Tropospheric Warming Research Articles

Using deep learning to emulate and accelerate a radiative-transfer model

AbstractThis paper describes the development of U-net++ models, a type of neural network that performs deep learning, to emulate the shortwave Rapid Radiative-transfer Model (RRTM). The goal is to emulate the RRTM accurately in a small fraction of the computing time, creating a U-net++ that could be used as a parameterization in numerical weather prediction (NWP). Target variables are surface downwelling flux, top-of-atmosphere upwelling flux (), net flux, and a profile of radiative-heating rates. We have devised several ways to make the U-net++ models knowledge-guided, recently identified as a key priority in machine learning (ML) applications to the geosciences. We conduct two experiments to find the best U-net++ configurations. In Experiment 1, we train on non-tropical sites and test on tropical sites, to assess extreme spatial generalization. In Experiment 2, we train on sites from all regions and test on different sites from all regions, with the goal of creating the best possible model for use in NWP. The selected model from Experiment 1 shows impressive skill on the tropical testing sites, except four notable deficiencies: large bias and error for heating rate in the upper stratosphere, unreliable for profiles with single-layer liquid cloud, large heating-rate bias in the mid-troposphere for profiles with multi-layer liquid cloud, and negative bias at lowzenith angles for all flux components and tropospheric heating rates. The selected model from Experiment 2 corrects all but the first deficiency, and both models run ~104 times faster than the RRTM. Our code is available publicly.

Climatological analysis of freezing level height over China and its implications using homogenized in‐situ data

ABSTRACTFreezing level height (FLH) is the altitude of a location where the air temperature is 0°C. The FLH denotes the approximate position for water phase transitions from liquid (or vapour) to solid but not vapour to liquid. Based on temperature and height data on the ground surface to heights with an atmospheric pressure of 400 hPa, the monthly FLH at 118 radiosonde stations located in China are derived for the 1958–2017 period. The monthly and annual climatology during 1981–2010 and the seasonal, intra‐annual and decadal variabilities of the FLH during 1958–2017, along with its long‐term trends, were quantitatively evaluated at the regional scale and for individual stations. FLH climatology is generally related to season, zone and topography. The annual FLH anomalies decline initially, followed by a rise, with synchronous changes in the temperature in the lower and middle troposphere; the anomalies in south‐eastern China are partly associated with El Niño–Southern Oscillation events. The increasing trends of FLH are closely associated with tropospheric warming and show evident regional and seasonal differences. Stronger increasing trends are observed in northern and western China in autumn or winter (particularly in recent decades). The changes in FLH have implications on long‐term trends in equilibrium line altitude (ELA) of glaciers in Tianshan Mountains, snow cover extent in melt season and hail frequency in China especially in high‐elevations. During recent decades, the evolution of the FLH and major composition in the cryosphere has accelerated as a response to tropospheric warming. The study on FLH change can provide valuable information for the evaluation of glaciers, snow cover extent and hail frequency in China from a new perspective; which is helpful for understanding regional hydrological processes and climate changes better. Further application of FLH in cryosphere monitoring can improve water management and flood forecasting in headwater regions in China.

A zonally-oriented teleconnection pattern induced by heating of the western Tibetan Plateau in boreal summer

The thermal effect of the Tibetan Plateau (TP) on the northern hemisphere climate has long been a hot topic of scientific research. However, the global effects of the TP heat source are still unclear. We investigate the teleconnection patterns coincident with the TP heat source in boreal summer using both observational data and numerical models including a linearized baroclinic model and an atmospheric general circulation model. The western TP shows the most intense variability in atmospheric heating and the most active connection to atmospheric circulations. The surface sensible heating component of the western TP heat source is associated with a high-latitude wave train propagating from North Japan to central North America through the Bering Sea and Canada. The radiative heating component is accompanied by a wavenumber-4 wave train over Eurasia. We focus on the global zonally-oriented pattern that is connected with the latent heat release from the western TP, referred to here as the TP–circumglobal teleconnection (TP-CGT). The TP-CGT pattern is triggered by the western TP latent heating in two parts starting from the TP: an eastward-propagating wave train trapped in the westerly jet stream and a westward Rossby wave response. The TP-CGT accounts for above 18% of the total variance of the circumglobal teleconnection pattern and modulates mid-latitude precipitation by superimposition. The western TP is the key region in which diabatic heating can initiate the two atmospheric responses concurrently, and the heating over northeastern Asia or the Indian Peninsula is unable to induce the circumglobal pattern directly. The unique geographical location and strong tropospheric heating also make the western TP as a “transit area” of transferring the indirect impact of the Indian summer monsoon (ISM) to the TP-CGT. These results enhance our understanding of the relationship between the circumglobal teleconnection and the ISM and is helpful for improving the prediction of the circumglobal teleconnection variability.

