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The late Quaternary paleoclimate of the North Atlantic region has been widely studied, but the local terrestrial response to broader climatic variations remains underexplored. The Azores Archipelago, influenced by the North Atlantic Oscillation (NAO) and the Azores High, is a strategic target to investigate such interactions. Here, paleosols developed in equilibrium with the atmosphere recorded environmental variations in their geochemistry, and volcanic units sealing those paleosols allow for their precise dating. Clay mineralogical transfer functions from paleosol geochemistry and geochronological data were used to track paleoclimatic and paleoecological changes in this region over the past 1.3 Myr. Mean annual precipitation and air temperature reconstructions range from 620 to 1,520 mm yr−1 and 14–26°C, with the latter tightly coupled with previous reconstructions of sea surface temperature. New K‐Ar ages evidence pulsed soil formation periods under weathering‐favorable wet and warm conditions, suggesting periods of a persistent negative NAO with a weakened or more southern Azores High after glacial Terminations I, II, IV, V, IX, and X. Our humidity province reconstructions indicate a prevailing moist to wet forest under cool temperate to subtropical conditions, with less variability than continental Europe. A rapid paleoecological shift occurred at ∼430 ka in São Miguel Island, probably associated with the high amplitude of Termination V. Paleoecological changes younger than 430 ka could be related to local, not large‐scale, climate changes. Average past precipitations were ∼170 mm yr−1 lower than in the present, which suggests that modern weathering rates are higher than observed in our record. Plain Language Summary: In this work, we studied the local climate of the Azores volcanic islands in response to broader climatic changes over the past 1 million years. By studying the chemistry of paleosols–ancient soils incorporated into the geological record–we reconstructed the conditions of precipitation and temperature at the time of their formation. Air temperature reconstructions are similar to previous reconstructions of sea surface temperatures, suggesting a close relationship between atmospheric and sea surface temperatures in the North Atlantic region. The age determination of volcanic units allowed us to constrain the ages of paleosols. We observed that most paleosols developed quickly after the end of glacial periods, under wet and warm conditions that favored the fast transformation of volcanic products into soils. This indicates periods over which the Azores High–an atmospheric high‐pressure system that impacts the Azores' climate–was weakened or centered farther to the south of its current position. The chemistry of the paleosols registered fast environmental changes and suggests that the present annual precipitations are higher than over the past million years, which could impact the local economic activities and even the global climate, due to the capture of atmospheric CO2 through the weathering of volcanic rocks. Key Points: Precise K‐Ar geochronology revealed pulses of soil formation over the past 1 Myr in the Azores, Central North AtlanticPaleosols' geochemistry recorded paleoclimatic conditions and revealed fast environmental changesWet and warm climate resulted from persistent negative North Atlantic Oscillation conditions, favoring fast weathering of volcanic rocks [ABSTRACT FROM AUTHOR]

The upper water‐column dynamics and surface productivity variability in the Northeastern Atlantic Ocean across the Marine Isotope Stage (MIS) 7 interglacial complex is not well understood. Here, we present high‐resolution planktic foraminiferal proxies combined with Artificial Neural Network based‐sea‐surface temperature (SST) and ice‐rafted detritus records from International Ocean Discovery Program Site U1385, SW Iberian Margin for the intervals representing MIS 8 deglaciation, MIS 7 interglacial complex and MIS 6 glacial inception. The long‐term SST pattern is modulated by insolation and precession parameters across the MIS 7 interglacial complex and is superimposed by the millennial‐scale variability (stadials at ∼250, ∼243, ∼230, ∼221, ∼203, ∼196 and ∼192 ka). The regional SST records indicate high temperature gradient (∼6°C) between the sub‐polar North Atlantic Ocean and the SW Iberian Margin during MIS 7d which enhanced the moisture transport from mid‐to‐high latitudes. Further, low obliquity with low insolation induced cooling at high latitudes and promoted the expansion of ice‐sheets during MIS 7d. Comparison of our faunal proxies with the published marine and terrestrial records from SW Europe and western Mediterranean Sea suggested a weakening and southward shift of Azores High (AH) pressure system, similar to the present‐day (−) NAO‐like atmospheric configuration during the early phases of MIS 7e, MIS 7c and MIS 7a, resulting reduced surface productivity of SW Iberian Margin. During the late phases of MIS 7e, MIS 7c and MIS 7a, the AH pressure system strengthened and shifted northward causing the intensification of the trade winds, a scenario similar to present‐day (+) NAO‐like atmospheric configuration, which resulted in high surface productivity of SW Iberian Margin. Key Points: Long‐term sea‐surface temperature (SST) trend across the Marine Isotope Stage (MIS) 7 interglacial complex is modulated by insolation and precessionHigh SST gradient between sub‐polar North Atlantic and SW Iberian Margin and low obliquity enhanced the expansion of ice‐sheets during MIS 7dSurface productivity during MIS 7e, MIS 7c, & MIS 7a is driven by a NAO‐like atmospheric configuration [ABSTRACT FROM AUTHOR]

In deserts and dry areas, sudden changes in temperature cause strong winds and as a result cause the phenomenon of dust. The purpose of this research is to investigate the relationship between the occurrence of dust storms and the North Atlantic Oscillation (NAO) index at selected stations in the west and southwest of Iran (Abadan, Bostan, Ilam, Dehloran, Kermanshah and Sarpol-E-Zahab). For this purpose, we used the codes of days with dust storms, horizontal visibility of less than 1 km, data related to index values (NAO) during the statistical period 1987-2022, and satellite data including dust optical depth index (AOD) obtained from the productions of the gauges of 2000-2017 MISR and 2002-2022 MODIS periods. The results of the correlation coefficients showed that there is a relatively strong inverse relationship between the NAO index in the negative phase and a weak direct relationship with dust in the positive phase, and between 51% and 70% of the dust occurrences were simultaneous with the positive phase. The graph of changes in the time series of the indices in the two negative and positive phases showed that the value of the AOD index increased from west to southwest, which is evident in the Abadan station. According to the index produced by the MISR sensor in the years 2011 and 2015 in the positive phase and in the year 2008 in the negative phase, the peak of dust storm activity in all stations and its lowest value are related to the Sarpol-E-Zahab and Kermanshah stations in the years 2002 and 2004, respectively. According to the Deep Blue index obtained from the MODIS sensor, the positive phase of 2022 and the negative phase of 2009 can be separated as the peak of dust storms. The results of the Wind rose and HYSPLIT models showed that the direction of dust storms entering Abadan, Bostan and Ilam stations was from the northwest and southwest and in Dehlorn, Kermanshah, and Sarpol-E-Zahab from the west, and the storms with external origin in these stations were influenced more by the countries of Iraq, Syria and Saudi Arabia. [ABSTRACT FROM AUTHOR]

Arctic warming has significant environmental and social impacts. Arctic long‐term warming trend is modulated by decadal‐to‐multidecadal variations. Improved understanding of how different external forcings and internal variability affect Arctic surface air temperature (SAT) is crucial for explaining and predicting Arctic climate changes. We analyze multiple observational data sets and large ensembles of climate model simulations to quantify the contributions of specific external forcings and various modes of internal variability to Arctic SAT changes during 1900–2021. We find that the long‐term trend and total variance in Arctic‐mean SAT since 1900 are largely forced responses, including warming due to greenhouse gases and natural forcings and cooling due to anthropogenic aerosols. In contrast, internal variability dominates the early 20th century Arctic warming and mid‐20th century Arctic cooling. Internal variability also explains ∼40% of the recent Arctic warming from 1979 to 2021. Unforced changes in Arctic SAT are largely attributed to two leading modes. The first is pan‐Arctic warming with stronger loading over the Eurasian sector, accounting for 70% of the unforced variance and closely related to the positive phase of the unforced Atlantic Multidecadal Oscillation (AMO). The second mode exhibits relatively weak warming averaged over the entire Arctic with warming over the North American‐Pacific sector and cooling over the Atlantic sector, explaining 10% of the unforced variance and likely caused by the positive phase of the unforced Interdecadal Pacific Oscillation (IPO). The AMO‐related changes dominate the unforced Arctic warming since 1979, while the IPO‐related changes contribute to the decadal SAT changes over the North American‐Pacific Arctic. Plain Language Summary: The Arctic warms much faster than the rest of the world, leading to significant local and remote influences. Warming in the Arctic is not uniform over time, with decadal‐to‐multidecadal variations upon the long‐term trend. The changes in Arctic surface air temperature (SAT) can be attributed to either instrinct variability within the climate system or external forcings including anthropogenic factors such as greenhouse gases emission and natural factors such as volcanic eruptions. Understanding of the relative contributions of internal variability and external forcing to observed changes in Arctic SAT is crucial for improving Arctic climate projections in coming decades. By synthesizing multiple observational data sets and large‐ensemble climate simulations, we find that the Arctic experienced long‐term warming with some periods of slowdown in response to external forcing, which largely explains the overall change since 1900. Internal variability, particularly the multidecadal oscillation in the North Atlantic, dominates the early 20th century warming and mid‐20th century cooling and significantly contributes to the recent rapid warming since 1979. A regression‐based rescaling method removes systematic biases in model‐simulated response (in comparison with observations), ensuring that our results are not influenced by the choice of climate models, as long as they are under the same historical forcing. Key Points: Externally forced Arctic warming dominates the trend and variance in Arctic surface air temperature during 1900–2021Most of the internally generated Arctic temperature changes are related to the unforced Atlantic Multidecadal OscillationInternal variability explains 40% of the Arctic warming from 1979 to 2021, while greenhouse gases and natural forcings account for the rest [ABSTRACT FROM AUTHOR]

