Search

The observed partitioning of poleward heat transport between atmospheric and oceanic heat transports (AHT and OHT) is compared to that in coupled climate models. Model ensemble mean poleward OHT is biased low in both hemispheres, with the largest biases in the Southern Hemisphere extratropics. Poleward AHT is biased high in the Northern Hemisphere, especially in the vicinity of the peak AHT near 40°N. The significant model biases are persistent across three model generations (CMIP3, CMIP5, CMIP6) and are insensitive to the satellite radiation and atmospheric reanalyzes products used to derive observational estimates of AHT and OHT. Model biases in heat transport partitioning are consistent with biases in the spatial structure of energy input to the ocean and atmosphere. Specifically, larger than observed model evaporation in the tropics adds excess energy to the atmosphere that drives enhanced poleward AHT at the expense of weaker OHT. Plain Language Summary: The equator‐to‐pole contrast of solar radiation entering the climate system drives the large‐scale oceanic and atmospheric circulations that, in turn, move heat from the equator to the poles to moderate latitudinal temperature contrasts. The ocean moves the majority of heat in the tropics whereas the atmosphere moves the vast majority of heat in the mid‐ and polar‐latitudes. We demonstrate that, on average, state‐of‐the‐art climate models representing both oceanic and atmospheric circulations simulate too little oceanic heat transport and too much atmospheric heat transport relative to observational estimates. These model biases in the atmosphere‐ocean partitioning of poleward heat transport are persistent across three generations of climate model ensembles spanning 20 years of progress in climate modeling and are insensitive to the choice of datasets used to calculate observed heat transports. The model biases are consistent with stronger than observed surface evaporation in the tropics which enhances atmospheric heat transport at the expense of oceanic heat transport. Key Points: Climate models simulate too little poleward oceanic heat transport and too much poleward atmospheric heat transport in the extratropicsModel biases in heat transport partitioning are persistent across model generation and are insensitive to the observational data sets usedStronger than observed evaporation in models enhances atmospheric heat transport at the expense of oceanic heat transport [ABSTRACT FROM AUTHOR]

Population growth, arable land and fresh water limits, and climate change have profound implications for the ability of agriculture to meet this century's demands for food, feed, fiber, and fuel while reducing the environmental impact of their production. Success depends on the acceptance and use of contemporary molecular techniques, as well as the increasing development of farming systems that use saline water and integrate nutrient flows.

Arctic warming under increased CO2 peaks in winter, but is influenced by summer forcing via seasonal ocean heat storage. Yet changes in atmospheric heat transport into the Arctic have mainly been investigated in the annual mean or winter, with limited focus on other seasons. We investigate the full seasonal cycle of poleward heat transport modeled with increased CO2 or with individually applied Arctic sea‐ice loss and global sea‐surface warming. We find that a winter reduction in dry heat transport is driven by Arctic sea‐ice loss and warming, while a summer increase in moist heat transport is driven by sub‐Arctic warming and moistening. Intermodel spread in Arctic warming controls spread in seasonal poleward heat transport. These seasonal changes and their intermodel spread are well‐captured by down‐gradient diffusive heat transport. While changes in moist and dry heat transport compensate in the annual‐mean, their opposite seasonality may support non‐compensating effects on Arctic warming. Plain Language Summary: The Arctic is warming much faster than the rest of the planet in response to rising greenhouse gas concentrations. Because Arctic warming peaks in winter, many studies have focused on the wintertime processes amplifying Arctic warming. However, others have found that summer atmospheric heating also contributes to winter warming by melting sea ice and storing heat in the ocean until it is released to the atmosphere in winter. Here we study changes in all seasons for one source of atmospheric heating in the Arctic—atmospheric heat transport from lower latitudes. Using climate model simulations, we find that heat and moisture are transported away from the regions that warm and moisten the most in response to rising greenhouse gas concentrations. The Arctic warms more than lower latitudes in winter, which reduces heat transport to the Arctic in winter. Atmospheric moisture increases most in late summer at lower latitudes, driving increased moisture transport in late summer from lower latitudes to the Arctic. We suggest that changes in heat and moisture transport may impact Arctic warming differently due to their opposite seasonality: by producing a larger change in surface solar absorption, summer changes in moisture transport may outweigh winter changes in heat transport. Key Points: Sea‐ice loss reduces dry heat transport to the Arctic in winter; sub‐Arctic warming increases latent heat transport to the Arctic in summerIntermodel spread in Arctic warming controls intermodel spread in seasonal heat transport changesThe seasonal pattern of poleward heat transport change is well‐captured by down‐gradient diffusion of temperature and moisture anomalies [ABSTRACT FROM AUTHOR]

