), while the El Niño‐like warming reduces it in two basinsThe different responses are due to the different tropical zonal SST gradients associated with the two warming patternsChina and North America would experience more landfalling TCs in La Niña‐like warming pattern than in El Niño‐like warming pattern [ABSTRACT FROM AUTHOR]

The Atlantic coast of North America north of Cape Hatteras has been proposed as a "hotspot" of late 20th century sea‐level rise. Here we test, using salt‐marsh proxy sea‐level records, if this coast experienced enhanced sea‐level rise over earlier multidecadal‐centennial periods. While we find in agreement with previous studies that 20th century rates of sea‐level change were higher compared to rates during preceding centuries, rates of 18th century sea‐level rise were only slightly lower, suggesting that the "hotspot" is a reoccurring feature for at least three centuries. Proxy sea‐level records from North America (Iceland) are negatively (positively) correlated with centennial changes in the North Atlantic Oscillation. They are consistent with sea‐level "fingerprints" of Arctic ice melt, and we therefore hypothesize that sea‐level fluctuations are related to changes in Arctic land‐ice mass. Predictions of future sea‐level rise should take into account these long‐term fluctuating rates of natural sea‐level change. Plain Language Summary: Measurements of sea‐level change have shown that during the 20th century sea‐level rise along the Atlantic coast of North America between Cape Hatteras and Nova Scotia has been faster than the global average. We investigated whether this anomaly also occurred earlier by reconstructing historical sea‐level changes from salt‐marsh sediments and microscopic salt‐marsh fossils (foraminifera). We found evidence in three locations (Nova Scotia, Maine, and Connecticut) for rapid sea‐level rise in the 18th century, which was almost as rapid as the 20th century sea‐level rise. Using additional sea‐level reconstructions from across the North Atlantic, we propose an explanation for the periods of enhanced sea‐level rise. We hypothesize that they occur during distinct phases of the North Atlantic Oscillation and during periods of enhanced ice melt in the Arctic. The fluctuations are a reoccurring feature and should be considered in planning for future sea‐level rise and coastal hazards. Key Points: An episode of rapid sea‐level rise occurred in the 18th century along the Atlantic coast of North America (Nova Scotia to Cape Hatteras)The 18th century and previously identified 20th century hotspots occur in similar geographic regionsWe hypothesize that these centennial sea‐level changes are related to variations in Arctic land‐ice mass [ABSTRACT FROM AUTHOR]

Sedimentary records of lake‐level fluctuations have long provided unambiguous evidence of past hydrologic changes. Here, we synthesize geomorphic and sedimentological evidence of late‐Quaternary lake‐level changes across North America and show that currently moist regions were commonly drier than today. Many lakes across humid or snow‐dominated areas of North America were lower than today or totally desiccated for prolonged periods but rose to their current levels following climate changes in recent millennia. Maps of 173 records show that hydroclimates responded to deglacial dynamics and orbital changes but that regional hydrology changed even under the modest forcing and temperature variability of the past 8,000 years. Submerged paleoshorelines, ubiquitous across humid regions, reveal that North America's ecosystems and critical zone existed in a drier‐than‐modern state for much of the Quaternary and that continental moisture gradients often differed from today. The hydrologic changes underscore the climatic risk to many societally critical water supplies. Key Points: A synthesis of water‐level changes in 173 lakes across North America shows extensive hydroclimate change during the late QuaternaryCurrently humid or snow‐dominated regions were commonly drier than today since the last glacial maximumAridity was extensive across North America from 13–6 ka but shifted in distribution as factors like storm tracks responded to deglaciation [ABSTRACT FROM AUTHOR]

