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Climate Dynamics
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ISSN (Print) 1432-0894 - ISSN (Online) 0930-7575
Published by Springer-Verlag Homepage  [2467 journals]
  • Correction to: Evaluation of Alpine-Mediterranean precipitation events in
           convection-permitting regional climate models using a set of tracking
           algorithms

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      PubDate: 2022-12-03
       
  • Subseasonal strength reversal of the East Asian winter monsoon

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      Abstract: Abstract As one of the most significant circulation systems over the Northern Hemisphere in the cold season, the East Asian winter monsoon (EAWM) has been broadly investigated from the seasonal-mean perspective, while subseasonal variations in the EAWM still remain ambiguous. Based on Season-reliant Empirical Orthogonal Function (S-EOF) analysis, this study shows that the subseasonal strength reversal of the EAWM (SR-EAWM), featuring a weaker (or stronger) EAWM in early winter (December) and a stronger (or weaker) EAWM in late winter (January–February), is a distinct leading mode of the month-to-month variation of the EAWM. The weak-to-strong SR-EAWM is characterized by an anomalous low over Eurasia and a weakened East Asian major trough (EAT) in early winter, with an intensified Siberian High and a deepened EAT in late winter. The SR-EAWM is preceded by surface air temperature anomalies over Davis Strait (DST) and those over central-eastern North America (CENAT) in September–October. The DST mainly influences the SR-EAWM in early winter through a “sea ice bridge” of the November Baffin Bay sea ice concentration anomaly (BBSIC). The BBSIC could intensify the DST in December by altering surface heat flux, thus exciting a downstream atmospheric response and modulating the strength of the EAT in early winter. The CENAT affects the SR-EAWM in late winter by inducing an “ocean bridge” of the western North Atlantic sea surface temperature anomaly (WNASST). The WNASST can persist into late winter and then significantly affects the SR-EAWM by regulating Eurasian circulation anomalies and the downstream EAT. The bridge roles of the BBSIC and WNASST can be further verified by a linear baroclinic model. Finally, two physical-empirical models are established using the DST/BBSIC and the CENAT indices, respectively. Both exhibit promising prediction skills. The results highlight that the DST, BBSIC, and CENAT are crucial predictability sources for the SR-EAWM.
      PubDate: 2022-12-02
       
  • Understanding extremely pluvial winters over Yangtze–Huia river basin in
           China: their complexity and tropical oceans influences

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      Abstract: Abstract In 2018/19 winter, the Yangtze–Huai River Basin (YHRB, one of the most developed areas in China) experienced an extremely pluvial condition with excessive and long-duration precipitation. Other three extremely pluvial winters during 1979/80–2018/19: 1997/98, 2002/03 and 1989/90 winters were also found. In 2018/19, 2002/03 and 1997/98 winters, similar low-level circulation pattern of anomalous southwesterlies (southeasterlies) over the southern (northern) YHRB was found. However, we found different tropical ocean states for the events. In 2018/19 winter, a combination of a moderate El Niño in the tropical Pacific and warm sea surface temperature (SST) anomalies in the tropical western Indian Ocean (IO) and Atlantic was seen. In 1997/98 winter, we found an extreme eastern Pacific El Niño with warm SST anomalies in the western tropical IO. In 2002/03 winter, a central Pacific El Niño occurred. In contrast, no significant warm SST anomalies were seen in 1989/90 winter. Based on coupled model numerical experiments, the tropical SST anomalies contributed almost half of the YHRB precipitation anomalies in 2018/19 and 1997/98 winters. In particular, the tropical IO warm SST anomalies induced anomalous anticyclones over the South China Sea and northeastern China, with moisture convergence over the YHRB. In 2002/03, the tropical Pacific SST anomalies contributed almost half of the YHRB precipitation anomalies, although its contribution could be reduced due to the inter-basin interactions. For 1989/90 winter, we discussed possible impacts of mid-high latitude wave trains. The results indicate the complexity and diverse potential predictability among the four extreme pluvial winters.kindly check and conform the processed affliations was correct [1,2]Yes, we have checked that. The affliations are correct.
      PubDate: 2022-12-01
       