The occurrence of multiple precipitation events not‐independent in time, that is, a temporal clustering, is an example of a temporal compounding event. This type of forcing is of great relevance for the occurrence of different natural hazards, like floods and deep‐seated landslides, for which previous soil saturation plays an important role in shaping the associated hazard. Using ERA5‐Land data set and E‐OBS, we firstly investigate the spatial and temporal characteristics of temporal clustering of precipitation over the Italian territory, and we relate it with two oscillation patterns, namely North Atlantic Oscillation (NAO) and Mediterranean Oscillation Index (MOI), and with common synoptic conditions. Then, we explore the role of temporal compounding of precipitation in the generation of different movement types (complex, debris flow, fall, flow, and sliding) using the database of landslides from the Aree Vulnerate Italiane project (in Italian AVI, meaning Areas Affected by Landslides or Floods). From this study it emerges that below average values of NAO and MOI increase the probability of having clustered precipitation events. For all types of landslides, except rock falls, we observed that the majority of the events are preceded by a temporal clustering of precipitation, over longer time windows for complex events, shorter for debris flows. For rock falls, we found also a link with low minimum temperature and freeze‐thaw cycles for winter events and high maximum temperature for summer events. This work contributes to the investigation of temporal clustering of precipitation in connection with natural hazards characterized by a mechanism of saturation. Plain Language Summary: Most natural catastrophic events are caused by a sequence in time of multiple not‐independent precipitation events, also called temporal clustering of precipitation. This is related to the process of saturation of the soil that in most cases is not saturated by a single precipitation event. For example, soil moisture is important in the occurrence of landslides, since it causes instability of the slope, or in floods, since it prevents water from infiltrating. When an extreme event is caused or amplified by the occurrence of multiple meteorological events in time or space we talk about climate‐related compound events. In this work we look at the characteristics of temporal clustering of precipitation in Italy, where and when it occurs and its relation with large scale circulations. Then, we investigate its role, together with the role of single intense precipitation events and temperature, as a trigger of different landslide types (complex, debris flow, fall, flow, and sliding). In this work we bring a clearer understanding of the trigger of landslides in Italy, and we highlight the role of temporal clustering of precipitation for hazards related with a saturation process. Key Points: We introduced a statistical method to detect temporal clustering of events, for example, precipitationNegative dependence between some teleconnection indices and temporal clustering of precipitation in winter in ItalyTemporal clustering of precipitation is a significant trigger of landslides in Italy. Temperature is also relevant for rock falls [ABSTRACT FROM AUTHOR]

This paper describes a framework for identifying dominant atmospheric drivers of ocean variability. The method combines statistics of atmosphere–ocean fluxes with physics from an ocean general circulation model to derive atmospheric patterns optimized to excite variability in a specified ocean quantity of interest. We first derive the method as a weighted principal components analysis and illustrate its capabilities in a toy problem. Next, we apply our analysis to the problem of interannual upper ocean heat content (HC) variability in the North Atlantic Subpolar Gyre (SPG) using the adjoint of the MITgcm and atmosphere–ocean fluxes from the ECCOv4-r4 state estimate. An unweighted principal components analysis reveals that North Atlantic heat and momentum fluxes in ECCOv4-r4 have a range of spatiotemporal patterns. By contrast, dynamics-weighted principal components analysis collapses the space of these patterns onto a small subset—principally associated with the North Atlantic Oscillation—that dominates interannual SPG HC variance. By perturbing the ECCOv4-r4 state estimate, we illustrate the pathways along which variability propagates from the atmosphere to the ocean in a nonlinear ocean model. This technique is applicable across a range of problems across Earth system components, including in the absence of a model adjoint. Significance Statement: While the oceans have absorbed 90% of the excess heat associated with human-forced climate change, the change in the ocean's heat content is not steady, with peaks and troughs superimposed upon a general increase. These fluctuations come from chaotic changes in the atmosphere and ocean and can be hard to disentangle. We use this case of ocean heat content variability to introduce a new method for determining the patterns of weather and climate in the atmosphere that are most effective at generating fluctuations in the ocean. To do this, we combine the statistics of recent atmospheric activity with output from a state-of-the-art numerical ocean model that reveals physical processes driving changes in ocean quantities including ocean heat content. This approach suggests that the atmospheric patterns that stimulate the most energetic changes in ocean heat content in the northern North Atlantic are not necessarily the most energetic patterns present in the atmosphere. We test our findings by preventing these patterns from affecting the ocean in our numerical model and measure a strong reduction in ocean heat content fluctuations. [ABSTRACT FROM AUTHOR]

Recently, a connection between the quasi‐biennial oscillation (QBO) and the Madden–Julian oscillation (MJO) was found in observations. The boreal winter seasonal mean amplitude of the daily MJO is anticorrelated with the QBO winds measured at 50 hPa. We investigate whether this relationship is captured in reforecasts of past winters in a seasonal prediction system, and find the ensemble mean response does not show this association, with essentially zero correlation. By repeated subsampling of the ensemble reforecast data, we also show that the observed correlation is very unlikely to arise merely as the result of sampling in the relatively short observational record. Instead, we conclude that the reforecasts genuinely cannot capture the observed QBO–MJO seasonal relationship. We also consider the boreal winter seasonal mean response of tropical convection to the QBO and its influence on the subtropics. We find that the reforecasts are able to capture the stationary pattern found in the observed outgoing long‐wave radiation (OLR) over the tropical west Pacific in response to the QBO. We also find that the associated upper tropospheric divergence in observations interacts with the west Pacific subtropical jet to produce atmospheric Rossby wave patterns that propagate across the midlatitude Pacific Ocean. These extratropical waves have the potential to interfere with the climatological waves in midlatitudes, modifying the strength of the midlatitude annular mode through interactions with the extratropical stratosphere. However, the observed influence on wavenumber 1 is not captured by the reforecasts, suggesting a weak extratropical influence and a partial explanation for the weaker‐than‐observed QBO teleconnection to the Arctic Oscillation and North Atlantic Oscillation. [ABSTRACT FROM AUTHOR]

The European Centre for Medium‐range Weather Forecasts (ECMWF) extended‐range forecasts display large errors in the representations of Madden–Julian Oscillation (MJO) teleconnections over the North Atlantic with a strong underestimation of the impact of the MJO on the North Atlantic Oscillation (NAO) following an MJO in Phase 2–3. The origin of this error was investigated using a large set of re‐forecasts covering 20 years where part of the atmosphere was relaxed towards the ECMWF Reanalysis v5 (ERA5) reanalysis. These relaxation experiments show that relaxing the Tropics significantly improves the MJO teleconnections associated with Phase 2–3, with a reduction of about 50% in the amplitude error of the teleconnections. However, model errors outside the Tropics also play an important role. Results also suggest that errors in local processes reduce by about 20% the amplitude of the MJO teleconnections over the North Atlantic. Another example of a source of MJO teleconnection errors is the near surface over the Tibetan and Mongolian Plateaux. Nudging this area towards ERA5 improves the representation of the Pacific subtropical jet and the amplitude of MJO teleconnections associated with Phase 2–3 in the extended‐range re‐forecasts. Nudging the stratosphere exerts a comparatively weaker impact on the MJO teleconnections 11–15 days after an MJO in Phase 2–3. Overall, these experiments indicate that there are multiple sources of MJO teleconnection errors, making the representation of realistic MJO teleconnection by dynamical models a particularly challenging task. [ABSTRACT FROM AUTHOR]

This article explores the potential impact of climate change on loggerhead sea turtles in the Alboran Sea. The study focuses on sporadic nesting events in the western Mediterranean, which may indicate a colonization process. Genetic studies have shown past instances of colonization, and current climatic conditions in the western Mediterranean are suitable for nesting. The article also discusses the role of the North Atlantic Oscillation (NAO) in influencing sea turtle migration patterns. The findings have implications for loggerhead turtle conservation and highlight the expansion of nesting range within the Mediterranean basin. In Peru, a citizen science project has observed nesting behavior in two sea turtle species, despite nesting beaches not traditionally being found in the country. However, it is suggested that this may be due to inadequate beach surveys rather than a shift in sea turtle behavior. The absence of similar occurrences outside the Mediterranean suggests that additional factors beyond climate change may be involved. The NAO, which is sensitive to climate change, is expected to contribute to an increase in sporadic nesting on the beaches of the Alboran Sea in the future. [Extracted from the article]