Quantifying the Mechanisms of Atmospheric Circulation Response to Greenhouse Gas Increases in a Forcing-Feedback Framework

AbstractWhile there is substantial evidence for tropospheric jet shift and Hadley cell expansion in response to greenhouse gas increases, quantitative assessments of individual mechanisms and feedback for atmospheric circulation changes remain lacking. We present a new forcing-feedback analysis on circulation response to increasing CO2 concentration in an aquaplanet atmospheric model. This forcing-feedback framework explicitly identifies a direct zonal wind response by holding the zonal mean zonal wind exerting on the zonal advection of eddies unchanged, in comparison with the additional feedback induced by the direct response in zonal mean zonal wind. It is shown that the zonal advection feedback accounts for nearly half of the changes to the eddy-driven jet shift and Hadley cell expansion, largely contributing to the subtropical precipitation decline, when the CO2 concentration varies over a range of climates. The direct response in temperature displays the well-known tropospheric warming pattern to CO2 increases, but the feedback exhibits negative signals. The direct response in eddies is characterized by a reduction in upward wave propagation and a poleward shift of midlatitude eddy momentum flux (EMF) convergence, likely due to an increase in static stability from moist thermodynamic adjustment. In contrast, the feedback features a dipole pattern in EMF that further shifts and strengthens midlatitude EMF convergence, resulting from the upper-level zonal wind increase seen in the direct response. Interestingly, the direct response produces an increase in eddy kinetic energy (EKE), but the feedback weakens EKE. Thus, the forcing-feedback framework highlights the distinct effect of zonal mean advecting wind from direct thermodynamic effects in atmospheric response to greenhouse gas increases.

Stratospheric contraction caused by increasing greenhouse gases

Rising emissions of anthropogenic greenhouse gases (GHG) have led to tropospheric warming and stratospheric cooling over recent decades. As a thermodynamic consequence, the troposphere has expanded and the rise of the tropopause, the boundary between the troposphere and stratosphere, has been suggested as one of the most robust fingerprints of anthropogenic climate change. Conversely, at altitudes above ∼55 km (in the mesosphere and thermosphere) observational and modeling evidence indicates a downward shift of the height of pressure levels or decreasing density at fixed altitudes. The layer in between, the stratosphere, has not been studied extensively with respect to changes of its global structure. Here we show that this atmospheric layer has contracted substantially over the last decades, and that the main driver for this are increasing concentrations of GHG. Using data from coupled chemistry-climate models we show that this trend will continue and the mean climatological thickness of the stratosphere will decrease by 1.3 km following representative concentration pathway 6.0 by 2080. We also demonstrate that the stratospheric contraction is not only a response to cooling, as changes in both tropopause and stratopause pressure contribute. Moreover, its short emergence time (less than 15 years) makes it a novel and independent indicator of GHG induced climate change.

Quantifying and attributing time step sensitivities in present-day climate simulations conducted with EAMv1