The impact of the Arctic stratospheric polar vortex on persistent weather regimes over North America is so far underexplored. Here we show the relationship between four wintertime North American weather regimes and the stratospheric vortex strength using reanalysis data. We find that the strength of the vortex significantly affects the behavior of the regimes. While a regime associated with Greenland blocking is strongly favored following weak vortex events, it is not the primary regime associated with a widespread, elevated risk of extreme cold in North America. Instead, we find that the regime most strongly associated with widespread extremely cold weather does not show a strong dependency on the strength of the lower stratospheric zonal mean zonal winds. We also suggest that stratospheric vortex morphology may be particularly important for cold air outbreaks during this regime. Plain Language Summary: During winter, the strength of the winds 10–50 km above the Arctic can affect the weather patterns at the surface. Generally, this influence is strongest over the North Atlantic and Europe. However, we show that the strength of stratospheric winds has a significant impact on weather patterns across North America. Our results indicate that knowledge of the stratospheric winds can provide a greater understanding of the evolution of likely weather in this region on longer time periods, including both severely cold weather (and its associated impacts on energy consumption, transport, and human health) and an unusual absence of severe cold. Key Points: The behavior of three of four regimes over North America is significantly linked to the strength of the lower stratospheric polar vortexA regime associated with Greenland blocking shows the strongest relationship with the stratospheric polar vortex strengthThe regime most strongly associated with widespread severe North American cold does not show a dependency on stratospheric vortex strength [ABSTRACT FROM AUTHOR]

Atmospheric inversions allow us to estimate the terrestrial carbon sink by combining atmospheric observations with atmospheric transport models. However, these inverse estimates remain highly uncertain. Here we quantify uncertainties in simulations of North American atmospheric CO2 concentrations using a probabilistic approach. We demonstrate that uncertainty in fossil fuel emissions is a key factor in the uncertainty surrounding biospheric flux estimates. We show that atmospheric transport uncertainties in state‐of‐the‐art numerical weather models diminish when averaged over time, while uncertainties in large‐scale CO2 boundary inflow considerably impair our ability to quantify regional fluxes. Current estimates of the North America land sink that neglect the uncertainties in CO2 boundary inflow and fossil fuel emissions are likely overconfident. Our findings suggest that targeted use of new atmospheric observations and improved quantification of uncertainty components are a promising avenue to improve atmospheric inversions with the goal to refine estimates of biospheric CO2 fluxes on regional and continental scales. Plain Language Summary: The uncertainty in biospheric carbon dioxide (CO2) flux estimates drives divergent projections of future climate and uncertainty in prescriptions for climate mitigation. The terrestrial carbon sink can be inferred from atmospheric CO2 observations with transport models via inversion methods. Regional CO2 flux estimates remain uncertain due to the mixture of uncertainties caused by transport models, prior estimates of biospheric fluxes, large‐scale CO2 boundary inflow, the assumptions in the inversion process, and the limited density of atmospheric CO2 observations. Understanding the characteristics of these uncertainties in space and time is essential for accurate biospheric CO2 flux estimates. Here we identify the terms that most confound biospheric flux estimates. Our results show that, over North America, (i) biospheric fluxes dominate the model uncertainty over all timescales. (ii) Contrary to expectation, fossil fuel emissions are the second largest source of uncertainty at all timescales. (iii) Transport uncertainties are large at short timescales but act like random errors decreasing with time averaging. (ix) Continental boundary inflow uncertainties are large near the boundaries and become significant at seasonal to annual timescales. We propose sampling and analysis strategies that can better quantify and reduce uncertainties in both fossil fuel emission and biospheric flux estimates. Key Points: We quantify biospheric flux, fossil fuel emission, atmospheric transport, and boundary inflow uncertainties in modeled atmospheric CO2Biospheric fluxes and fossil fuel emissions are the largest contributors to atmospheric CO2 uncertainty over North AmericaTransport uncertainties can be approximated by random errors, while boundary inflow uncertainties are persistent and can be sizeable [ABSTRACT FROM AUTHOR]