Long‐term variabilities of monthly zonal (U) and meridional winds (V) in northern polar mesosphere and lower thermosphere (MLT, ∼80–100 km) are investigated using meteor radar observations during 1999–2022 over Esrange (67.9°N, 21.1°E). The summer (June‐August) mean zonal winds are characterized by westward flow up to ∼88–90 km and eastward flow above this height. The summer mean meridional winds are equatorward with strong jet at ∼85–90 km and it weakens above this height. The U and V exhibit strong interannual variability that varies with altitude and month or season. The responses of U and V anomalies (from 1999 to 2003) to solar cycle (SC), Quasi Biennial Oscillation at 10 and 30 hPa, El Niño‐Southern Oscillation, North Atlantic Oscillation, ozone (O3) and carbon dioxide (CO2) are analyzed using multiple linear regression. From analysis, significant regions of correlations between MLT winds and above potential drivers vary with altitude and month. The positive responses of U and V to SC (up to 15 m/s/100 sfu) indicates the strengthening of eastward winds in mid‐late winter, and poleward winds in late autumn and early winter. The O3 likely intensifies the eastward and poleward winds (∼100 m/s/ppmv) in winter and early spring. The CO2 significantly influence the eastward flow in late winter and summer (above ∼90–95 km) and strengthen the meridional circulation. The significant positive trend in U peaks in summer, late autumn and early winter (∼0.6 m/s/year), the negative trend in V is more prominent in summer above ∼90–95 km. Plain Language Summary: The transition region between middle atmosphere and thermosphere is known as the mesosphere and lower thermosphere (MLT). The dynamics and circulation in this region are significant for global transport of important trace chemical species. Further the MLT winds play crucial role for the dynamical coupling of the middle and upper atmosphere. In the present study, the long‐term variability and tendencies in monthly mean zonal and meridional winds are investigated in the Arctic MLT between ∼80 and ∼100 km from meteor radar observations during 1999–2022 over Esrange (67.9°N, 21.1°E) in Sweden. The ability of radar provided the unique, consistent, and long‐term data set of polar MLT winds for duration of about two solar cycles. The MLT winds show important characteristic features and significant interannual variability that vary with altitude and month or season. In addition, the possible influence of climate forcings viz., solar activity, Quasi Biennial Oscillation (at 10 and 30 hPa), El Niño‐Southern Oscillation, North Atlantic Oscillation, ozone, and carbon dioxide on the variabilities of polar MLT winds has been analyzed using multiple linear regression. The significant interannual variabilities and tendencies in northern polar MLT zonal and meridional winds can be attributed to the above potential drivers. Key Points: The long‐term variabilities in arctic mesosphere and lower thermosphere winds in response to potential climate forcings have been investigated for 1999–2022 over EsrangeThe variability in U and V significantly correlated with O3 in winter and early spring, and with CO2 in summer based on altitudeThe interannual variability in U and V is found to vary with altitude and month or season [ABSTRACT FROM AUTHOR]

Dust plays an important role in influencing global weather and climate via impacting the Earth's radiative balance. Based on the atmospheric and oceanic datasets during 1980–2022, the impacts of preceding winter North Atlantic Oscillation (NAO) and El Niño-Southern Oscillation (ENSO) on the following spring dust activities over North China are explored. It is found that both NAO and ENSO exert significant effects in influencing the dust activities over North China, particularly during their negative phases. A synergistic influence on the dust activities in North China is observed when both NAO and ENSO are in negative phase, with their combined impacts exceeding that of either factor alone. The previous winter NAO exhibits significant impacts on the sea surface temperatures (SST) in the North Atlantic, associating with an anomalous SST tripole pattern. Owing to the persistence of SST, these anomalies can extend into the following spring, when anomalous atmospheric teleconnection wave trains would be induced, thereby influencing the dust activities in North China. ENSO, on the one hand, directly impacts dust activities in North China by modulating the circulation in the Western North Pacific (WNP). Moreover, ENSO enhances the NAO's effect on the North Atlantic SST, explaining their synergistic effects on the dust activities over North China. This study explains the combined role of NAO and ENSO on the dust weather over North China, providing one season ahead signals for the forecast of spring dust activities in North China. [ABSTRACT FROM AUTHOR]

The North Atlantic Oscillation (NAO) is a dominant atmospheric variability in the Northern Hemisphere that has a substantial impact on weather and climate on various time scales. Here, we investigate the role of energy conversion terms day‐by‐day during the NAO life cycle. The relative timing and contribution of the energy conversion terms are quantified by projecting the terms onto the eddy total energy, eddy kinetic energy (EKE), and eddy available potential energy (EAPE) patterns associated with the NAO. The results show a remarkable phase dependence in the growth and maintenance mechanisms. For positive NAOs, the initial growth is driven by the barotropic interactions between the eddies propagating over North America. This suggests that a non‐local growth mechanism plays an important role in the initiation of positive NAOs. In contrast, it is the baroclinic processes that initiate negative NAOs. The conversion of the mean APE into the EAPE and then into the EKE, processes which include eddy heat fluxes and vertical motions, results in the relatively local growth of negative NAOs. During the mature phase, the largest source of energy is the conversion of the mean APE into EAPE. Part of this is converted to EKE, making a substantial contribution to the maintenance of the EKE. This process is particularly important for negative NAOs. [ABSTRACT FROM AUTHOR]

We develop and compare model‐error representation schemes derived from data assimilation increments and nudging tendencies in multidecadal simulations of the Community Atmosphere Model, version 6. Each scheme applies a bias correction during simulation runtime to the zonal and meridional winds. We quantify the extent to which such online adjustment schemes improve the model climatology and variability on daily to seasonal timescales. Generally, we observe about a 30% improvement to annual upper‐level zonal winds, with largest improvements in boreal spring (around 35%) and winter (around 47%). Despite only adjusting the wind fields, we additionally observe around 20% improvement to annual precipitation over land, with the largest improvements in boreal fall (around 36%) and winter (around 25%), and around 50% improvement to annual sea‐level pressure, globally. With mean‐state adjustments alone, the dominant pattern of boreal low‐frequency variability over the Atlantic (the North Atlantic Oscillation) is significantly improved. Additional stochasticity increases the modal explained variances further, which brings the variability closer to the observed value. A streamfunction tendency decomposition reveals that the improvement is due to an adjustment to the high‐ and low‐frequency eddy–eddy interaction terms. In the Pacific, the mean‐state adjustment alone led to an erroneous deepening of the Aleutian low, but this was remedied with the addition of stochastically selected tendencies. Finally, from a practical standpoint, we discuss the performance of using data assimilation increments versus nudging tendencies for an online model‐error representation. [ABSTRACT FROM AUTHOR]

The sea surface temperature (SST) is not only a crucial external factor in the evolution of the atmosphere, but also a primary factor and premonition signal used in climate prediction. It is challenging to obtain a precise SST for generating accurate initial and boundary conditions in numerical models. This study employs a machine learning approach, that is, a convolutional neural network (CNN) algorithm, to predict SST on a seasonal scale. In particular, the subsurface ocean temperature (OT) and ocean salinity (OS) at depths of 5.02, 15.08, 25.16, 35.28, 45.45 and 76.55 m were used as training factors in developing a CNN prediction model. The results indicate that subsurface OT and OS can persist for 6 months or longer, with a maximum persistence of up to 12 months. Using the CNN prediction model, the SST can be reliably predicted 6 months in advance in most cases. The predicted SST has a mean bias of approximately 0–0.8 K on the globe. The bias is small (below 0.5 K) in the open ocean. The root mean square errors (RMSEs) of hindcasting for Interdecadal Pacific Oscillation, North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation indices are all less than 1.0 K. Specifically, the RMSE for El Niño prediction is less than 0.5 K. This study provides a viable method for establishing initial and boundary conditions for climate prediction. [ABSTRACT FROM AUTHOR]

It is increasingly recognized that the assumption of stationarity is not always appropriate for estimating return periods from meteorological time series under the effects of climate change and climate variability. Here, we assessed the capability of a non-stationary framework for modeling seasonal precipitation series using Generalized Additive Models for Location, Scale and Shape (GAMLSS), a well-established approach for modeling hydro-meteorological variables under non-stationary conditions. Seasonal precipitation series were considered from 95 stations covering the period of 1970–2021 across Turkey. Four widely used homogeneity tests showed that the seasonal precipitation series were generally reliable, with no obvious anthropogenic influences or errors. The parameters of the fitted distributions were modeled as a function of large-scale oscillation indices known to control precipitation across Turkey, namely the North Atlantic Oscillation (NAOI), North Sea-Caspian Pattern (NCPI), Mediterranean Oscillation (MOI), and Southern Oscillation (SOI). The model with oscillation indices performed better at 85%, 79%, 76%, and 54% of sites for autumn, winter, spring, and summer, respectively, than the model with no covariates. In particular, the NCPI was seen as a significant predictor during the winter, while the MOI captured precipitation variability well across the country during autumn and summer. The NAOI appeared as another important predictor during all seasons except summer, while the SOI appeared as a significant explanatory variable in certain regions. Using the non-stationary models, we then computed seasonal precipitation estimates for different return periods (i.e., 20, 50, and 100 years) and considered the minimum and maximum possible extreme precipitation scenarios at each site. We show how the use of simple minimum/maximum values derived from the non-stationary models can help provide water resource managers and policy makers with a plausible range of extreme values, rather than the single deterministic value obtained from the traditional stationary approach. [ABSTRACT FROM AUTHOR]