  • Impact of global warming on summertime submonthly wave patterns and
           tropical cyclone activity in the western North Pacific

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      Abstract: Abstract This study analyzes the re-analysis and model simulations to determine changes in circulation features for submonthly (6 − 20 days) wave patterns and tropical cyclones (TCs) in the western North Pacific (WNP) under global warming. The High Resolution Atmospheric Model (HiRAM) that can produce tropical cyclones is adopted to perform present (1979 − 2008) and future (2075 − 2100) simulations under the Representative Concentration Pathway (RCP) 8.5 scenarios. In the present simulation, the wave pattern is effectively simulated in the WNP. However, in the future simulation, the monsoon trough is weakened and fewer cases are detected. The intraseasonal oscillation (ISO) mostly exhibits a westward propagation tendency in the present simulation but northward movement tendency in the future simulation. Although the background seasonal fields are weakened in the future, the growth rate of the submonthly wave pattern in the westerly phase of the ISO is enhanced in the future simulation. A kinetic energy analysis reveals that the barotropic conversion will supply more kinetic energy from the background flow to the wave pattern in the future westerly phase. This enhancement from barotropic conversion is attributable to the northward displacement of the monsoon trough caused by the northward propagating ISO; this northward monsoon trough displacement and the northwest − southeast-oriented submonthly perturbations and TCs are more strongly coupled. Therefore, the submonthly cases in the westerly phase of the future simulation are unique because of the higher efficiency of kinetic energy transfer from the background flow to submonthly perturbations which provides a more favorable environment for stronger cases in the future westerly phase.
      PubDate: 2022-12-01
       
  • On deep learning-based bias correction and downscaling of multiple climate
           models simulations

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      Abstract: Abstract Bias correcting and downscaling climate model simulations requires reconstructing spatial and intervariable dependences of the observations. However, the existing univariate bias correction methods often fail to account for such dependences. While the multivariate bias correction methods have been developed to address this issue, they do not consistently outperform the univariate methods due to various assumptions. In this study, using 20 state-of-the-art coupled general circulation models (GCMs) daily mean, maximum and minimum temperature (Tmean, Tmax and Tmin) from the Coupled Model Intercomparison Project phase 6 (CMIP6), we comprehensively evaluated the Super Resolution Deep Residual Network (SRDRN) deep learning model for climate downscaling and bias correction. The SRDRN model sequentially stacked 20 GCMs with single or multiple input-output channels, so that the biases can be efficiently removed based on the relative relations among different GCMs against observations, and the intervariable dependences can be retained for multivariate bias correction. It corrected biases in spatial dependences by deeply extracting spatial features and making adjustments for daily simulations according to observations. For univariate SRDRN, it considerably reduced larger biases of Tmean in space, time, as well as extremes compared to the quantile delta mapping (QDM) approach. For multivariate SRDRN, it performed better than the dynamic Optimal Transport Correction (dOTC) method and reduced greater biases of Tmax and Tmin but also reproduced intervariable dependences of the observations, where QDM and dOTC showed unrealistic artifacts (Tmax < Tmin). Additional studies on the deep learning-based approach may bring climate model bias correction and downscaling to the next level.
      PubDate: 2022-12-01
       
  • Southern Ocean sea ice concentration budgets of five ocean-sea ice
           reanalyses