We examine the evidence for large‐scale tropical hydroclimate changes over the Common Era based on a compilation of 67 tropical hydroclimate records from 55 sites and assess the consistency between the reconstructed hydroclimate changes and those simulated by transient model simulations of the last millennium. Our synthesis of the proxy records reveals several regionally coherent patterns on centennial time scales. From 800 to 1000 CE, records from the eastern Pacific and parts of Mesoamerica indicate a pronounced drying event relative to background conditions of the Common Era. In addition, 1400–1700 CE is marked by pronounced hydroclimate changes across the tropics, including dry and/or isotopically enriched conditions in South and East Asia, wet and/or isotopically depleted conditions in the central Andes and southern Amazon in South America, and fresher and/or isotopically depleted conditions in the Maritime Continent. We find notable dissimilarities between the regional hydroclimate changes and global‐scale and hemispheric‐scale temperature reconstructions, indicating that more work needs to be done to understand the mechanisms of the widespread tropical hydroclimate changes during the LIA. Apropos to previous interpretations of large‐scale reorganization of tropical Pacific climate during the LIA, we do not find support for a large‐scale southward shift of the Pacific Intertropical Convergence Zone, while evidence for a strengthened Pacific Walker Circulation and/or an equatorward contraction of the monsoonal Asian‐Australian rain belt exists from limited geographic regions but require additional paleoclimate constraints. Transient climate model simulations exhibit weak forced long‐term tropical rainfall changes over the last millennium but provide several important insights to the proxy reconstructions. Key Points: A synthesis of 67 tropical hydroclimate records from 55 sites indicate several regionally coherent patterns of change in the Common EraRobust patterns include a regional drying event from 800 to 1000 CE and a range of tropical hydroclimate changes ∼1400–1700 CETransient model simulations provide several new insights to the proxy reconstructions [ABSTRACT FROM AUTHOR]

The impact of global orography on Northern Hemisphere wintertime climate is revisited using the Whole Atmosphere Community Climate Model, version 6 (WACCM6). A suite of experiments explores the roles of both resolved orography and the parameterized effects of unresolved orographic drag (hereafter parameterized orography), including gravity waves and boundary layer turbulence. Including orography reduces the extratropical tropospheric and stratospheric zonal mean zonal wind U ¯ by up to 80%; this is substantially greater than previous estimates. Ultimately, parameterized orography accounts for 60%–80% of this reduction; however, away from the surface most of the forcing of U ¯ by parameterized orography is accomplished by resolved planetary waves. We propose that a catalytic wave–mean-flow positive feedback in the stratosphere makes the stratospheric flow particularly sensitive to parameterized orography. Orography and land–sea contrast contribute approximately equally to the strength of the midlatitude stationary waves in the free troposphere, although orography is the dominant cause of the strength of the Siberian high and Aleutian low at the surface and of the position of the Icelandic low. We argue that precisely quantifying the role of orography on the observed stationary waves is an almost intractable problem, and in particular should not be approached with linear stationary wave models in which U ¯ is prescribed. We show that orography has less impact on stationary waves, and therefore on U ¯ , on a backward-rotating Earth. Last, we show that atmospheric meridional heat transport shows remarkable constancy across our simulations, despite vastly different climates and stationary wave strengths. [ABSTRACT FROM AUTHOR]

The increasing frequency of very high temperatures driven by global warming has motivated growing interest in how the probability distribution of summertime temperatures will evolve in the future. Climate models forced by increasing CO2 simulate increasing monthly‐averaged temperature variance across the midlatitudes. In this study we present evidence that these projections are credible and driven primarily by the magnitude of local warming. A first‐principles analytic theory reproduces the increased midlatitude summertime temperature variance in climate models extremely well by considering only the warming‐induced change in the climatological vapor pressure deficit. The impacts of local warming on saturation specific and relative humidity are shown to have roughly equal contributions to increases in summertime temperature variance. The vegetation response to increasing CO2 is found to be an important contributor to the uncertainty in modeled temperature variance change, highlighting the role of plants in shaping the summertime temperature distribution. Plain Language Summary: Extreme summertime temperatures are a focal point for the impacts of climate change. Climate models driven by increasing CO2 emissions project increasing summertime temperature variability by the end of the 21st century. If credible, these increases imply that extreme summertime temperatures will become even more frequent than a simple shift in the contemporary probability distribution would suggest. Given the impacts of extreme temperatures on public health, food security, and the global economy, it is of great interest to understand whether the projections of increased temperature variance are credible. In this study, we use a theoretical model of the land surface to demonstrate that the large increases in summertime temperature variance projected by climate models are credible, predictable from first principles, and driven by the effects of warmer temperatures on evapotranspiration. We also find that the response of plants to increased CO2 and mean warming is important to the projections of increased temperature variability. Key Points: Summertime temperature variance over land increases with local mean temperature in contemporary global climate modelsA theoretical model captures these increases using only projected changes in temperature and relative humidity from global climate modelsUncertainties in climate sensitivity and plant processes control the spread of climate model summertime temperature variance change [ABSTRACT FROM AUTHOR]