This study investigates the relationship between the uncertainty of empirical orthogonal function (EOF) modes and sampling size in climate models, using simulated results of preindustrial control (piControl) experiments in phase 6 of the Coupled Model Intercomparison Project (CMIP6), and taking the North Atlantic Oscillation (NAO) and El Niño–Southern Oscillation (ENSO) as examples. The results indicate that this relationship can be quantified by a concise fitting function [i.e., y = a/(x − b)]. Here, y is the 5%–95% confidence interval of congruence coefficient, x is the sampling size, and a and b are two parameters depending on models or observations. As compared with b, which modulates the sampling size in the fitting function, the parameter a scales the sampling size and thus plays a much more important role. Further analysis indicates that the parameter a, or the uncertainty of EOF1 mode, decreases dramatically with the increase of the difference between variance fractions of EOF1 and EOF2 modes, approximately in the form of a power function. The minimum sampling size to ensure a reliable EOF mode can also be estimated by the fitting function and shows a great diversity among models both for the NAO and ENSO. The diversity suggests the importance of estimating the minimum sampling size before model evaluations on climate variability modes and projections on the future change in modes, particularly when the EOF2 mode explains the variance close to EOF1 mode. Significance Statement: Empirical orthogonal function (EOF) analysis, principal component analysis, or eigenvector analysis has been widely used in various research fields. However, it remains as an open question as to how large the sampling size is required to be to obtain reliable modes through the EOF method. In this study, we investigate the relationship between the uncertainty of EOF results and sampling size in current climate models, using adequately long simulated data, and we find that this relationship can be depicted by the fitting function y = a/(x − b). Here, y represents the uncertainty, x is the sampling size, and a and b are parameters. The parameter a is closely related to the difference between variance fractions of first and second EOF modes and plays a more important role in the fitting function. The minimum sampling sizes that are required to obtain reliable EOF modes can also be estimated by the fitting function and vary greatly from model to model. The results provide a basis for judging the reliability of EOF modes, particularly when the first and second EOF modes explain similar variance fractions. [ABSTRACT FROM AUTHOR]

We investigate the impact of seasonal forecast biases in the Tropical Atlantic on the North Atlantic. The analysis uses a novel ensemble-based method to estimate the impact of tropical rainfall bias on forecasts of the Extratropical North Atlantic. The inter-ensemble spread of the forecast model is used to estimate the impact of the bias in Tropical Atlantic rainfall on the North Atlantic by selecting model members that happen to produce forecast anomalies that most closely resemble the tropical rainfall bias and using these as a proxy for the model error. The Tropical Atlantic rainfall bias impacts Rossby wave sources over the Subtropical Atlantic and there is a clear Rossby wave pattern originating from this area which is comparable to the mean bias in hindcasts. We argue that Tropical Atlantic rainfall errors explain a significant amount of the bias in seasonal forecasts over the Extratropical North Atlantic. [ABSTRACT FROM AUTHOR]

The mid-latitude climate responses to Arctic sea ice loss remain unclear, partly because the atmospheric responses depend sensitively to the location of sea ice anomalies. Evaluating the role of regional Arctic sea ice extent anomalies is therefore essential to appreciate the extent of atmospheric responses. We investigated these responses but also the atmospheric precursors to regional Arctic sea ice extent anomalies in long pre-industrial control simulations from 36 CMIP6 climate models. This study examines changes in various atmospheric variables at different lead and lag times by performing a composite analysis between years of low and high sea ice extents in different Arctic seas. Stronger and more statistically significant relationships are found when the atmosphere leads to changes in sea ice than in the reverse direction, suggesting that in the CMIP6 models the atmosphere drives the sea ice rather than the opposite. The atmospheric circulation is found to be relatively insensitive to regional sea ice anomalies except in the Barents–Kara (BAKA) Seas, where the negative anomalies are followed by a robust negative winter North Atlantic Oscillation (NAO)-like pattern. Consistent with the so-called stratospheric pathway, a weakening of the stratospheric polar vortex (SPV) is simulated 1 month prior to the NAO change and can be partially explained by the sea ice anomaly in BAKA in preceding months. The magnitude of this weakening in models depends on other factors, such as the Siberian snow cover, the El Niño–Southern oscillation, and the quasi-biennial oscillation. Moreover, the SPV and the NAO index responses scale approximately linearly with the magnitude of the BAKA sea ice anomaly. These results highlight the inter-model consistency of the role played by the BAKA sea ice extent anomaly for the atmospheric responses under pre-industrial conditions. In upcoming work, the sea ice loss in this region should therefore primarily be considered, but under future conditions. [ABSTRACT FROM AUTHOR]

Climate change is contributing to rapid changes in lake ice cover across the Northern Hemisphere, thereby impacting local communities and ecosystems. Using lake ice cover time‐series spanning over 87 yr for 43 lakes across the Northern Hemisphere, we found that the interannual variability in ice duration, measured as standard deviation, significantly increased in only half of our studied lakes. We observed that the interannual variability in ice duration peaked when lakes were, on average, covered by ice for about 1 month, while both longer and shorter long‐term mean ice cover duration resulted in lower interannual variability in ice duration. These results demonstrate that the ice cover duration can become so short that the interannual variability rapidly declines. The interannual variability in ice duration showed a strong dependency on global temperature anomalies and teleconnections, such as the North Atlantic Oscillation and El Niño–Southern Oscillation. We conclude that many lakes across the Northern Hemisphere will experience a decline in interannual ice cover variability and shift to open water during the winter under a continued global warming trend which will affect lake biological, cultural, and economic processes. [ABSTRACT FROM AUTHOR]

In the northeastern tropical Atlantic, a region of high potential vorticity (PV) determines the size of the exchange window for the interior thermocline flow of the subtropical cell via its variations in strength and extent. Variability of this PV barrier has the potential to impact the ventilation of the tropical Atlantic on decadal timescales. Here, the impact of the North Atlantic Oscillation (NAO) on the PV barrier related to isopycnals within the thermocline of the subtropical‐tropical Atlantic Ocean is assessed from Argo observations for the time period of 2006–2022. Relative to the negative NAO phase (2009–2010), during the positive NAO phase (2014–2019), the North Atlantic subtropical high and the northeast trades are intensified. Satellite‐derived wind stress curl shows increased upwelling/downwelling on the equatorward/poleward side of the trade wind zone, respectively. In the subtropical‐tropical Atlantic, a symmetric pattern of isopycnal heave is observed: rising isopycnals within 20°N and 20°S and sinking poleward of that. With rising isopycnals, the PV barrier in the northeastern tropical Atlantic becomes stronger. Analyses of geostrophic velocities and the Sverdrup streamfunction show that during the positive NAO phase there are increased equatorward velocities at thermocline level along the western boundary and reduced velocities through the interior as a result of intensified northeast trades and therefore a strengthened PV barrier. Intensified trades lead to enhanced subduction of thermocline waters and, independent of that, to a strengthened Equatorial Undercurrent transport as observed at the mooring site at 0°, 23°W, likely via the pulling effect of the subtropical cells. [ABSTRACT FROM AUTHOR]

Paleoclimate reconstructions across Iceland provide a template for past changes in climate across the northern North Atlantic, a crucial region due to its position relative to the global northward heat transport system and its vulnerability to climate change. The roles of orbitally driven summer cooling, volcanism, and human impact as triggers of local environmental changes in the Holocene of Iceland remain debated. While there are indications that human impact may have reduced environmental resilience during late Holocene summer cooling, it is still difficult to resolve to what extent human and natural factors affected Iceland's late Holocene landscape instability. Here, we present a continuous Holocene fire record of northeastern Iceland from proxies archived in Stóra Viðarvatn sediment. We use pyrogenic polycyclic aromatic hydrocarbons (pyroPAHs) to trace shifts in fire regimes, paired with continuous biomarker and bulk geochemical records of soil erosion, lake productivity, and human presence. The molecular composition of pyroPAHs and a wind pattern reconstruction indicate a naturally driven fire signal that is mostly regional. Generally low fire frequency during most of the Holocene significantly increased at 3 ka and again after 1.5 ka BP before known human settlement in Iceland. We propose that shifts in vegetation type caused by cooling summers over the past 3 kyr, in addition to changes in atmospheric circulation, such as shifts in North Atlantic Oscillation (NAO) regime, led to increased aridity and biomass flammability. Our results show no evidence of faecal biomarkers associated with human activity during or after human colonisation in the 9th century CE. Instead, faecal biomarkers follow the pattern described by erosional proxies, pointing toward a negligible human presence and/or a diluted signal in the lake's catchment. However, low post-colonisation levels of pyroPAHs, in contrast to an increasing flux of erosional bulk proxies, suggest that farming and animal husbandry may have suppressed fire frequency by reducing the spread and flammability of fire-prone vegetation (e.g. heathlands). Overall, our results describe a fire frequency heavily influenced by long-term changes in climate through the Holocene. They also suggest that human colonisation had contrasting effects on the local environment by lowering its resilience to soil erosion while increasing its resilience to fire. [ABSTRACT FROM AUTHOR]

