Surface Air Temperature Anomalies Research Articles

Advancing Subseasonal Surface Air Temperature and Heat Wave Prediction Skill in China by Incorporating Scale Interaction in a Deep Learning Model

AbstractAccurate subseasonal predictions of high surface air temperature (SAT) and heat wave events 10–30 days in advance are crucial for mitigating the risks of extreme weather; however, they pose a challenge for current operational models. In this study, we trained a convolutional neural network (CNN)‐based deep learning model to exploit the modulations in China's SAT by precursor signals across different timescales to improve predictions of future SAT and heat wave events. This CNN model demonstrated superior capability in capturing the evolution of SAT anomalies and the occurrence of heat wave events with forecast lead times beyond 20 days, compared with that of the operational models of the China Meteorological Administration and European Centre for Medium‐Range Weather Forecasts. Explainability analysis highlighted that subseasonal SAT predictability in China is driven primarily by large‐scale intraseasonal perturbations from both lower‐ and higher‐latitude regions of Eurasia and interannual variability.

Comparing MJO Intensity over the Tropical Western Pacific during Mega and Equatorial La Niña Winters

Abstract This study investigated variations in the Madden–Julian oscillation (MJO) behavior between two types of La Niña winters: mega and equatorial La Niñas. Results of this work show that in contrast to mega conditions, accompanied by more abundant intraseasonal column-integrated moisture anomalies over the planetary boundary layer, intensities of intraseasonal outgoing longwave radiation anomalies are stronger over the tropical western Pacific (WP) at MJO phases 5–6 under equatorial conditions. The occurrence of the moisture anomaly change averaged over the tropical WP is supported by a distinct moisture tendency difference. Moisture budget and multiscale interaction diagnoses underscore the pivotal effect of an area-averaged vertical gradient change in low-frequency background moisture in driving such tendency change. Considering notable MJO convection variations, the teleconnections associated with MJO phases 5–6 over East Asia (EA) were also explored, including warmer surface air temperature anomalies and stronger positive geopotential height anomalies at 200 hPa on the intraseasonal time scale under equatorial conditions. Results from a linear baroclinic model demonstrate that such teleconnection changes could be effectively explained by the linear response to the MJO’s diabatic heating anomaly. Roles of mean state and MJO itself variations in the linear response change are also discussed. These findings provide novel insights into MJO activity and offer potential improvements for subseasonal forecasting in EA. Significance Statement The relationship between El Niño–Southern Oscillation (ENSO) and MJO has been extensively explored recently. However, variations in the MJO behavior and its climate effects under mega and equatorial La Niña winters have not been thoroughly investigated. In this work, we utilized reanalysis datasets and an idealized linear baroclinic model to address these questions. We observed more robust and abundant intraseasonal convection activity and planetary boundary layer–integrated moisture anomaly averaged over the tropical WP at MJO phases 5–6 under equatorial conditions than mega conditions, which is closely linked with the vertical gradient change in low-frequency background moisture. Additionally, MJO-related teleconnections also exhibit some changes. This work may provide a new perspective on understanding the relationship between the MJO and ENSO and offer potential improvements for the subseasonal forecast.

Internal Variability Dominated the Extreme Cold Wave Over North America in December 2022

AbstractIn December 2022, North America experienced an unprecedented extreme cold event. However, the underlying physical mechanisms of this cold wave, and the extent to which it is driven by internal variability or external forcing, are not fully understood. Using ERA5 reanalysis data and the HadGEM3‐A‐N216 attribution simulations, we identified internal variability as the main cause, contributing −5.14 K to surface air temperature (SAT) anomalies in North America. External forcing slightly mitigated the cold by 0.42 K. An internally generated wave train from the North Pacific, influenced in combination by Pacific‐North American (PNA) and North Pacific Oscillation (NPO) teleconnection patterns, initiated this intense cyclonic event, contributing −2.18 K and −2.12 K to SAT anomalies, respectively. La Niña‐like sea surface temperature anomalies amplified this wave train and resultant cold wave. Additionally, excessive snow cover in the previous November also intensified the December cold anomalies by enhancing surface albedo and reducing solar radiation.