Abstract. This study assesses the relative importance of time integration error in present-day climate simulations conducted with the atmosphere component of the Energy Exascale Earth System Model version 1 (EAMv1) at 1∘ horizontal resolution. We show that a factor-of-6 reduction of time step size in all major parts of the model leads to significant changes in the long-term mean climate. Examples of changes in 10-year mean zonal averages include the following: up to 0.5 K of warming in the lower troposphere and cooling in the tropical and subtropical upper troposphere, 1 %–10 % decreases in relative humidity throughout the troposphere, and 10 %–20 % decreases in cloud fraction in the upper troposphere and decreases exceeding 20 % in the subtropical lower troposphere. In terms of the 10-year mean geographical distribution, systematic decreases of 20 %–50 % are seen in total cloud cover and cloud radiative effects in the subtropics. These changes imply that the reduction of temporal truncation errors leads to a notable although unsurprising degradation of agreement between the simulated and observed present-day climate; to regain optimal climate fidelity in the absence of those truncation errors, the model would require retuning. A coarse-grained attribution of the time step sensitivities is carried out by shortening time steps used in various components of EAM or by revising the numerical coupling between some processes. Our analysis leads to the finding that the marked decreases in the subtropical low-cloud fraction and total cloud radiative effect are caused not by the step size used for the collectively subcycled turbulence, shallow convection, and stratiform cloud macrophysics and microphysics parameterizations but rather by the step sizes used outside those subcycles. Further analysis suggests that the coupling frequency between the subcycles and the rest of EAM significantly affects the subtropical marine stratocumulus decks, while deep convection has significant impacts on trade cumulus. The step size of the cloud macrophysics and microphysics subcycle itself appears to have a primary impact on cloud fraction in the upper troposphere and also in the midlatitude near-surface layers. Impacts of step sizes used by the dynamical core and the radiation parameterization appear to be relatively small. These results provide useful clues for future studies aiming at understanding and addressing the root causes of sensitivities to time step sizes and process coupling frequencies in EAM. While this study focuses on EAMv1 and the conclusions are likely model-specific, the presented experimentation strategy has general value for weather and climate model development, as the methodology can help researchers identify and understand sources of time integration error in sophisticated multi-component models.

A single column simulation-based decomposition of the tropical upper tropospheric warming

AbstractAn atmospheric global climate model (GCM) and its associated single-column model are used to study the tropical upper tropospheric warming and elucidate how different processes drive this warming. In this modeling framework, on average the direct radiative process accounts for 13% of the total warming. The radiation increases the atmospheric lapse rate and triggers more convection, which further produces 74% of the total warming. The rest 13% is attributable to the circulation adjustment. The relative importance of these processes differs in different regions. In the deep tropics, the radiative-convective adjustment produces the most significant warming and accounts for almost 100% of the total warming. In the subtropics, the radiative-convective adjustment accounts for 73% of the total warming and the circulation adjustment plays a more important role than in the deep tropics, especially at the levels above 200 hPa. When the lateral boundary conditions, i.e. the temperature and water vapor advections, are held fixed in single-column simulations, the tropospheric relative humidity significantly increases in the radiative-convective adjustment in response to the surface warming. This result, in contrast to the relative humidity conservation behavior in the GCM, highlights the importance of circulation adjustment in maintaining the constant relative humidity. The tropical upper tropospheric warming in both the full GCM and the single-column simulations is found to be less strong than the warming predicted by reference moist adiabats. This evidences that the sub-moist-adiabat warming occurs even without the dilution effect of the large-scale circulation adjustment.

The Contribution of Internal Variability to Asian Midlatitude Warming

AbstractThe tropospheric warming in the Northern Hemisphere (NH) midlatitudes has been an important factor in regulating weather and climate since the twentieth century. Apart from anthropogenic forcing leading to the midlatitude warming, this study investigates the possible contribution of internal variability to Asian midlatitude warming and its role in East Asian circulation changes in boreal summer, using four reanalysis datasets in the past century and a set of 1800-yr preindustrial control simulations of the Community Earth System Model version 1 large ensemble (CESM-LE). The surface and tropospheric warming in the Asian midlatitudes is associated with a strong upper-level geopotential height rise north of the Tibetan Plateau (TP). Linear trends of 200-hPa geopotential height (Z200) confirm a dipole of an anomalous high north of the TP and an anomalous low over the Iranian Plateau in 1958–2017. The leading internal circulation mode bears a striking resemblance to the Z200 trend in the past 60 and 111 years, indicating that the long-term trend may be partially of internal origin. The Asian midlatitude warming is also found in preindustrial simulations of CESM-LE, further suggesting that internal variability explains at least part of the temperature change in the Asian midlatitudes, which is in a chain of wave trains along the NH midlatitudes. The Asian warming decreases the meridional gradient of geopotential height, resulting in the weakening of westerly winds over the TP and the TP thermal forcing. Thus, it is essential to consider the role of internal variability in shaping East Asian surface temperature and East Asian summer monsoon changes in the past decades.