El Niño-Southern Oscillation (ENSO) teleconnection to the Euro-Atlantic exhibits strong subseasonal variations, as the North Atlantic Oscillation (NAO) response systematically reverses its phase from early to late winter. Based on two sets of atmospheric model simulations in CMIP6 forced by historical and projected SST, we report a future disappearance of this teleconnection reversal, with the positive early winter ENSO-NAO correlation turning negative and the negative late winter correlation becoming stronger. We suggest that this negative NAO tendency is associated with the strengthening and eastward shift of the ENSO atmospheric teleconnection towards the Euro-Atlantic due to parallel changes in the ENSO tropical convection. While the early winter NAO phase transition is further facilitated by the mean state change in the North Atlantic jet meridional shear, which shifts the ENSO-driven Rossby wave-propagating direction, an intensified stratospheric pathway is demonstrated to play an additional role in strengthening the late winter NAO response. [ABSTRACT FROM AUTHOR]

Gaining a comprehensive understanding of the characteristics and propagation of precipitation-based meteorological drought to terrestrial water storage (TWS)-derived hydrological drought is of the utmost importance. This study aims to disentangle the frequency–time relationship between precipitation-derived meteorological and TWS-based hydrological drought from June 2002 to June 2017 based on the Standardized Precipitation Index (SPI) and Standardized Terrestrial Water Storage Index (STI) by employing wavelet coherence rather than a traditional correlation coefficient. The possible influencing factors on drought propagation in 28 regions across the world are examined. The results show that the number of drought months detected by the STI is higher than that detected by the SPI worldwide, especially for slight and moderate drought. Generally, TWS-derived hydrological drought is triggered by and occurs later than precipitation-based meteorological drought. The propagation characteristics between meteorological and hydrological droughts vary by region across the globe. Apparent intra-annual and interannual scales are detected by wavelet analysis in most regions, but not in the polar climate region. Drought propagation differs in phase lags in different regions. The phase lag between hydrological and meteorological drought ranges from 0.5 to 4 months on the intra-annual scale and from 1 to 16 months on the interannual scale. Drought propagation is influenced by multiple factors, among which the El Niño–Southern Oscillation, North Atlantic Oscillation, and potential evapotranspiration are the most influential when considering one, two, or three factors, respectively. The findings of this study improve scientific understanding of drought propagation mechanisms over a global scale and provide support for water management in different subregions. [ABSTRACT FROM AUTHOR]

In this study, cold waves (CW) were set and classified in different severity levels by applying standardized index (Z score) of minimum temperature (MT) in the period of 1951–2019, and also, the frequency of monthly, annual and decadal occurrence of CW was calculated and analyzed. The results showed that there was a decreasing linear trend in distribution of temporal changes of CW. The probability for annual occurrence of CW (which annually reaches 42 times or less) was 99%, and the probability of its occurrence for more than 42 waves was 1%. The trend analysis of time series component represented a significant reduction in the cold waves frequency (CWF) in the study area. Also the results of the trend analysis for the severity levels of occurrence of different cold waves showed that despite the significant reduction in the CWF, none of the classes had a significant trend except the reducing trend of annual cold waves frequency (ACWF). Investigating the decadal condition of CWF displays an increased incidence of CW from 70 s and stepwise reduction in the CWF from the 1981 to 2010, and consequently, the general trend of decadal occurrence of CW was decreasing. The twenty-first-century prediction of the CWF shows the CWF will decrease significantly compared to the long-term average of the twentieth century. The results obtained from the correlation and Morlet wavelet analysis (MWA) between the levels of CW severities with Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) teleconnections indicated that the correlation between CWF is in-phase, positive and incremental. [ABSTRACT FROM AUTHOR]

This study focuses on the interannual variability of winter mean surface air temperature (SAT) and subseasonal SAT reversal in North China, which have profound impacts on national life and economic activities. The analysis explores the temporal and spatial distribution characteristics of these variations and their relationship with the Eurasian Zonal Circulation (EZ). The findings reveal distinct interdecadal changes in North China's winter mean SAT, along with a general subseasonal reversal of cold and warm conditions. In comparison to the North Atlantic oscillation/Arctic Oscillation, the EZ has a more significant influence on the interannual variability of winter and subseasonal reversals. Strong EZ years are associated with the weakening of the Siberian High and the Ural Blocking High, causing the East Asian Trough to move eastward, which leads to the retention of cold air from the north and increased SAT in North China. Conversely, weak EZ years exhibit opposite circulation patterns. The winter mean EZ is influenced by preceding autumn Arctic sea ice concentrations (SIC). Reduced autumn SIC results in decreased heat flux and reduced eddy energy, leading to substantial attenuation of the winter storm track across Scandinavia and Western Asia. This attenuation, mediated through the interaction between eddies and mean flow, triggers a slowdown of Eurasian zonal circulation and intensification of the Siberian High. These conditions favor a positive phase of the winter Scandinavia pattern, facilitating the transport of cold Arctic air into North China. Likewise, subseasonal reversals in SIC influence corresponding alterations in the EZ. Plain Language Summary: The winter surface air temperatures (SAT) in North China exhibit notable interdecadal variations and distinct intra‐seasonal changes. These intra‐seasonal variations lead to subseasonal reversals, where early winter may be unusually warm, followed by later winter turning unusually cold, or vice versa. These subseasonal SAT fluctuations are influenced by atmospheric circulation anomalies. In this study, we investigate the spatial and temporal patterns of the mean winter SAT and subseasonal SAT reversals in North China, focusing on their interannual variability. We emphasize the significant impact of anomalous zonal circulation over the middle and high latitudes in Eurasia and explore the relative influence of the Arctic as a boundary forcing factor in circulation anomalies. Key Points: The winter surface air temperature (SAT) and subseasonal SAT reversal in North China have noteworthy interannual variationsThe Eurasian Zonal Circulation (EZ) strongly influences winter SAT and subseasonal reversal variabilityThe physical mechanisms linking autumn Arctic sea ice concentrations to EZ is examined [ABSTRACT FROM AUTHOR]

We present a comprehensive analysis diagnosing the primary factors driving the observed changes in major atmospheric teleconnection patterns in the Northern Hemisphere winter, including the Pacific North American pattern (PNA), North Atlantic Oscillation (NAO), and North American winter dipole (NAWD), with particular focus on their roles in shaping anomalous weather across North America. Our investigation reveals a consistent influence of the NAWD over seven decades, contrasting with fluctuating impacts from PNA and minor impacts from NAO. In particular, an emergent negative correlation between the NAWD and PNA, signaling a shifted phase of teleconnection patterns, is identified. Such a relationship change is traced to enhanced upper-level ridges across western North America, reflecting a reinforced winter stationary wave. Through attribution analysis, we identify greenhouse gas emissions as a probable driver for the northward drift of the Asia-Pacific jet core, which, aided by orographic lifting over the Alaskan Range, subsequently amplifies the winter stationary wave across western North America. This work emphasizes the pronounced effect of human-induced global warming on the structure and teleconnection of large-scale atmospheric circulation in the Northern Hemisphere winter, providing vital perspectives on the dynamics of current climate trends. [ABSTRACT FROM AUTHOR]

In recent decades, winter Arctic sea-ice concentration (SIC) has experienced a most prominent decline over Barents-Kara Seas (BKS). However, what regulates the time scale and spatial structure of the SIC variability over the Eurasian Arctic is unclear. Here, we find that the SIC variability over the Eurasian Arctic exhibits two major modes: decadal dipole mode with antiphase variation between the BKS and East Greenland (EG), and interannual monopole mode with in-phase variation between the BKS and EG. This decadal mode mainly results from interdecadal changes in ocean heat transports (OHTs) through Barents Sea Opening (BSO) and EG, lagging the Atlantic Multidecadal Oscillation by 7–16 years. The positive SIC dipole mode with a decrease over the BKS and an increase over the EG is also tied to the negative Arctic Oscillation comprised of Ural blocking and the negative North Atlantic Oscillation (NAO). However, the SIC loss of the interannual monopole mode mainly stems from the positive Arctic dipole comprised of Ural blocking and positive NAO through interannual changes in the BSO OHT and atmospheric moisture or heat transport. We further highlight that interannual atmospheric transports and BSO OHT associated with the Arctic dipole contribute to ~66% and ~34% of the interannual variability of the Eurasian Arctic SIC during 1960-2017, respectively. On decadal timescales, the relative contributions of atmospheric transports associated with Arctic Oscillation and OHT to the Eurasian Arctic SIC variability are ~19% and ~81%, respectively. Especially, the contribution of decadal atmospheric transports is significantly intensified during 2000–2017. [ABSTRACT FROM AUTHOR]