Causes of the extreme cold event in December 2023 on Eastern China

Abstract An extreme cold event outbreaks in Eastern China (EC) in December 16–22, 2023. Its maximum intensity (−8.30 °C) and duration (7 days) are in the second place in December during 1980–2023. In Early Stage (December 6–10), surface air temperature (SAT) anomalies reach the highest at 6.77 °C, exceeding mean value by two standard deviations. The variation of SAT anomalies (differences of SAT anomalies between the last day and the first day for a given period) is 0.60 °C. In Development Stage (December 11–15), SAT anomalies begin to decline but remain positive. In Outbreak Stage (December 16–22), the variation of SAT anomalies reaches a minimum of −3.17 °C, reflecting the cooling of EC. From December 1, cold air gradually gathers in Siberia under the influence of Arctic high moving southward. Cold air is locked in Siberia due to negative anomalies of geopotiential height (GH) and the westerlies anomalies between 40°–50°N. On December 11, these negative GH anomalies begin to move southeastward, and the westerlies anomalies weaken to a easterlies. From December 16–22, EC experiences an extreme cold event due to the southward of Arctic high and the eastward of Ural and Okhotsk high. On the basis of the zonal wind index (ZI) phase changes from negative to positive and the jet stream moves southeastward, the strong (weak) jet stream is spotted to block (promote) the southward of cold air. Linear regression shows that negative Arctic Oscillation (AO) conducts to the concentration of cold air in Siberia. Positive Siberia High (SH) pushes cold air to EC. SAT anomalies decrease by 2.29 °C in EC with the increase of 1 unit for SH. In empirical orthogonal function (EOF) analysis, EOF1 (28.07%) is characterized by warm Arctic and cold Siberia (WA-CS), which reflects the effect of SH on the occurrence of extreme cold events.

Subseasonal variability of sea level pressure and its influence on snowpack over mid-high-latitude Eurasia during boreal winter

The atmospheric circulation significantly influences the snowpack over mid-high-latitude Eurasia. This study examines the characteristics of the leading subseasonal variability mode of boreal winter sea level pressure (SLP) with 20-80-day period and its relationship with snowpack over mid-high-latitude Eurasia, using the fifth generation of European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) data and different snowpack datasets. The SLP leading mode, characterized by a monopole pattern with a strong surface anomalous high centered near the Ural Mountains, exhibits a barotropic structure and extends from the surface to the tropopause. Above SLP and geopotential height anomalies propagate southeastward from the Barents-Kara Sea to East Asia. This leading SLP mode contributes to surface air temperature (SAT) and snowfall circulation anomalies over mid-high-latitude Eurasia. The latter two both directly influence on snowpack anomalies in situ. Over high latitude region, snowfall circulation anomaly is the dominant factor to control the snow depth anomaly. Over middle latitude region, both SAT and snowfall circulation anomalies lead to the snowpack anomaly. Furthermore, the response of snow depth to the leading subseaonal SLP mode occurs 2–5 days earlier than the response of snow cover to the same mode over middle latitude region. In addition, it is suggested that the Arctic Oscillation (AO), East Atlantic/West Russia (EAWR) and Polar/Eurasia (PEU) pattern may contribute to the development of the leading SLP mode and subsequently influence snowpack anomalies.

Interannual variation of the westward ridge point of the Western Pacific subtropical high in boreal winter