The lapse rate feedback is the dominant driver of stronger warming in the Arctic than the Antarctic in simulations with increased CO2. While Antarctic surface elevation has been implicated in promoting a weaker Antarctic lapse rate feedback, the mechanisms in which elevation impacts the lapse rate feedback are still unclear. Here we suggest that weaker Antarctic warming under CO2 forcing stems from shallower, less intense climatological inversions due to limited atmospheric heat transport above the ice sheet elevation and elevation‐induced katabatic winds. In slab ocean model experiments with flattened Antarctic topography, stronger climatological inversions support a stronger lapse rate feedback and annual mean Antarctic warming comparable to the Arctic under CO2 doubling. Unlike the Arctic, seasonality in warming over flat Antarctica is mainly driven by a negative shortwave cloud feedback, which exclusively dampens summer warming, with a smaller contribution from the winter‐enhanced lapse rate feedback. Plain Language Summary: Models project stronger surface warming in the Arctic than the Antarctic under climate change. A climate feedback in which more warming occurs near the surface than at higher altitudes in the atmosphere promotes this stronger Arctic warming. Antarctica's surface elevation is thought to weaken this feedback in comparison to the Arctic, but how this occurs is unclear. Here we show that Antarctic elevation weakens surface warming by changing the base state vertical temperature structure. When Antarctic topography is flattened in model experiments, Antarctica experiences more warming under climate change, resembling Arctic warming. Similarly to the Arctic, flat Antarctica warms most during the winter, but this seasonality is driven by different climate feedbacks in the Arctic versus Antarctic. These results indicate the importance of base state temperatures for warming under climate change and suggest that strong polar amplification is possible without local sea ice loss. Key Points: Antarctic elevation causes asymmetry in climatological inversions, lapse rate feedbacks, and warming between the polesIn model experiments with flattened Antarctica, strengthened inversions lead to stronger lapse rate feedback and Antarctic amplificationShortwave cloud feedback promotes seasonality in flat Antarctic warming; lapse rate feedback more strongly promotes seasonality in Arctic [ABSTRACT FROM AUTHOR]

Downward longwave radiation (DLR) is often assumed to be an independent forcing on the surface energy budget in analyses of Arctic warming and land‐atmosphere interaction. We use radiative kernels to show that the DLR response to forcing is largely determined by surface temperature perturbations. We develop a method by which vertically integrated versions of the radiative kernels are combined with surface temperature and specific humidity to estimate the surface DLR response to greenhouse forcing. Through a decomposition of the DLR response, we estimate that changes in surface temperature produce at least 63% of the clear‐sky DLR response in greenhouse forcing, while the changes associated with clouds account for only 11% of the full‐sky DLR response. Our results suggest that surface DLR is tightly coupled to surface temperature; therefore, it cannot be considered an independent component of the surface energy budget. Plain Language Summary: Longwave radiation, often referred to as "thermal" or "infrared" radiation, emitted downward by Earth's atmosphere is a primary contributor to the surface energy budget. Numerous studies have invoked longwave radiation as a driver of surface warming. This paper shows that this line of reasoning fails to account for the strong control surface temperature exerts on longwave radiation. Using radiative kernels, matrices that quantify the longwave radiation response to a climate perturbation (like global warming), we argue that any surface temperature anomaly will generate a downward longwave radiation response. This constitutes a feedback between the Earth's surface and its atmosphere. The kernels show large longwave responses to perturbations in the lowest part of the atmosphere and almost no response to perturbations at high levels; by vertically integrating the kernels, we can ignore the vertical structure of climate perturbations. Using this modification, we predict the longwave radiation response to a warming world using only the surface changes. Our prediction agrees with climate model output, suggesting that the longwave radiation response is determined primarily by surface temperature. Further, the cloud contribution to changes in longwave radiation is small. These results provide clarity on how changes in the surface energy budget should be analyzed. Key Points: The surface downward longwave radiation (DLR) response to forcing is determined by surface temperatureVertically integrated radiative kernels explain 71–84% of the variance in the DLR response to greenhouse gas forcingChanges in cloud cover contribute only 11% of the DLR response to greenhouse gas forcing [ABSTRACT FROM AUTHOR]