Observed Large‐Scale Structures and Diabatic Heating Profiles of Precipitation Over the Tibetan Plateau and South China

AbstractThis study investigates and compares large‐scale dynamic and thermodynamic structures of precipitation over the Tibetan Plateau (TP) and South China (SC). Vertical velocity, temperature and moisture advection, apparent heat source Q1 and apparent moisture sink Q2 are analyzed for strong, moderate, and light/no‐rain based on the measurements. The principal findings are as follows. (1) Both regions showed the strongest upward motion at around 400 hPa for strong and moderate rainfall, but the intensity of upward motion in SC was twice that of the TP. (2) Horizontal temperature advection over the TP showed advective warming in the lower troposphere and advective cooling in the upper troposphere, but this advection over SC was weaker and had a different vertical structure. Moreover, the strong‐rain horizontal temperature advection was the strongest over the TP but the weakest over SC. (3) The moisture advection over the TP reached its maximum at around 400 hPa, showing a one‐peak structure; while that over SC was stronger in the mid‐to‐low troposphere, showing a multi‐peak structure. (4) The Q1 profiles of the three rain categories in the TP were top‐heavy, with two heating peaks in the mid‐to‐upper troposphere. In contrast, the Q1 profiles in SC only had a heating peak in the middle troposphere. Correlations between the vertically integrated <Q1> and <Q2> were significant in SC but weakened in the TP. (5) Diurnal cycles of large‐scale structures in the TP were strong from the late afternoon to early morning, while those in SC were strong in the afternoon.

Natural variability contributes to model–satellite differences in tropical tropospheric warming

A long-standing discrepancy exists between general circulation models (GCMs) and satellite observations: The multimodel mean temperature of the midtroposphere (TMT) in the tropics warms at approximately twice the rate of observations. Using a large ensemble of simulations from a single climate model, we find that tropical TMT trends (1979-2018) vary widely and that a subset of realizations are within the range of satellite observations. Realizations with relatively small tropical TMT trends are accompanied by subdued sea-surface warming in the tropical central and eastern Pacific. Observed changes in sea-surface temperature have a similar pattern, implying that the observed tropical TMT trend has been reduced by multidecadal variability. We also assess the latest generation of GCMs from the Coupled Model Intercomparison Project Phase 6 (CMIP6). CMIP6 simulations with muted warming over the central and eastern Pacific also show reduced tropical tropospheric warming. We find that 13% of the model realizations have tropical TMT trends within the observed trend range. These simulations are from models with both small and large climate sensitivity values, illustrating that the magnitude of tropical tropospheric warming is not solely a function of climate sensitivity. For global averages, one-quarter of model simulations exhibit TMT trends in accord with observations. Our results indicate that even on 40-y timescales, natural climate variability is important to consider when comparing observed and simulated tropospheric warming and is sufficiently large to explain TMT trend differences between models and satellite data.

Dominant Characteristics of Early Autumn Arctic Sea Ice Variability and Its Impact on Winter Eurasian Climate

AbstractThe present study investigated dominant characteristics of autumn Arctic sea ice concentration (SIC) interannual variations and impacts of September–October (SO) mean SIC anomalies in the East Siberian–Chukchi–Beaufort (EsCB) Seas on winter Eurasian climate variability. Results showed that the decreased SO EsCB sea ice is favorable for tropospheric warming and positive geopotential height anomaly over the Arctic region one month later through transporting much more heat flux to the atmosphere from the open water. When entering the early winter (November–January), enhanced upward propagation of quasi-stationary planetary waves in the mid-high latitudes generates anomalous Eliassen–Palm flux convergence in the upper troposphere, which decelerates the westerly winds and maintains the positive geopotential height anomaly in the Arctic region. This anticyclonic anomaly extends southward into central-western Eurasia and leads to evident surface cooling there. Two months later, it further develops downstream accompanied by a deepened trough, making northeastern China experience a colder late winter (January–March). Meanwhile, an anticyclonic anomaly over the eastern North Pacific excites a horizontal eastward wave train and contributes to a positive (negative) geopotential height anomaly around Greenland (Europe), favoring a negative surface temperature anomaly over western Europe. In addition, the stratospheric polar vortex is also significantly weakened in the wintertime, which is attributed to a decreased meridional temperature gradient, and decelerated westerly winds provide a favorable condition for more quasi-stationary planetary waves propagating into the stratosphere. Some major features of atmospheric responses to EsCB sea ice loss are well reproduced in the CAM4 sensitivity experiments.