During past glacial periods, extensive areas of North America were covered by permafrost. The timing and extent of these paleo‐permafrost conditions, however, remains ambiguous. Here we present a 250,000‐year record of speleothem growth from a midlatitude North American cave and report 141 U‐Th ages with hiatuses in growth that reflect the development of temporally continuous permafrost. Combined with U‐Th ages from other speleothem studies, we demonstrate that regional permafrost conditions occurred during both of the prior two glacial maxima but were markedly shorter in duration during the penultimate (Marine Isotope Stage 6, MIS 6) versus the last (MIS 2) glacial period. Notably, a network of sea surface temperatures indicates that mid‐ and low‐latitude temperatures were 0.9 °C ± 0.2 °C warmer during the culmination of MIS 6 versus MIS 2. Our results illustrate the importance of developing regional paleo‐permafrost records and highlight the sensitivity of permafrost conditions during glacial periods to relatively small differences in global‐scale temperature. Plain Language Summary: Permafrost is an important reservoir of carbon during glacial periods. Our understanding of the timing and spatial extent of permafrost conditions during recent glaciations, however, is lacking because of a paucity of paleo‐permafrost indicators. Here we present a new, high‐precision chronology of speleothem growth in a Wisconsin (USA) cave that constrains permafrost conditions during the last two glacial periods. The new record suggests that permafrost conditions endured for 18 ka during the culmination of the last glacial period but were ephemeral during the penultimate glacial period. Our findings are supported by sea surface temperature reconstructions that indicate the penultimate glaciation was nearly 1 °C warmer than the last glacial period. These results provide useful information for quantifying the sensitivity of permafrost to past temperature changes. Key Points: A 250,000‐year record of speleothem growth from Wisconsin (USA) reveals differing permafrost persistence in the last two glacial periodsPaleo‐permafrost indicators across the American Midwest suggest that the penultimate glaciation was warmer than the last glacial periodA network of midlatitude and tropical sea surface temperatures demonstrates a 1 °C difference between the last two glacial periods [ABSTRACT FROM AUTHOR]

Hydroclimate extremes in North America, Europe, and the Mediterranean are linked to ocean and atmospheric circulation anomalies in the Atlantic, but the limited length of the instrumental record prevents complete identification and characterization of these patterns of covariability especially at decadal to centennial time scales. Here we analyze the coupled patterns of drought variability on either sides of the North Atlantic Ocean basin using independent climate field reconstructions spanning the last millennium in order to detect and attribute epochs of coherent basin‐wide moisture anomalies to ocean and atmosphere processes. A leading mode of broad‐scale moisture variability is characterized by distinct patterns of North Atlantic atmosphere circulation and sea surface temperatures. We infer a negative phase of the North Atlantic Oscillation and colder Atlantic sea surface temperatures in the middle of the fifteenth century, coincident with weaker solar irradiance and prior to strong volcanic forcing associated with the early Little Ice Age. Key Points: Atlantic ocean‐atmosphere circulation links drought in North America, Europe, and the MediterraneanCoupled modes of drought variability can be used to infer atmospheric circulation and ocean temperature anomaliesCoupled terrestrial drought patterns reveal the start of the Little Ice Age to be characterized by a negative NAO and cold Atlantic SSTs [ABSTRACT FROM AUTHOR]

Western North American (WNA) glaciers outside of Alaska cover 14,384 km2 of mountainous terrain. No comprehensive analysis of recent mass change exists for this region. We generated over 15,000 multisensor digital elevation models from spaceborne optical imagery to provide an assessment of mass change for WNA over the period 2000–2018. These glaciers lost 117 ± 42 gigatons (Gt) of mass, which accounts for up to 0.32 ± 0.11 mm of sea level rise over the full period of study. We observe a fourfold increase in mass loss rates between 2000–2009 [−2.9 ± 3.1 Gt yr−1] and 2009–2018 [−12.3 ± 4.6 Gt yr−1], and we attribute this change to a shift in regional meteorological conditions driven by the location and strength of upper level zonal wind. Our results document decadal‐scale climate variability over WNA that will likely modulate glacier mass change in the future. Plain Language Summary: Glaciers in western North America provide important thermal and flow buffering to streams when seasonal snowpack is depleted. We used spaceborne optical satellite imagery to produce thousands of digital elevation models to assess recent mass loss for glaciers in western North America outside of Alaska. Our analysis shows that glacier loss over the period 2009–2018 increased fourfold relative to the period 2000–2009. This mass change over the last 18 years is partly explained by changes in atmospheric circulation. Our results can be used for future modeling studies to understand the fate of glaciers under future climate change scenarios. Key Points: We provide a comprehensive assessment of mass change for western North American (WNA) glaciers excluding those in AlaskaWNA glaciers lost 117 +/‐ 42 gigatons (Gt) of mass over the period 2000‐2018 and could account for 0.32 +/‐ 0.11 mm of sea level riseRegional changes in glacier mass are partly explained by decadal scale changes in atmospheric circulation [ABSTRACT FROM AUTHOR]