This study adopts a novel dynamic index of the westward ridge point (WRP) of the western Pacific subtropical high (WPSH) to investigate the interannual variation of the WPSH in boreal winter. The WRP index based on the theory of gradient wind approximation is particularly suitable for boreal winters. The WRP index comprises two dimensions that depict the zonal and meridional movement of the WPSH, respectively. There is a significant positive correlation between the zonal WRP index and the meridional WRP index. When the WPSH gets stronger, the WRP and the WPSH shift equatorward while advancing westward, and vice versa. The zonal and meridional shifts of the WPSH have distinct impact on the climate anomalies in the western North Pacific and East Asia. The northward shift of the WPSH characterizes a cyclonic-anticyclonic pair over western North Pacific, whereas the eastward shift of the WPSH characterizes a cyclonic anomaly over the subtropical western North Pacific. The anomalies of precipitation and surface air temperature vary accordingly. The meridional shift of the WPSH is closely associated with the El Niño-Southern Oscillation (ENSO) Sea Surface Temperature (SST) anomalies. The WRP index also demonstrates robust predictability in the hindcast data from ENSEMBLES, suggesting its far-reaching potential for climate prediction.

The Subseasonal Feedback of Extreme Anomalous Tibetan Plateau Snow Cover Events on the Atmosphere

Abstract The Tibetan Plateau snow cover exhibits notable subseasonal variability and plays a crucial role in influencing the atmosphere. This study employs numerical experiments to investigate the atmospheric feedback resulting from extreme anomalous snow cover events on the Tibetan Plateau, with a focus on both local and nonlocal atmospheric temperatures. The findings reveal that diabatic heating, directly induced by these events, leads to a local surface energy cooling response over the Tibetan Plateau, contributing to a reduction in local temperatures. This cooling effect amplifies local atmospheric temperature anomalies associated with extreme anomalous Tibetan Plateau snow cover events, constituting approximately 50% of the total final local surface air temperature anomalies. Furthermore, the Tibetan Plateau snow cover, through adiabatic processes, exerts a nonlocal influence on atmospheric temperature and circulation. The atmospheric temperature responses downstream of the Tibetan Plateau vary at different heights and regions, featuring both cold and warm anomaly responses. These variations depend on the relative contributions of horizontal advection and vertical advection in adiabatic heating. Significance Statement Snow cover is influenced by the atmosphere, and in turn, it affects the atmosphere. This study examines the feedback of extreme anomalous Tibetan Plateau snow cover events on the atmosphere at the subseasonal time scale, with a focus on atmospheric temperature. Our findings indicate that Tibetan Plateau snow cover has both local and nonlocal feedback effects on the atmosphere. In particular, extreme anomalous Tibetan Plateau snow cover amplifies local surface air temperature anomalies. The feedback contributes to approximately 50% of the total final local surface air temperature anomalies.

A Transmitted Subseasonal Mode of the Winter Surface Air Temperature in the Mid‐ and High‐Latitudes of the Eurasia and Contributions From the North Atlantic and Arctic Regions

AbstractA significant and striking seesaw pattern of winter surface air temperature (SAT) has emerged, featuring pronounced warming Arctic and cooling Eurasian (referred to as WACE). This study investigates the subseasonal SAT modes across the mid‐ and high‐latitudes of Eurasia and their possible mechanisms based on daily reanalysis data from 1979 to 2022. Our results reveal that Eurasian winter SAT exhibits two distinct subseasonal modes, characterized by a correlated southeastward propagation of temperature and geopotential height anomalies (GHAs) in the middle and lower troposphere. Notably, the subseasonal SAT anomalies with eight phases constrained by the hydrostatic equilibrium, originate from the GHAs in the Arctic stratosphere and then transfer to the East Asia. The sixth phase of the transmitted subseasonal SAT mode is proved to be the key transition phase from the WACE pattern to its counterpart. Further analysis indicates that the strength of the transmitted subseasonal SAT mode is controlled by the tripolar sea surface turbulent heat flux anomalies over the north Atlantic.