Projected changes in the characteristics of the East Asian summer monsoonal front and their impacts on the regional precipitation

Summer monsoonal rainfall over East Asia is dominated by precipitation associated with the East Asian summer monsoonal front (EASMF). A Community Atmospheric Model (CAM5.1) with a high horizontal resolution of 50 km is employed in this study to investigate the interannual variability as well as projected future trends in the EASMF under the Representative Concentration Pathway 8.5 scenario. Seasonal march of the EASMF is reproduced reasonably well in the model’s present-day simulation despite a northward shift of the simulated front from its observed position. Based upon a suite of objectively-defined daily indices of the EASMF, we show that the EASMF in the late twenty-first century will be more intense and displaced eastward and southward from its present-day mean location. Moreover, EASMF events will exhibit a wider meridional expansion and a longer duration. Monsoonal precipitation over East Asia is particularly sensitive to the meridional displacements of EASMF. In conjunction with the projected southward shift of EASMF, an enhanced rain band is seen to extend northeastward from southern China to the northwestern Pacific south of Japan. This precipitation feature is associated with strengthened and southward-shifted westerly jet streams at 250 and 700 hPa, which are respectively linked to tropical warming in the upper troposphere and warming over the South China Sea in the lower troposphere during the twenty-first century. Within the latitudinal “gap” south of the upper-level jet and north of the lower-level jet, the local vorticity tendencies are maintained by upper-level divergence and lower-level convergence, thus accompanied by enhanced upward motion and precipitation. The site at which this “jet stream-precipitation” relationship prevails is notably modulated by long-term trends in the temperature and circulation patterns associated with climate change.

Quantifying the Tropical Upper Tropospheric Warming Amplification Using Radio Occultation Measurements

AbstractIn the upper troposphere (UT), Global Navigation Satellite System (GNSS) Radio Occultations (ROs) provide accurate air temperatures (<0.5 K) every ∼200 m vertically. We use RO observations from 01/2002 until 12/2018 at 300, 250, and 200 hPa to quantify the tropical upper tropospheric amplification (defined as the ratio of temperature trends in the UT relative to the surface). We compare the RO‐derived results against Atmospheric Infrared Sounder (AIRS), Coupled Model Intercomparison Project Phase 6 (CMIP6) Atmospheric Model Intercomparison Project (AMIP) models, Modern‐Era Retrospective Analysis for Research and Applications version 2 (MERRA‐2), and European Center for Medium‐range Weather Forecasts Re‐Analysis Interim (ERA‐Interim) data. We find that CMIP6 AMIP models show excellent agreement with independent AIRS and RO observations on the magnitude of the UT warming and show warming that is significantly faster than both reanalyses above 250 hPa. Additionally, AIRS and CMIP6 AMIP present excellent agreement with the RO‐measured tropical tropospheric amplification.

Cold‐Season Arctic Amplification Driven by Arctic Ocean‐Mediated Seasonal Energy Transfer

AbstractThe Arctic warming response to greenhouse gas forcing is substantially greater than the rest of the globe. It has been suggested that this phenomenon, commonly referred to as Arctic amplification, and its peak in boreal fall and winter result primarily from the lapse‐rate feedback, which is associated with the vertical structure of tropospheric warming, rather than from the sea‐ice albedo feedback, which operates mainly in summer. However, future climate model projections show consistently that an overall reduction of sea‐ice in the Arctic region leads to a gradual weakening of Arctic amplification, thereby implying a key role for sea‐ice albedo feedback. To resolve this apparent contradiction, we conduct a comprehensive analysis using atmosphere/ocean reanalysis data sets and a variety of climate model simulations. We show that the Arctic Ocean acts as a heat capacitor, storing anomalous heat resulting from the sea‐ice loss during summer, which then gets released back into the atmosphere during fall and winter. Strong air‐sea heat fluxes in fall/winter in sea‐ice retreat regions in conjunction with a stably stratified lower troposphere lead to a surface‐intensified warming/moistening, augmenting longwave feedback processes to further enhance the warming. The cold‐season surface‐intensified warming/moistening is found to virtually disappear if ocean‐atmosphere‐sea ice interactions are suppressed, demonstrating the importance of ice insulation effect and ocean heat uptake/release. These results strongly suggest that the warm‐season ocean heat recharge and cold‐season heat discharge link and integrate the warm and cold season feedbacks, and thereby effectively explain the predominance of the Arctic amplification in fall and winter.