The interannual variability of deep convection in the Irminger Sea is considered essential in controlling the intensity of the Atlantic Meridional Overturning Circulation (AMOC). Based on the ARMOR-3D oceanic dataset and the Era-Interim atmospheric reanalysis, we suggest that the oceanic heat advection in the sea of the Irminger Current governs the interdecadal variability of deep convection of the Irminger Sea by modulating the upper ocean heat content in the basin. This forms a negative feedback that stabilizes the AMOC: an increase in the advection of oceanic heat into the Irminger Sea leads to a decrease in the convection depth, which further reduces the northward heat transport by the AMOC. We suggest that, on interdecadal time scales, the effect of ocean–atmosphere heat exchange on deep convection is relatively small due to a much lower interannual variability of the former compared to that of the oceanic heat convergence. During the positive phase of the North Atlantic Oscillation (NAO), the Irminger Current becomes colder, which allows an alternative explanation for the intermittently observed intensification of convection in the Irminger Sea with an increase in the NAO index. Thus, the long-period variability of the convection intensity in the Irminger Sea is associated not with local atmospheric influence, but rather with remote atmospheric forcing. [ABSTRACT FROM AUTHOR]

Variability in seasonal weather in north‐west Europe is substantially determined by jet stream variability. The North Atlantic Oscillation (NAO) has been well studied as a key representation of this jet stream variability, but other circulation indices are also important. Here the first three principal component empirical orthogonal functions (EOFs) of 500 hPa geopotential height (GPH), which broadly correspond to the NAO, the East Atlantic pattern (EA) and Scandinavian pattern (SCA), as well as jet speed and latitude, are correlated with temperature and precipitation anomalies over Europe with a focus on north‐west Europe, as well as measures of persistence and variability. In high summer (July and August), all three of the principal EOFs are significantly correlated with extreme temperatures in large areas of northern Europe. In winter, for much of north‐west Europe, both temperatures and precipitation are positively correlated with the jet speed, and precipitation is negatively correlated with EOF3. There is some non‐stationarity in some of the relationships, notably between winter precipitation and EOF1, and between July/August precipitation and EOF2. In addition to single variate correlations, multiple correlation coefficients are also used to determine areas of significant correlation when combining two or three of the circulation indices. The multiple correlation analyses show that combining the three EOFs produces significant correlations with temperature and precipitation over much of Europe. These analyses provide scope for using seasonal forecasts to predict likely temperature and precipitation anomalies based on predicting the atmospheric circulation anomalies and downscaling them. Improved seasonal forecasts of temperature and precipitation, including persistence and variability, will be useful to a number of users, such as agrifood, transport, energy supply and insurance. [ABSTRACT FROM AUTHOR]

Empirical orthogonal function (EOF) analysis is widely adopted for identifying spatial patterns in regular latitude‐longitude gridded geographical data. However, a standard EOF analysis could underestimate variances from low latitudes due to the increasing grid cell areas when moving toward the equator. A broadly adopted compensating approach is the area‐weighted EOF, where the grid data are multiplied by a factor proportional to the grid area, thereby forcing each datum to represent identical areas. In this article, we revisit the area‐weighting scheme and discuss its potential drawbacks. In particular, we show that along with compensating the unequal areas, another unaddressed issue is which region of the data is more relevant to our focused problem. We propose a regional attentive EOF (RA‐EOF) method, which resolves area‐compensation and region selection simultaneously. We conduct case studies of the North Atlantic Oscillation (NAO) analysis and perform RA‐EOF to the sea level pressure variability in the North Atlantic sector. Experiments show that our method evidently detects the NAO pattern in the leading mode of sea level pressure variability and suggests the southwestward movement of the southern action centre of NAO during the summers. Our findings clarify the ambiguity about the summer NAO in previous studies. The proposed methodology provides a potent instrument for discerning the spatial distribution and analysing the temporal variability of atmospheric teleconnections and oscillations, particularly in cases characterized by weak signal strength. [ABSTRACT FROM AUTHOR]

The synoptic cold surge is a weather event that frequently occurs in the Northern Hemisphere, often causing severe damage accompanied with intense winds and precipitation. It is reported that the Arctic Oscillation and the North Atlantic Oscillation can affect the cold surge. However, on interacting with tropical circulation, the cold surge potentially links to the Antarctic sea‐ice loss with influence extending to Tropics. Therefore, this research aims to investigate the potential link between Antarctic sea‐ice loss and Northern Hemisphere cold surges and corresponding wind–precipitation compound events. To study this link, twin numerical experiments using a physically based atmosphere model with a prescribed 30% Antarctic sea‐ice loss scenario were used. The results show that the cold surge occurs more frequently in most regions of the world, except for central North America and central Eurasia. In Europe, the cold surges occur closer to the lower latitudes in the Antarctic sea‐ice loss scenario. Additionally, the Antarctic sea‐ice loss can result in intensified wind and precipitation extremes within compound events. The intensified precipitation extremes are caused by increased moisture transportation by tropical easterlies and higher local temperature after the cold surge occurrence, providing more moisture available for precipitation in East Asia and East North America, respectively. Overall, the results of the numerical experiments provide evidence that Antarctic sea‐ice loss influences cold surges in Northern Hemisphere through atmospheric teleconnections. It is important to continue investigating the link between Antarctic sea‐ice loss and weather events like cold surges, as this research can help improve our understanding of the impacts of polar climate change on lower latitudes. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) anomalies often display a tripole pattern in the North Atlantic Ocean. This study investigates the formation of large amplitude North Atlantic tripole SST anomaly pattern during boreal spring when the El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) act individually and synthetically. It is found that the tripole SST anomalies have a larger amplitude when ENSO and NAO act together than when ENSO or NAO acts alone. The effects of ENSO are mainly in the tropics. The effects of NAO are larger in the high latitude. The large amplitude tripole SST anomaly pattern is associated with a similar tripole pattern in surface latent heat flux anomalies. Surface sensible heat flux and ocean horizontal advection also contribute to the high-latitude SST anomalies and have a supplementary effect on the mid-latitude SST anomalies. Latent heat lux anomalies are attributed to the effect of wind speed in the tropics and middle latitude and sea-air humidity difference in the high latitude. Sensible heat flux anomalies are mainly attributed to the effect of sea-air temperature difference. [ABSTRACT FROM AUTHOR]

The interaction between the North Atlantic Oscillation (NAO) and the latitudinal shifts of the Gulf Stream sea surface temperature front (GSF) has been the subject of extensive investigations. There are indications of nonstationarity in this interaction, but differences in the methodologies used in previous studies make it difficult to draw consistent conclusions. Furthermore, there is a lack of consensus on the key mechanisms underlying the response of the GSF to the NAO. This study assesses the possible nonstationarity in the NAO–GSF interaction and the mechanisms underlying this interaction during 1950–2020, using reanalysis data. Results show that the NAO and GSF indices covary on the decadal time scale but only during 1972–2018. A secondary peak in the NAO–GSF covariability emerges on multiannual time scales but only during 2005–15. The nonstationarity in the decadal NAO–GSF covariability is also manifested in variations in their lead–lag relationship. Indeed, the NAO tends to lead the GSF shifts by 3 years during 1972–90 and by 2 years during 1990–2018. The response of the GSF to the NAO at the decadal time scale can be interpreted as the joint effect of the fast response of wind-driven oceanic circulation, the response of deep oceanic circulation, and the propagation of Rossby waves. However, there is evidence of Rossby wave propagation only during 1972–90. Here it is suggested that the nonstationarity of Rossby wave propagation caused the time lag between the NAO and the GSF shifts on the decadal time scale to differ between the two time periods. [ABSTRACT FROM AUTHOR]