Attribution of the concurrent extreme heatwaves in Northern Europe and Northeast Asia in July 2018

In July 2018, Northern Europe and Northeast Asia experienced unprecedented, concurrent extreme heatwaves. Utilizing the HadGEM3-A-N216 attribution model and dynamical adjustment method, we highlight that the internally-generated circulation was the predominant factor triggering these heatwaves both in Northern Europe and Northeast Asia. External forcings further amplify these heatwaves and contribute about 1 K surface air temperature (SAT) anomalies both in Northern Europe and Northeast Asia. Moreover, there is a statistically significant positive correlation of SAT between Northern Europe and Northeast Asia. An internal-generated zonal wave train spanning Eurasia was identified as a key circulation facilitating the concurrent heatwaves by instigating the simultaneous anomalous anticyclones in these two regions. Positive sea surface temperature (SST) anomalies in the North Atlantic plays a pivotal role in the formation of this zonal wave train. The negative SST anomalies in the northern Indian Ocean and tropical central Pacific can further amplify the magnitude of wave train, particularly in the Northeast Asia. In additions, the internal thermodynamic processes, especially soil moisture-SAT feedback can slightly damp the heatwaves in both regions. It is due to enhanced evaporative cooling and a reduction in upward sensible heat flux, linked to increased soil moisture in previous month.

Winter “warm Arctic-cold Eurasia” pattern and its statistical linkages to oceanic precursors during the era of satellite observations

A striking recurrent feature of winter climate variability is the “warm Arctic-cold Eurasia” (WACE) pattern of opposite sign anomalies of surface air temperature (SAT) in the Barents Sea region and midlatitude Eurasia. Its origins and mechanisms are hotly debated, and its predictability remains unknown. This study investigates statistical relationships of the winter WACE dipole with concurrent anomalies of atmospheric circulation and oceanic precursors during the era of satellite observations. The results reveal a high potential for seasonal predictability of not only the WACE dipole but also several related indicators of winter climate variability, including the Arctic and Eurasian SAT anomalies. During subperiods of extreme covariability between the Arctic and Eurasian SATs around the early 1980s and late 2000s, most of the WACE variability is explained by ocean temperature and surface turbulent heat flux anomalies in the Barents Sea region during the preceding months. Anomalies in summer Atlantic water temperature (AWT) and autumnal sea surface temperature (SST) in this region explain about 70–80% of the variance of the following winter WACE variability during all events of strong Arctic-Eurasian SAT covariability. Analysis of SST variability in the Arctic-North Atlantic region suggests that the winter WACE link to the summer AWT anomalies reflects an atmospheric response to a large-scale surface reemergence of ocean temperature anomalies. However, this linkage had been robust only until the early 2000s. Since then, the winter WACE variability has been strongly related to autumnal SST anomalies in the Barents Sea region and the North Pacific.

The out‐of‐phase variation of surface air temperature anomalies over northern Central Asia between January and February: The role of intraseasonal variations in tropical convection

AbstractThe out‐of‐phase mode of winter surface air temperature anomalies (SATAs) over northern Central Asia (NCA; 45°–65°N, 40°–100°E) between January and February is investigated in this study. This mode corresponds to warm (cold) SATAs in January (February) over NCA and is mainly modulated by the enhanced tropical convection anomalies over the Maritime Continent in previous late December, associated with MJO phase 4. These tropical convection anomalies can induce anomalous tropospheric Rossby‐wave sources over the North Pacific in late December. The eastward‐propagating Rossby‐wave train, triggered by these anomalous Rossby‐wave sources, can cause negative and positive tropospheric geopotential height anomalies over the Greenland–Scandinavia region and NCA in the following early–mid‐January, subsequently leading to warm SATAs over NCA in January. The negative geopotential height anomalies over the Greenland–Scandinavia region in early–mid‐January can trigger upward‐propagating wave activity fluxes (WAFs) into the stratosphere, resulting in negative stratospheric geopotential height anomalies in late January–early February. These stratospheric anomalies, by triggering downward‐propagating WAFs, can in turn lead to positive tropospheric geopotential height anomalies over the Greenland–Scandinavia region in early February. These anomalies over the Greenland–Scandinavia region can maintain themselves in the following mid‐ and late February by feedback of anomalous storm tracks, and cause negative geopotential height anomalies and subsequently cold SATAs over NCA in February by triggered southeastward‐propagating Rossby‐wave train.