Slow feedbacks resulting from strongly enhanced atmospheric methane mixing ratios in a chemistry–climate model with mixed-layer ocean

Abstract. In a previous study the quasi-instantaneous chemical impacts (rapid adjustments) of strongly enhanced methane (CH4) mixing ratios have been analysed. However, to quantify the influence of the respective slow climate feedbacks on the chemical composition it is necessary to include the radiation-driven temperature feedback. Therefore, we perform sensitivity simulations with doubled and quintupled present-day (year 2010) CH4 mixing ratios with the chemistry–climate model EMAC (European Centre for Medium-Range Weather Forecasts, Hamburg version – Modular Earth Submodel System (ECHAM/MESSy) Atmospheric Chemistry) and include in a novel set-up a mixed-layer ocean model to account for tropospheric warming. Strong increases in CH4 lead to a reduction in the hydroxyl radical in the troposphere, thereby extending the CH4 lifetime. Slow climate feedbacks counteract this reduction in the hydroxyl radical through increases in tropospheric water vapour and ozone, thereby dampening the extension of CH4 lifetime in comparison with the quasi-instantaneous response. Changes in the stratospheric circulation evolve clearly with the warming of the troposphere. The Brewer–Dobson circulation strengthens, affecting the response of trace gases, such as ozone, water vapour and CH4 in the stratosphere, and also causing stratospheric temperature changes. In the middle and upper stratosphere, the increase in stratospheric water vapour is reduced with respect to the quasi-instantaneous response. We find that this difference cannot be explained by the response of the cold point and the associated water vapour entry values but by a weaker strengthening of the in situ source of water vapour through CH4 oxidation. However, in the lower stratosphere water vapour increases more strongly when tropospheric warming is accounted for, enlarging its overall radiative impact. The response of the stratosphere adjusted temperatures driven by slow climate feedbacks is dominated by these increases in stratospheric water vapour as well as strongly decreased ozone mixing ratios above the tropical tropopause, which result from enhanced tropical upwelling. While rapid radiative adjustments from ozone and stratospheric water vapour make an essential contribution to the effective CH4 radiative forcing, the radiative impact of the respective slow feedbacks is rather moderate. In line with this, the climate sensitivity from CH4 changes in this chemistry–climate model set-up is not significantly different from the climate sensitivity in carbon-dioxide-driven simulations, provided that the CH4 effective radiative forcing includes the rapid adjustments from ozone and stratospheric water vapour changes.

Interdecadal Change of the Winter-Spring Tropospheric Temperature Over Asia and its Impact on the South China Sea Summer Monsoon Onset

The interdecadal change of the winter-spring tropospheric temperature over Asia and its impact on the South China Sea summer monsoon (SCSSM) onset are investigated in this study. The SCSSM onset experiences a significant advance around the mid-1990s, which is attributed to the positive interdecadal anomaly of meridional temperature gradient in the mid-upper troposphere in May over the South China Sea monsoon region. This positive interdecadal anomaly can be traced back to the previous winter and spring, mainly associated with the mid-upper tropospheric warming in the East Asian subtropics. During the interdecadal anomalous seasonal evolution of the East Asian subtropical mid-upper tropospheric temperature, advection of the warm temperature anomaly by climatological mean wind overtakes the effects of anomalous adiabatic cooling and diabatic cooling, leading to a net interdecadal seasonal warming in winter. In spring, the adiabatic heating caused by the interdecadal anomalous subsidence flow cannot offset the effects of interdecadal anomalous cold advection and diabatic cooling, resulting in a net interdecadal seasonal cooling. However, the interdecadal seasonal cooling in spring is not strong enough to offset the interdecadal seasonal warming in winter, preserving an interdecadal mid-upper tropospheric warming over the subtropical East Asia in late spring. This interdecadal warming provides a favorable condition for the interdecadal advance of the SCSSM onset. Both observational analyses and numerical experiments suggest that the interdecadal change of atmospheric thermodynamic processes in winter−spring is related to the interdecadal warming in the tropical western Pacific. The enhanced convection and condensation heating over the regions from the South China Sea to the Philippines, forced by the tropical western Pacific warming, stimulates an anomalous anticyclone in the upper troposphere and warming in the whole troposphere over the subtropical East Asia.