The Pacific North American (PNA) pattern is an internal mode of the Northern Hemisphere atmosphere, which strongly affects the hydroclimate and ecosystems of the Pacific‐North American sector. Recent studies have suggested a more positive PNA pattern in response to greenhouse warming, but how extreme PNA events will change remains unclear. Based on results from the Coupled Model Intercomparison Project Phase 5 multimodel ensemble, we find a 50% increase in the frequency of extreme positive PNA events and a 32% increase in extreme negative PNA events in the 21st century relative to the twentieth century. The increased frequency arises from a shift toward a more positive PNA pattern and is enhanced by more frequent ENSO events under greenhouse warming. Our study suggests that a continued increase in greenhouse‐gas emissions is likely to cause increased occurrence of extreme PNA events, thus increasing the risk of extreme weather and climate in North America. Key Points: Increased frequency of extreme positive and negative PNA events is detected in response to greenhouse warmingIncreased extreme PNA events are associated with a shift toward a positive PNA pattern and more frequent ENSO eventsIncreased extreme PNA occurrences aggravate the risk of weather and climate extremes in North America [ABSTRACT FROM AUTHOR]

Given the critical role that snow plays in modulating current climate, it is vital to examine the contribution of the projected snow decline to the future warming. This is investigated over North America using regional climate modeling experiments with prescribed current and future snow conditions. Results indicate that the decline in snow contributes to future warming by up to 2°C and 4°C, respectively, to annual and spring temperature increases. Results also show that decline in snow depth and cover lead to the attenuation (amplification) of cold (warm) extremes. The rate of intensification of warm extremes is higher over the snow‐abundant regions of northern Canada—where current snow cover fraction is close to one. The largest attenuation in cold extremes, on the other hand, is over snow marginal regions of the Great Plains and Prairies. These results demonstrate the spatiotemporal differences in projected changes exclusively associated with snow cover and depth decline. Plain Language Summary: Climate models suggest a decline in snow cover and snow depth in future warmer climate. This decline in snow cover in turn warms the climate further through the snow‐albedo feedback. However, the contribution of extra warming due to snow loss is highly uncertain and needs further investigation. The aim of this study is to quantify the role of projected snow cover decline on the overall future warming of North America using regional climate model simulations prescribed with current and future snow depth/cover conditions. Our results indicate that snow loss‐related share to future warming is maximum over the present‐day snow‐atmosphere coupling hot‐spot regions and contributes up to 3–4°C to the overall warming. Our results also highlight the sensitivity of temperature extremes and their spatial distribution to the decline in snow cover and depth. The rate of intensification of warm extremes is higher over snow abundant regions of northern Canada—where snow cover fraction is close to one. The largest attenuation in cold extremes, on the other hand, is over snow marginal regions of the Great Plains and Prairies. These results demonstrate the importance of snow‐atmosphere feedbacks on future regional extremes. Key Points: A detailed investigation of the contribution of projected snow decline to the overall future warming over North America is presentedSnow depth/cover decline‐related contribution to overall warming is assessed and it reaches up to 4°C in springFuture snow depth/cover changes contribute to intensifying (attenuating) warm (cold) extremes over snow abundant (marginal) regions [ABSTRACT FROM AUTHOR]