Joint influence of the North Atlantic sea surface temperature and the Barents sea-ice concentration on the dipole pattern of Eurasian surface air temperature in March

In recent decades, an increase in the frequency of extreme events in early spring across Eurasia has resulted in significant impacts on crop yields and socio-economic development. However, the variations of March surface air temperature (SAT) over Eurasia and mechanisms are still unclear. This study aims to investigate the dominant mode of the March surface air temperature (SAT) over Eurasia during 1979–2022, and its relationship with sea surface temperature (SST) and sea ice concentration (SIC). The results reveal the prevalence of a dipole mode of the Eurasian SAT anomalies (SATAs) in March characterized by positive SATAs in northern (50°N–80°N) Eurasia and negative ones in southern (10°N–45°N) region. Further analyses demonstrate that this dipole mode of Eurasian SATAs shows significant relationships with a positive phase of the North Atlantic tripole SST anomalies (NAT SSTAs), as well as with the decline of Barents Sea SIC anomalies (SICAs). Associated with the NAT SSTAs and the Barents Sea SICAs during March, two Rossby wave trains propagate towards Eurasia through different ways, one is propagating eastwards from the mid-latitude of the North Atlantic to Eurasia, and another one spreads from the polar region to Eurasia. Modulated by these two wave trains, anomalous near-surface southwesterly (northeasterly) wind induces more (less) warm air towards northern Eurasia, while more (less) cloud emerges in southern Eurasia and causes less (more) solar radiation reaching southern Eurasia, leading to the dipole mode of the Eurasian SATAs in March. These results are further confirmed by numerical experiments from the version 5 of the Community Atmosphere Model forced by the above mentioned SSTAs and SICAs.

Asian-Australian monsoon as a mediator on North American surface air temperature anomalies

The Asian-Australian monsoon (AAM) significantly shapes global weather and climate patterns, yet existing research has primarily focused on regional monsoon subsystems rather than adopting a comprehensive global perspective on the AAM. This study addresses this gap by elucidating the interannual variability of the AAM during boreal summer and underscoring the pivotal mediating role of AAM precipitation anomalies as a driving force behind surface air temperature anomalies (SATA) over central and southern North America (CSNA). To investigate AAM characteristics, we employ an empirical orthogonal function analysis to identify the primary mode of the AAM. The corresponding principal component serves as a representative measure for the AAM. Our findings reveal a distinct ‘wet (dry) Maritime Continent-dry (wet) Western North Pacific (WNP)’ dipole pattern in strong (weak) AAM years. Vertical motion anomalies emerge as the dominant contributors to these precipitation anomalies. Both observed data and model results indicate that the sea surface temperature anomalies (SSTA) in the central and eastern Pacific (CEP) play a crucial role in explaining the vertical motion anomalies. This explanation is facilitated through two key systems: the east–west Walker circulation-like anomaly and the anomalous equatorially symmetric anticyclonic pair in the Pacific. AAM precipitation anomalies act as a mediator between SSTA in the CEP and SATA over the CSNA. These generate Rossby wave source over the WNP, triggering Rossby waves that propagate to CSNA along a great circle path. This process results in upper-level positive geopotential height anomalies, thereby influencing SATA positively over the CSNA, as illustrated by the linear baroclinic model. These findings not only deepen our understanding of the AAM but also offer novel insights into the mechanisms influencing SATA formation over North America.

Iceberg danger in the seas of the Russian Federation Arctic Zone under modern climate change conditions