Nonlinear Response of Atmospheric Blocking to Early Winter Barents–Kara Seas Warming: An Idealized Model Study

AbstractWintertime Ural blocking (UB) has been shown to play an important role in cold extremes over Eurasia, and thus it is useful to investigate the impact of warming over the Barents–Kara Seas (BKS) on the behavior of Ural blocking. Here the response of UB to stepwise tropospheric warming over the BKS is examined using a dry dynamic core model. Nonlinear responses are found in the frequency and local persistence of UB. The frequency and local persistence of the UB increase with the strength of BKS warming in a less strong range and decrease with the further increase of BKS warming, which is linked to the UB propagation influenced by upstream background atmospheric circulation. For a weak BKS warming, the UB becomes more persistent due to its less westward movement associated with intensified upstream zonal wind and meridional potential vorticity gradient (PVy) in the North Atlantic mid-high latitudes, which corresponds to a negative height response over the North Atlantic high latitudes. When BKS warming is strong, a positive height response appears in the early winter stratosphere, and its subsequent downward propagation leads to a negative NAO response or increased Greenland blocking events, which reduces zonal wind and PVy in the high latitudes from North Atlantic to Europe, thus enhancing the westward propagation of UB and reducing its local persistence. The transition to the negative NAO phase and the retrogression of UB are not found when numerically suppressing the downward influence of weakened stratospheric polar vortex, suggesting a crucial role of the stratospheric pathway in nonlinear responses of UB to the early winter BKS warming.

日本初のアイウォール貫通型航空機観測により捉えられた2017年台風第21号（ラン）の二重暖気核構造

Upper-tropospheric aircraft reconnaissance was carried out for Typhoon Lan (2017) using a civil jet with a newly developed dropsonde system. This was the first time a Japanese research group observed the inner core of an intense typhoon using dropsondes. This paper describes the warm-core structure in the eye and the associated thermodynamic and kinematic features of the eyewall. During two days of reconnaissance, the typhoon preserved its peak intensity in an environment with strengthening vertical shear. The dropsondes captured a double warm-core structure with a higher perturbation temperature in the middle and upper troposphere, which persisted between the two missions. The two warm cores showed a difference in equivalent potential temperature (θe) of more than 10 K, suggesting different air origins. Saturation-point analysis suggests that the air observed in the upper warm core was entrained from the eyewall. The eyewall updraft in the left-of-shear semicircle had a two-layer structure with a higher θe and lower absolute angular momentum on the inner side of the updraft core. Analyses of the saturation point and parcel method suggest that the warmer air with a θe exceeding 370 K on the inner side of the updrafts originated from the eye boundary layer and was eventually transported into the upper warm core. These results led us to hypothesize that the vertical transport of high-θe air from the eye boundary layer contributed to the continuous eye warming in the upper troposphere against the negative effect of strengthening environmental wind shear on storm intensity. This study demonstrates the significance of eyewallpenetrating upper-tropospheric reconnaissance for monitoring the warm-core structure in the present situation, where accurate measurements of both humidity and temperature for calculating θe can only be made with dropsonde-type expendables.

Seasonality in the Vertical Structure of Long-Term Temperature Trends Over North America

ABSTRACT The surface warming of northern continents during the twentieth century is not uniform across seasons. Surface warming is particularly pronounced over northwestern Canada, where winter trends are much larger than summer ones. The upper-air temperature trends over the region are analyzed in three radiosonde datasets from 1958 to 2012 to assess their seasonal structure. The seasonal variation of upper-air trends can provide insights into the dynamical and thermodynamical processes generating these trends, including warming at the surface. The focus is not on the canonical structure of secular (i.e., long-term) trends—tropospheric warming and stratospheric cooling—but its seasonal variation. We find the boreal winter-minus-summer difference in trends over northwestern Canada to be positive and large in the lower troposphere (p ≳ 500 hPa) and lower stratosphere (50 hPa ≲ p ≲ 150 hPa); it is largest at the surface and smallest at the tropopause. The decreasing seasonality of the tropospheric trend with height supports the attribution of the notable seasonality of surface warming in this region to both land–surface–hydroclimate interactions and changes in winter circulation. In the lower stratosphere, a cooling trend is evident in all seasons, not unexpectedly, but a pronounced seasonality is again apparent, with the strongest cooling in summer. The near-zero trend tropopause region is a rare point of confluence for seasonal trends.

Significant Increase in the Water Vapour Over India and Indian Ocean: Implications for Tropospheric Warming and Regional Climate Forcing

Significant Increase in the Water Vapour Over India and Indian Ocean: Implications for Tropospheric Warming and Regional Climate Forcing