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      Abstract: Abstract In this study, sea ice concentration (SIC) budgets were calculated for five ocean-sea ice reanalyses (CFSR, C-GLORSv7, GLORYS12v1, NEMO-EnKF and ORAS5), in the Southern Ocean and compared with observations. Benefiting from the assimilation of SIC, the reanalysis products display a realistic representation of sea ice extent as well as sea ice area. However, when applying the SIC budget diagnostics to decompose the changes in SIC into contributions from advection, divergence, thermodynamics, deformation and data assimilation, we find that both atmospheric and oceanic forcings and model configurations are significant contributors on the budget differences. For the CFSR, the primary source of deviation compared to other reanalyses is the stronger northward component of ice velocity, which results in stronger sea ice advection and divergence. Anomalous surface currents in the CFSR are proposed to be the main cause of the ice velocity anomaly. Furthermore, twice the mean ice thickness in the CFSR compared to other reanalyses makes it more susceptible to wind and oceanic stresses under Coriolis forces, exacerbating the northward drift of sea ice. The C-GLORSv7, GLORYS12v1 and NEMO-EnKF have some underestimation of the contribution of advection and divergence to changes in SIC in autumn, winter and spring compared to observations, but are more reasonable in summer. ORAS5, although using the same coupled model and atmospheric forcing as C-GLORSv7 and GLORYS12v1, has a more significant underestimation of advection and divergence to changes in SIC compared to these two reanalyses. The results of the SIC budgets of five ocean-sea ice reanalyses in the Southern Ocean suggest that future reanalyses should focus on improving the modelling of sea ice velocities, for example through assimilation of sea ice drift observations.
      PubDate: 2022-12-01
       
  • Uncertainty in the projected changes of Sahel summer rainfall under global
           warming in CMIP5 and CMIP6 multi-model ensembles

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      Abstract: Abstract The Sahel summer rainfall is of great significance to the local social, economic, and cultural environment. In the context of a long Sahel megadrought in the last thirty years of the twentieth century, the future change of Sahel summer rainfall under global warming has aroused wide attention. Based on the historical simulations and high emission scenario experiments from 20 Coupled Model Intercomparison Project phase-5 (CMIP5) models and 22 CMIP6 models, this study investigates the future projections of Sahel summer rainfall under global warming. The results show that the multi-model ensemble (MME) mean projects a slight increase (1–2%/℃) of summer rainfall over the Sahel in the future which seems to be due to the thermodynamic changes and opposed by the dynamic changes, but that the inter-model spread is due to the latter. We find that, in particular, the inter-model spreads in the extratropical northern and tropical Atlantic sea surface temperature (SST) changes are two important sources of the uncertainty in the Sahel summer rainfall projections via two different atmospheric teleconnection processes. On the one hand, a warmer northern Atlantic SST would induce an anomalous large-scale cyclone over North Africa and Europe, and the southern branch would strengthen the western African monsoonal circulation, leading to a wetter Sahel. On the other hand, a warmer tropical Atlantic SST would weaken the regional circulation, resulting in a drier Sahel. Our results suggest that an improved projection of the future Atlantic warming, especially the differential warming between the northern and tropical Atlantic, is a priority for the reliable future projection of Sahel summer rainfall.
      PubDate: 2022-12-01
       
  • The synergistic effect of the preceding winter Northern Hemisphere annular
           mode and spring tropical North Atlantic SST on spring extreme cold events
           in the mid-high latitudes of East Asia

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      Abstract: Abstract In this paper, the synergistic effect of the preceding winter positive Northern Hemisphere annular mode (pNAM) and spring negative tropical North Atlantic (nTNA) sea surface temperature anomaly (SSTA) on spring extreme cold events in the mid-high latitudes of East Asia (MHEA) is investigated. The results show that the co-occurrence of the two factors is unfavorable for extreme cold events during spring in the MHEA via the snow cover and atmospheric bridges. Over the Atlantic, the spring nTNA SSTA can lead to an atmospheric response that is similar to the North Atlantic Oscillation, which enhances the persistence of the pNAM and in turn amplifies the negative spring Eurasian snow cover extent (EASCE) anomaly caused by the preceding winter pNAM. Meanwhile, the spring EASCE is closely related to the spring MHEA anomalous anticyclone. In addition to storing its signal in the spring EASCE, the spring nTNA SSTA can also lead to the spring MHEA anomalous anticyclone via the eastward Rossby wave train. The evidence shows that the Rossby wave energy can propagate eastward to the MHEA as a result of the enhanced negative spring EASCE anomaly and Rossby wave induced by the spring nTNA SSTA, and the two factors have an obvious synergistic effect on the spring MHEA anomalous anticyclone. This anomalous MHEA anticyclone becomes a barrier that can hinder the intrusion of cold air from the polar region and can increase the thickness of the atmospheric layer. The anomalous sinking motion of the spring MHEA anomalous anticyclone can also lead to an increase in net radiation received at the surface and increase the air temperature through the vertical motion of air. The southerly wind over the west side of the spring MHEA anomalous anticyclone leads to horizontal warm advection. All of the above processes favor an increase in air temperature and dampen extreme cold events, implying the synergistic effect of the preceding winter pNAM and spring nTNA SSTA on spring extreme cold events in the MHEA.
      PubDate: 2022-12-01
       