The link between weather regimes (WRs) and polar low (PL) activity is examined over the North Atlantic and North Pacific basins. Compared to earlier studies based on limited, regional PL datasets, our study conducts a more complete evaluation of regional WR–PL relationships using an expanded PL climatology. Our findings show that PL activity is increased over the Norwegian and Barents Seas during the Atlantic ridge regime and decreased over the former region during the Scandinavian blocking regime, with negative impacts also stretching to the Irminger Sea. Over the Labrador and Irminger Seas, PL activity is modulated strongest by the North Atlantic Oscillation (NAO), with positive impacts during the positive phase and vice versa. Over the North Pacific, the Arctic low contributes to increased PL activity over most regions, while the opposite is true for the Pacific wave train regime. The variability of PL activity associated with WRs is strongly related to changes in key environmental conditions. In general, regions of enhanced (reduced) PL activity are coincident with anomalous low-level northerly (southerly) flow and reduced (increased) static stability. Further analysis shows that certain persistent WRs can strongly modulate PL activity over some regions, either due to the amplification or propagation of favorable or unfavorable conditions, which cautions the limitation of regarding WRs as stationary patterns. A previously developed PL genesis potential index is shown to represent the observed impacts well, which serves to confirm the robustness of our findings and suggests the potential applicability of WRs to the subseasonal prediction of PLs. Significance Statement: Polar lows are intense small-scale (∼300 km) cyclones that form over high-latitude oceanic regions. The hazardous impacts they pose to coastal communities and maritime and air operations in the Arctic motivate their skillful prediction, which remains a major challenge at lead times beyond a few days. In this study, we relate PLs and the key environmental conditions that favor their development to weather regimes, which are recurrent large-scale circulation patterns that can persist for weeks at a time. We find that weather regimes have strong impacts on polar low activity through the modulation of key environmental conditions. These relationships can potentially be utilized in the extended-range prediction of polar lows. [ABSTRACT FROM AUTHOR]

We investigate how the ocean responds to 10-yr persistent surface heat flux forcing over the subpolar North Atlantic (SPNA) associated with the observed winter NAO in three CMIP6-class coupled models. The experiments reveal a broadly consistent ocean response to the imposed NAO forcing. Positive NAO forcing produces anomalously dense water masses in the SPNA, increasing the southward lower (denser) limb of the Atlantic meridional overturning circulation (AMOC) in density coordinates. The southward propagation of the anomalous dense water generates a zonal pressure gradient overlying the models' North Atlantic Current that enhances the upper (lighter) limb of the density-space AMOC, increasing the heat and salt transport into the SPNA. However, the amplitude of the thermohaline process response differs substantially between the models. Intriguingly, the anomalous dense-water formation is not primarily driven directly by the imposed flux anomalies, but rather dominated by changes in isopycnal outcropping area and associated changes in surface water mass transformation (WMT) due to the background surface heat fluxes. The forcing initially alters the outcropping area in dense-water formation regions, but WMT due to the background surface heat fluxes through anomalous outcropping area decisively controls the total dense-water formation response and can explain the intermodel amplitude difference. Our study suggests that coupled models can simulate consistent mechanisms and spatial patterns of decadal SPNA variability when forced with the same anomalous buoyancy fluxes, but the amplitude of the response depends on the background states of the models. [ABSTRACT FROM AUTHOR]

Understanding the association of climatic oscillations (COs) and meteorological parameters (MPs) with rainfall is of considerable significance in the management of water resources. This study used bivariate wavelet coherence (BWC), partial wavelet coherence (PWC) and multiple wavelet coherence (MWC) formulations for investigating the multiscale coherence of monthly mean rainfall of Calicut, Kerala, India with diverse sets of COs and local MPs. Firstly, the multiscale association between rainfall of 1970 and 2019 with four COs, viz., El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), Indian Ocean Dipole (IOD) and North Atlantic Oscillation (NAO) are investigated using different WC formulations. The BWC and PWC analyses detected PDO and NAO as the significant COs influencing rainfall of Calicut, strongly modulated by ENSO and IOD. MWC analysis with 11 combinations of COs revealed the highest coherence for ENSO–IOD and ENSO–PDO–NAO, indicating an equally strong influence of different COs upon the rainfall of Calicut. Further, the teleconnections of rainfall with local MPs, viz., maximum temperature (Tmax), minimum temperature (Tmin), wind speed (U) and evaporation (E) over Calicut are also analyzed. The BWC analysis detected annual periodicity in all the time series, with an additional band at the scale of six months in Tmin series. The coherence strength quantified in terms of average wavelet coherence (AWC) and percentage of significant coherence (PSC) showed that evaporation was the most significant MP (AWC of 0.66 and PSC of 54%) modulated strongly by wind speed. The MWC analysis of rainfall with MPs displayed the highest coherence for Tmin–E and U–Tmin–E combinations in the rainfall of Calicut. [ABSTRACT FROM AUTHOR]

The weak stratospheric polar vortex (SPV) is usually linked to Northern Hemisphere cold spells. Based on the fifth generation of ECMWF atmospheric reanalysis and WACCM model experiments, we use K-means cluster analysis to extract the zonally asymmetric pattern of October-February stratospheric variability, which involves a stretched SPV and hence leads to cold surges in Northern Hemispheric mid-latitudes. There are contrasting effects and mechanisms between autumn (October–November) and late winter (February) SPV stretching events. In October, anomalies in the stratospheric circulation affect the near-surface 16–20 days after the weakening of the SPV. This contributes to a shift in the North Atlantic Oscillation (NAO) towards its negative phase, leading to cold anomalies over northern Eurasia. Together with the weakening of planetary wave-1 during days 31–40, the second stratosphere–troposphere coupling strengthens the East Asian trough and the Siberian high, resulting in Eurasian high-latitude cooling. For November, the suppressed upward propagation of wave-1 during days 11–15 is conducive to anomalous high pressure over northern Europe and thereby European cooling through a stratospheric pathway, while for days 21–30, the weakening of propagating wave-2 over Eastern Europe intensifies the mid-latitude wave train through a tropospheric pathway, favorable for cold temperatures in mid-latitude East Asia. In contrast, the late winter SPV stretching events and the attendant Eurasian coldness during 11–25 days are likely to have been simultaneously driven by the long-lived European high anomaly, which enhanced the upward-propagating tropospheric waves into the stratosphere and thus favored SPV stretching. It indicates that the tropospheric pathway, rather than the stratospheric pathway, plays a dominant role in cold Eurasia following February SPV stretching events. [ABSTRACT FROM AUTHOR]

In this research, we initially examined the key atmospheric circulation pattern influencing the occurrence of dust storms in Northwest China during spring (February–May). We then investigated the drivers impacting atmospheric circulation over the Mongolian Plateau and southern Central Siberia (MPCMS), using NCEP/NCAR reanalysis data and extensive ensemble simulations, and assessed the respective roles of external forces and internal variability. Our results validated a significant inverse correlation between the reduced frequency of spring dust storms in Northwest China post-mid-1980s and heightened geopotential height anomalies over the MPCMS. By scrutinizing five comprehensive ensemble model simulations, we demonstrated that the positive tendencies in atmospheric circulation anomalies over the MPCMS are largely triggered by external forces, accounting for roughly 69.3% of the observed augmentations in the atmospheric circulation index trend from 1954 to 2022. Although the North Atlantic Oscillation is a leading mode of internal variability associated with geopotential height anomalies over the MPCMS, its contribution is comparatively minor. Our findings underline that the primary cause of the decrease in dust storm frequency in Northwest China since the mid-1980s could be ascribed to global warming-related external forces. [ABSTRACT FROM AUTHOR]

The Atlantic multidecadal oscillation (AMO) is considered as one of the major drivers of Indian Summer monsoon (ISM) multidecadal variability. The teleconnection is thought to take place via two dominant pathways—by generating upper-level circulation and heating responses across Eurasia and via the Atlantic-Pacific atmospheric bridge mechanism and associated modulations of Hadley-Walker circulations. Using the PMIP3 Last millennium (LM) simulations, the current study investigates these pathways in five models which exhibit a significant positive correlation between AMO and ISM. In all the models, the upper-level wave responses associated with the AMO are found to be inadequate to induce positive tropospheric temperature (TT) anomalies over the ISM domain. In the four models which capture the AMO modulation of summer North Atlantic Oscillation (SNAO) variability, major discrepancies are observed in the SNAO downstream TT responses. The extratropical-tropical Pacific SST gradient is a critical aspect of the Atlantic-Pacific atmospheric bridge mechanism. A positive correlation is observed between the AMO and the Pacific SST gradient in all the models and the associated Walker circulation response is also captured in the LM simulations. Thus while the upper-level circulation and TT responses involved in the Eurasian pathway, are not captured by most of the models, the Pacific pathway emerges as a better represented teleconnection pathway in the LM simulations. Reliability of decadal climate predictions of ISM largely depends on the fidelity of global models in simulating these teleconnection mechanisms and the current study highlights some of the main deficiencies in the model simulated teleconnection processes. [ABSTRACT FROM AUTHOR]