The aim of the research was to study the relationship between the intensity of iceberg formation in the Russian Arctic including the number of icebergs calving annually from outlet glaciers and surface air temperature anomalies. The research was carried out on the basis of satellite monitoring using non-commercial, freely distributed satellite information from optical-electronic satellites Landsat-8 (spatial resolution 15 m) and Sentinel-2 (spatial resolution 10 m) and the radar satellite Sentinel-1 (pixel size 20×40 m). To achieve the aim, an iceberg detection technique was used based on statistical criteria for searching for gradient zones in the analysis of two-dimensional fields of satellite images. Based on the analysis of satellite data of the visible spectral range of the Landsat-8 and Sentinel-2 satellites and Sentinel-1 radar data the maximum spatial dimensions of icebergs formed by the outlet glaciers of Severnaya Zemlya, Franz Josef Land (ZFI) and Novaya Zemlya in 2012–2022 were estimated. Satellite monitoring of the Severnaya Zemlya region was carried out using visible range images in the spring season (March–May), characterized by the best observation conditions in terms of cloudiness, natural light, and monitoring of icebergs most of which are located in fast ice at this time. Monitoring of the ZFI area was carried out using radar data in the period August-September, corresponding to the minimal ice cover conditions. Satellite monitoring of the Novaya Zemlya region was carried out using visible images in the summer season. In total, about 500 satellite images were analyzed. The discusses the dependence of the intensity of the iceberg formation process on the ice shelf and outlet glaciers with a floating edge on the surface air temperature and the maximum thickness of fast ice. It is shown that the abnormally warm weather that set in 2020 during the period of ice melting led to a sharp intensification of the process of glacier melting in the Russian Arctic and the formation of almost 8,000 icebergs near Severnaya Zemlya, more than 6,600 icebergs near ZFI and over 1,000 icebergs near the western coast of Novaya Zemlya. For all the areas of the Russian Arctic studied in the period 2012–2022 an increase was noted in the maximum observed sizes of icebergs calving from glaciers. The largest iceberg, whose length was 5 km, broke off in 2020 from the Matusevich ice shelf.

Near-global summer circulation response to the spring surface temperature anomaly in Tibetan Plateau –– the GEWEX/LS4P first phase experiment

Subseasonal to seasonal (S2S) prediction of droughts and floods is one of the major challenges of weather and climate prediction. Recent studies suggest that the springtime land surface temperature/subsurface temperature (LST/SUBT) over the Tibetan Plateau (TP) can be a new source of S2S predictability. The project “Impact of Initialized Land Surface Temperature and Snowpack on Subseasonal to Seasonal Prediction (LS4P)” was initiated to study the impact of springtime LST/SUBT anomalies over high mountain areas on summertime precipitation predictions. The present work explores the simulated global scale response of the atmospheric circulation to the springtime TP land surface cooling by 16 current state-of-the-art Earth System Models (ESMs) participating in the LS4P Phase I (LS4P-I) experiment. The LS4P-I results show, for the first time, that springtime TP surface anomalies can modulate a persistent quasi-barotropic Tibetan Plateau-Rocky Mountain Circumglobal (TRC) wave train from the TP via the northeast Asia and Bering Strait to the western part of the North America, along with the springtime westerly jet from TP across the whole North Pacific basin. The TRC wave train modulated by the TP thermal anomaly play a critical role on the early summer surface air temperature and precipitation anomalies in the regions along the wave train, especially over the northwest North America and the southern Great Plains. The participant models that fail in capturing the TRC wave train greatly under-predict climate anomalies in reference to observations and the successful models. These results suggest that the TP LST/SUBT anomaly via the TRC wave train is the first order source of the S2S variability in the regions mentioned. Furthermore, the TP surface temperature anomaly can influence the Southern Hemispheric circulation by generating cross-equator wave trains. However, the simulated propagation pathways from the TP into the Southern Hemisphere show large inter-model differences. More dynamical understanding of the TRC wave train as well as its cross-equator propagation into the Southern Hemisphere will be explored in the newly launched LS4P phase II experiment.

Connection of Summer Surface Temperature Anomalies between the East European Plain–West Siberian Plain and North America on the Interannual Time Scale