Abstract: The impact of mountains on stratospheric circulation is explored using the Whole Atmosphere Community Climate Model. The “Mongolian mountains” decrease the boreal winter stratospheric jet strength by ∼1/3 and increase the frequency of major sudden stratospheric warmings from 0.08 year−1 to the observed 0.60 year−1. These changes are twice the magnitude of the impacts of the Tibetan plateau and Himalayas. Consistent with the decrease in the zonal jet, there is enhanced Eliassen‐Palm flux convergence; this is predominantly from changes in wave propagation pathways through changes to the upper troposphere circulation, not from an increased amplitude of planetary waves reaching the stratosphere. The Mongolian mountains have the greater impact on upper tropospheric circulation due to their meridional location. The Rocky Mountains have no significant impact on the stratospheric jet. Changes in wave propagation in response to the Mongolian mountains are similar to those associated with major sudden stratospheric warming events in observations. [ABSTRACT FROM AUTHOR]

Arctic warming under increased CO2 peaks in winter, but is influenced by summer forcing via seasonal ocean heat storage. Yet changes in atmospheric heat transport into the Arctic have mainly been investigated in the annual mean or winter, with limited focus on other seasons. We investigate the full seasonal cycle of poleward heat transport modeled with increased CO2 or with individually applied Arctic sea‐ice loss and global sea‐surface warming. We find that a winter reduction in dry heat transport is driven by Arctic sea‐ice loss and warming, while a summer increase in moist heat transport is driven by sub‐Arctic warming and moistening. Intermodel spread in Arctic warming controls spread in seasonal poleward heat transport. These seasonal changes and their intermodel spread are well‐captured by down‐gradient diffusive heat transport. While changes in moist and dry heat transport compensate in the annual‐mean, their opposite seasonality may support non‐compensating effects on Arctic warming. Plain Language Summary: The Arctic is warming much faster than the rest of the planet in response to rising greenhouse gas concentrations. Because Arctic warming peaks in winter, many studies have focused on the wintertime processes amplifying Arctic warming. However, others have found that summer atmospheric heating also contributes to winter warming by melting sea ice and storing heat in the ocean until it is released to the atmosphere in winter. Here we study changes in all seasons for one source of atmospheric heating in the Arctic—atmospheric heat transport from lower latitudes. Using climate model simulations, we find that heat and moisture are transported away from the regions that warm and moisten the most in response to rising greenhouse gas concentrations. The Arctic warms more than lower latitudes in winter, which reduces heat transport to the Arctic in winter. Atmospheric moisture increases most in late summer at lower latitudes, driving increased moisture transport in late summer from lower latitudes to the Arctic. We suggest that changes in heat and moisture transport may impact Arctic warming differently due to their opposite seasonality: by producing a larger change in surface solar absorption, summer changes in moisture transport may outweigh winter changes in heat transport. Key Points: Sea‐ice loss reduces dry heat transport to the Arctic in winter; sub‐Arctic warming increases latent heat transport to the Arctic in summerIntermodel spread in Arctic warming controls intermodel spread in seasonal heat transport changesThe seasonal pattern of poleward heat transport change is well‐captured by down‐gradient diffusion of temperature and moisture anomalies [ABSTRACT FROM AUTHOR]

Novel precipitation event data sets are created by tracking 3-hourly observed rainfall in both time and space in the TRMM 3B42 and ERA-Interim data sets. Relative to TRMM, ERA-Interim data undersimulate the total number of events (factor of ∼0.5), and oversimulate the frequency of events lasting >5 days (factor of 1.6). Longer-lasting events tend to have larger spatial footprints and higher intensity precipitation at any point in their lifetime, and thus contribute significantly more to total precipitation than shorter events. Precipitation changes in selected tropical and subtropical regions are attributed to different event characteristics: some to changes in event rainfall intensity, others to changes in the number of events. In the 40°S-40°N spatial average, the number of events lasting 1-5 days significantly increased from 1998 to 2014 in both the TRMM 3B42 and ERA-Interim data. The event data sets, analysis scripts, and selected processed data are freely available online. [ABSTRACT FROM AUTHOR]

Using four different climate models, we investigate sea level pressure variability in the extratropical North Atlantic in the preindustrial climate (1750 AD) and at the Last Glacial Maximum (LGM, 21 kyrs before present) in order to understand how changes in atmospheric circulation can affect signals recorded in climate proxies. In general, the models exhibit a significant reduction in interannual variance of sea level pressure at the LGM compared to pre-industrial simulations and this reduction is concentrated in winter. For the preindustrial climate, all models feature a similar leading mode of sea level pressure variability that resembles the leading mode of variability in the instrumental record: the North Atlantic Oscillation (NAO). In contrast, the leading mode of sea level pressure variability at the LGM is model dependent, but in each model different from that in the preindustrial climate. In each model, the leading (NAO-like) mode of variability explains a smaller fraction of the variance and also less absolute variance at the LGM than in the preindustrial climate. The models show that the relationship between atmospheric variability and surface climate (temperature and precipitation) variability change in different climates. Results are model-specific, but indicate that proxy signals at the LGM may be misinterpreted if changes in the spatial pattern and seasonality of surface climate variability are not taken into account. [ABSTRACT FROM AUTHOR]