The UK has experienced recurring periods of hydrological droughts in the past, including the drought declared in summer 2022. Seasonal hindcasts, consisting of a large sample of plausible weather sequences, can be used to create drought storylines and add value to existing approaches to water resources planning. In this study, the drivers of winter rainfall in the Anglian region in England are investigated using the ECMWF SEAS5 hindcast dataset, which includes 2850 plausible winters across 25 ensemble members and 3 lead times. Four winter clusters are defined using the hindcast winters based on possible combinations of various atmospheric circulation indices (such as the North Atlantic Oscillation, NAO; East Atlantic, EA, pattern; and El Niño–Southern Oscillation). Using the 2022 drought as a case study, we demonstrate how storylines representing alternative ways the event could have unfolded can be used to explore plausible worst-case scenarios over winter 2022/23 and beyond. The winter clusters span a range of temperature and rainfall response in the study region and represent circulation storylines that could have happened over winter 2022/23. River flow and groundwater level simulations with the large sample of plausible hindcast winters show that drier-than-average winters characterised by predominantly NAO - /EA - and NAO + /EA - circulation patterns could have resulted in the continuation of the drought with a high likelihood of below-normal to low river flows across all selected catchments and boreholes by spring and summer 2023. Catchments in Norfolk were particularly vulnerable to a dry summer in 2023 as river flows were not estimated to recover to normal levels even with wet winters characterised predominantly by NAO - /EA + and NAO + /EA + circulation patterns, due to insufficient rainfall to overcome previous dry conditions and the slow response nature of groundwater-dominated catchments. Through this analysis, we aim to demonstrate the added value of this approach to create drought storylines during an ongoing event. Storylines constructed in this way supplement traditional weather forecasts and hydrological outlooks, in order to explore a wider range of plausible outcomes. [ABSTRACT FROM AUTHOR]

High‐resolution Sr and Nd isotope compositions along with major and trace element abundances have been analyzed in silicate fraction of sediments core, SSD004‐GC03, from the Equatorial Indian Ocean (7.2°N and 77.9°E) at 1,540 m water depth with a depositional history of ∼38 ka to determine source variabilities and their controlling factors. 87Sr/86Sr (0.71978–0.72491), ƐNd (−14.8 to −21.9), and a couple of source diagnostic elements display profound variability over the depositional time scale and point toward major sediment contribution from the Peninsular Gneissic Complex (PGC) and the Deccan Basalts along with aeolian dust flux, their relative proportions being determined by climate variability. The cold/arid periods are characterized by an enhanced proportion of aeolian dust and the Deccan Basalts, whereas the sediment contribution from the PGC is augmented during the warm/humid periods. The sediment provenance variations at the Equatorial Indian Ocean coincide very well with known cold/arid (Heinrich Stadial events: HS 1–4, LGM, Younger Dryas, 8.2 ka, 5.2 ka, and 1.1 ka) and warm/humid (Early Deglacial, Holocene Intensified Monsoon) climatic events reported in the tropical region and sea‐level change which are strongly captured by the Sr‐Nd isotope and elemental composition of sediments. The present investigation underscores the significant role of climate, mainly the aridity, in modulating the dust fluxes and erosion intensity and the strong coupling between Indian monsoon and North Atlantic climatic oscillations and further demonstrates minimal time delay between the production and transport of sediment from source to sink. Key Points: High‐resolution 87Sr/86Sr, ƐNd and a few diagnostic elements track temporal variation of sediment sources in the Equatorial Indian OceanThe cold periods are characterized by enhanced proportions of aeolian dust and the Deccan Basalts, whereas the PGC dominates warm periodsNorth Atlantic climatic oscillations modulate Indian monsoon thereby controlling dust and erosional fluxes in the Equatorial Indian Ocean [ABSTRACT FROM AUTHOR]

Reconstructing of past hydroclimates at regional scales during the Common Era (CE) is necessary to place the current warming in the context of natural climate variability. Here we present a composite record of oxygen isotope variations during last 2500 years based on eight stalagmites from four caves in the central Pyrenees (NE Spain) dominated by temperature variations, with the amount of precipitation playing a minor role. The dataset is compared with other Iberian reconstructions that show a high degree of internal coherence with respect to variability at the centennial scale. The Roman Period (RP) (especially 0–200 CE), the Medieval Climate Anomaly (MCA), and part of the Little Ice Age (LIA) represent the warmest periods, while the coldest decades occurred during the Dark Ages (DA) and most of the LIA intervals (e.g., 520–550 CE and 1800–1850 CE). Importantly, the LIA cooling or the MCA warming were not continuous or uniform and exhibited high decadal variability. The Industrial Era (IE) shows an overall warming trend although with marked cycles and partial stabilization during the last 2 decades (1990–2010). The strong coherence between the speleothem data, European temperature reconstructions and global tree-ring data informs about the regional representativeness of this new record as Pyrenean past climate variations. Solar variability, likely through its impact on the North Atlantic Oscillation, and major volcanic eruptions appear to be the two main drivers of climate in southwestern Europe during the past 2.5 millennia. [ABSTRACT FROM AUTHOR]

Abstract The role of internal variability in generating an apparent link between autumn Barents‐Kara sea (BKS) ice and the winter North Atlantic Oscillation (NAO) has been intensely debated. In particular, the robustness and causality of the link has been questioned by showing that BKS‐NAO correlations exhibit nonstationarity in both reanalysis and climate model simulations. We show that the lack of ice observations means nonstationarity cannot be confidently assessed using reanalysis prior to 1961. Model simulations are used to corroborate an argument that forced nonstationarity could result from ice edge changes due to global warming. Consequently, the observed change in BKS‐NAO correlations since 1960 might not be purely a result of internal variability and may also reflect that the ice edge has moved. The change could also reflect the availability of more accurate ice observations. We discuss potential implications for analysis based on coupled climate models, which exhibit large ice edge biases.

Abstract A too‐weak eddy feedback in models has been proposed to explain the signal‐to‐noise paradox in seasonal‐to‐decadal forecasts of the winter Northern Hemisphere. We show that the “eddy feedback parameter” (EFP) used in previous studies is sensitive to sampling and multidecadal variability. When these uncertainties are accounted for, the EFP diagnosed from CMIP6 historical simulations generally falls within the reanalysis uncertainty. We find the EFP is not independent of the sampled North Atlantic Oscillation (NAO). Within the same dataset, a sample containing larger NAO variability will show a larger EFP, suggesting that the link between eddy feedbacks and the signal‐to‐noise paradox could be due to sampling effects with the EFP. An alternative measure of eddy feedback, the barotropic energy generation rate, is less sensitive to sampling errors and delineates CMIP6 models that have weak, strong, or unbiased eddy feedbacks, but shows little relation to NAO variability.

Abstract Ensemble forecasts have been shown to better predict observed Atlantic climate variability than that of their own ensemble members. This phenomenon—termed the signal‐to‐noise paradox—is found to be widespread across models, timescales, and climate variables, and has wide implications. The signal‐to‐noise paradox can be interpreted as forecasts underestimating the amplitude of predictable signals on seasonal‐to‐decadal timescales. The cause of this remains unknown. Here, we examine sea level pressure variability from a very large multi‐model ensemble of uninitialized atmosphere‐only simulations, focusing on boreal winter. To assess signal‐to‐noise errors, the ratio of predictable components (RPC) is examined globally, as well as for regional climate indices: the North Atlantic Oscillation, Arctic Oscillation, Southern Annular Mode, and an Arctic index. Our analyses reveal significant correlations between the multi‐model ensemble‐mean and observations over large portions of the globe, particularly the tropics, North Atlantic, and North Pacific. However, RPC values greater than one are apparent over many extratropical regions and in all four climate indices. Higher‐resolution models produce greater observation‐model correlations and greater RPC values than lower‐resolution models in all four climate indices. We find that signal‐to‐noise errors emerge more clearly at higher resolution, but the amplitudes of predictable signals do not increase with resolution, at least across the range of resolutions considered here. Our results suggest that free‐running atmospheric models underestimate predictable signals in the absence of sea surface temperature biases, implying that signal‐to‐noise errors originate in the atmosphere or in ocean–atmosphere coupling.

In the North Atlantic and the Northern Ocean, from the second half of 2010 to 2014, satellite imagery data showed increased surface water temperatures (in the Icelandic Depression area in September–October 2010, it was 1.3 °C higher than in 2009). The peak of the annual sum of mean monthly ocean surface temperatures near the Icelandic Depression in 2010 (109.3 °C), as well as the negative values of the monthly averaged North Atlantic Oscillation (NAO) indices, estimated in the second half of 2010 and until March 2011, can be explained by the appearance of an additional film of oil origin on the water surface, formed after an oil spill accident at the Deepwater Horizon drilling rig in the Gulf of Mexico. Insufficient evaporative cooling of surface waters near the Icelandic Depression related to the formation of an additive film due to the influence of pollution of the North Sea by oil can explain the earlier peak in the annual sum of mean monthly ocean surface temperatures near the Icelandic Depression in 2003 (107.2 °C). Although global warming is usually ascribed to increased greenhouse gases in the atmosphere, ocean surface water pollution could increase the heat content of the ocean and explain the steady temperature stratification and desalination of these waters due to the melting of Greenland’s glaciers. Thus, when analyzing the concept of global warming, it is necessary to take into account the aspects of pollution of the ocean surface waters to assess the changes in their capacity to accumulate solar radiation, as well as the changes in the heat content of the ocean mixing zone (~200 m).