Abstract This study identifies a significantly positive relationship between summer surface air temperature (SAT) anomalies over two remote regions in the Eurasian continent and North America during the period 1979–2021 on the interannual time scale. The former region includes the East European Plain and the West Siberian Plain, and the latter region includes central and eastern North America. The regionally averaged summer SAT anomalies show a correlation coefficient of 0.66 between these two regions, which is significant at the 99% confidence level. This intercontinental SAT relationship can be explained by a wavelike pattern of circulation anomalies, which is the leading mode of upper-tropospheric circulation anomalies over the middle and high latitudes of the Northern Hemisphere in summer. Further analysis suggests that the sea surface temperature (SST) anomalies over the Pacific and North Atlantic in the preceding spring, being coupled with the leading mode of atmospheric circulation anomalies over the Pacific–Atlantic sector, persist into summer and affect the SATs in the two remote regions, resulting in the intercontinental SAT connection. Significance Statement Summer surface air temperature (SAT) has profound effects on public health and agricultural production. Here we find a significantly positive relationship between interannual variations of summer SATs over two remote regions, one in the Eurasian continent and the other in North America. This intercontinental relationship in SATs can be explained as a result of atmosphere–ocean coupling over the Pacific and North Atlantic in the preceding spring. The result is likely to be a critical implication for the seasonal forecast of SAT variations over the two regions. In addition, the concurrence of higher or lower temperatures in these two regions may have impacts on global grain production, since these two regions include many major grain-producing areas in the world.

Impact of spring land-surface conditions over the Tibetan Plateau on the early summer Asian monsoon using an AGCM large ensemble

Influence of land-surface memory effects over the Tibetan Plateau on the Asian summer monsoon has long been studied, but not quantified because of the difficulty of extracting only these effects from observational data. This study examines the impact of spring land-surface conditions, including surface air temperature (SAT) and snow cover, over the Tibetan Plateau and its surrounding regions on the early stage of the Asian summer monsoon using large-ensemble experiments produced by an atmospheric global climate model (AGCM). The results show that the above-normal SAT over and around the Tibetan Plateau in May can enhance June monsoon circulation without the sea surface temperature (SST) forcing. The physical mechanism behind this involves warmed surface air over the plateau enhancing the north–south SAT gradient, leading to strengthening the Asian summer monsoon circulation and precipitation. Inter-ensemble correlation analysis indicated that SAT warming over the plateau is clearly associated with upper-tropospheric anti-cyclonic circulation anomalies in May. Also, although winter land-surface signals do not persist until late spring, SAT anomalies in March tend to persist until May, explaining 10–20% of the total SAT variance in May. Interestingly, the spring SAT impacts on June monsoon circulations vary interannually, indicating that the contribution of the land–atmosphere (L–A) coupling process to interannual monsoon variability differs from year to year. Active L–A coupling years tend to coincide with cooler SSTs with anomalous low-level divergence and upper-level convergence over and around the Maritime continent, implying that SST forcing is associated with the L–A coupling strength. Specifically, in May of the developing stage of monsoon circulation, the weak Walker circulation over the equatorial and northern Indian Ocean and undeveloped monsoon circulation are suitable for the L–A coupling to influence the development of the monsoon circulation effectively. However, the well-known oceanic events, such as El Nin~\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\widetilde{n}}$$\\end{document}o, are found to be less connected with the interannual L–A coupling variations.

Relative importance of global warming, the IPO, and the AMO in surface air temperature and terrestrial precipitation

AbstractThe relative importance of oceanic modes to global surface air temperature (SAT) and terrestrial precipitation during 1934–2020 was investigated using a variety of statistical and dynamical system methods. Through singular spectrum analysis, we present the distribution of decadal (10–20‐year), multidecadal (20–50‐year), and secular (>50‐year) variabilities of global SAT and terrestrial precipitation anomalies. Three sea surface temperature modes were identified by singular value decomposition that affect the low‐frequency variabilities of global SAT and terrestrial precipitation anomalies—namely, global warming (GW), the Interdecadal Pacific Oscillation (IPO), and the Atlantic Multidecadal Oscillation (AMO). The relative importance GW, the IPO, and the AMO in SAT and terrestrial precipitation was obtained through a normalized multivariate information flow analysis. Besides a high information flow percentage (60%) from GW to global SAT, especially over the northern Indian Ocean and tropical West Pacific, information flow from the AMO to the northwestern Pacific was also found. In terms of terrestrial precipitation, a large area with a wet trend was found over Eurasia at mid‐to‐high latitudes, and this trend was especially remarkable in the boreal winter half‐year (November–April), as compared with that in the boreal summer half‐year (May–October). By employing artificial neutral networks with a self‐organized map to cluster the key patterns of vertically integrated water vapour flux, we found that the synoptic circulation related to the wet trend is characterized by westerly flow that transports water vapour from the northeastern Atlantic to Eurasia, which is favourable for precipitation there both in the boreal winter and summer half‐year.

Impact of stratospheric variability on subseasonal temperature reversals during late winter over the mid-high latitudes of East Asia

Significant phase shifts in winter surface air temperature (SAT) anomalies have successively occurred in East Asia (EA) during recent years, leading to detrimental effects on socio-economic activities. In this study, we identify the fourth principal mode of month-to-month SAT variations over EA in winter (November–February) using the Season-reliant Empirical Orthogonal Function (S-EOF) analysis. The fourth S-EOF mode (S-EOF4) represents subseasonal SAT reversals over the mid-high latitudes of EA during late winter (January–February), with cold (warm) anomalies over Northeastern Asia in January and widespread warm (cold) anomalies over midlatitude EA in February. Results indicate that the cold-to-warm S-EOF4 is associated with a northward shift of the Siberian High in January and a quasi-barotropic low-pressure system over the Ural region in February. The formation of the S-EOF4 is accompanied by stratospheric temperature anomalies over eastern Siberia–Alaska in January. Stratospheric warming corresponds to cold-to-warm transitions over the mid-high latitudes of EA, and vice versa. Further investigation suggests that stratospheric warming would simultaneously lead to a weakened Arctic stratospheric polar vortex (SPV), which mainly stretches into Northern Eurasia and directly influences cold anomalies over Northeastern Asia in January. The delayed impact of stratospheric variability on the reversal of EA SAT anomalies is accomplished by an “ocean bridge” of the midlatitude North Atlantic sea surface temperature (SST) anomaly. The weakened SPV promotes anomalous westerly winds in the midlatitude North Atlantic and generates a significant SST cooling, which then regulates a cyclonic anomaly over the Ural region and results in warm anomalies over the midlatitude EA in February. The bridge role of the North Atlantic SST anomaly is well supported by a linear baroclinic model. In addition, the deepening of the Aleutian Low in December could significantly enhance the climatological wavenumber 1, serving as the tropospheric precursor of stratospheric warming. These findings contribute to an in-depth comprehension of EA subseasonal temperature variability, offering valuable insights for enhancing seasonal prediction strategies.

An Intraseasonal Dipole Mode in Summertime Surface Air Temperature over Eurasia and Its Association with Heat Wave Occurrence

Abstract A strong coherence of the summer (June–August) surface air temperature (SAT) anomalies exists over eastern Eurasia (EE) and central Eurasia (CE) in association with a remarkable intraseasonal (10–90-day) variability. The intraseasonal SAT over Eurasia shows a negative correlation between EE and CE, which is closely related to an intraseasonal wave train at the upper troposphere over the mid–high latitudes of Eurasia. The wave train propagates eastward and results in the intraseasonal variation of SAT in a seesaw pattern. The column-integrated temperature budget suggests that both the horizontal and vertical advection are important for the intraseasonal SAT dipole. More specifically, the climatological westerly, intraseasonal meridional wind anomaly, and the anomalous vertical motion play dominant roles in perturbing temperature over the CE and EE regions, while the diabatic heating is a negative feedback. The propagation of the Rossby wave train in the upper troposphere helps to maintain the circulation anomalies associated with the SAT seesaw. The leading modes of meridional wind anomalies at 200 hPa over the Eurasian high latitudes resemble the intraseasonal wave train, indicating that the wave train associated with the SAT seesaw is an atmospheric intrinsic mode over the mid–high latitudes. This study also investigates the impact of the intraseasonal SAT seesaw on regional heat wave events. It is suggested that the intraseasonal SAT seesaw has a major influence on the variability of heat wave events over both EE and CE, which may provide great implications for regional subseasonal forecasts of extreme weather events.