  • Assessing the large-scale drivers of precipitation in the northeastern
           United States via linear orthogonal decomposition

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      Abstract: Abstract This study examines the linear orthogonal modes associated with monthly precipitation in the northeastern United States, from CESM1 LENS (35 ensemble members, 1979–2005) and two reanalysis datasets (ERA5, 1950–2018 and NOAA-CIRES-DOE 20CRv3, 1950–2015). Calendar months are aggregated together, and any linear trends in data are removed. Using region-averaged precipitation anomaly time series and monthly anomalies for several global 2D atmospheric fields, a linear orthogonal decomposition method is implemented to iteratively extract time series (based on field and geographic location) of absolute maximum correlation. Linear modes associated with this method are then projected onto the full set of 2D fields to provide physical insight into the mechanisms involved in generating precipitation. In this region, the first mode is associated with vapor transport from the Atlantic seaboard, the second mode is characterized by westward vapor transport associated with extratropical cyclones, and the third mode captures vapor transport from the Gulf of Mexico during the fall and winter. However, the third mode is less robust in the spring and summer. Results are generally consistent across the datasets, and applying multiple linear regression with the linear modes to predict the precipitation anomalies produces R-squared values of around 0.54–0.65 for CESM1 LENS, and around 0.58–0.88 for reanalysis, with the lowest values generally in the spring and late summer. The influence of low-frequency climate variability on the modes is considered for CESM1 LENS, and the modes in late winter can be predicted with some success via a combination of several, prominent large-scale teleconnection patterns.
      PubDate: 2022-12-01
       
  • The effects of bias, drift, and trends in calculating anomalies for
           evaluating skill of seasonal-to-decadal initialized climate predictions

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      Abstract: Abstract In initialized seasonal to decadal (S2D) predictions, model hindcasts rapidly drift away from the initial observed state and converge toward a preferred state characterized by systematic error, or bias. Bias and drift are among the greatest challenges facing initialized prediction today. Differences in trends between initial states and drifted states, combined with bias and drift, introduce complexities in calculating anomalies to assess skill of initialized predictions. We examine several methods of calculating anomalies using the Decadal Prediction Large Ensemble (DPLE) using the Community Earth System Model (CESM) initialized hindcasts and focus on Pacific and Atlantic SSTs to illustrate issues with anomaly calculations. Three methods of computing anomalies, one as differences from a long term model climatology, another as bias-adjusted differences from the previous 15 year average from observations, and a third as differences from the previous 15 year average from the model, are contrasted and each is shown to have limitations. For the first, trends in bias and drift introduce higher skill estimates earlier and later in the hindcast period due to the trends that contribute to skill. For the second, higher skill can be introduced in situations where low frequency variability in the observations is large compared to the hindcasts on timescales greater than 15 years, while lower skill can result if the predicted signal is small and the bias-correction itself produces a transition of SST anomalies to the opposite sign of those that are observed. The third method has somewhat lower skill compared to each of the others, but has less difficulties with not only the long term trends in the model climatology, but also with the unrealistic situational skill from using observations as a reference. However, the first 15 years of the hindcast period cannot be evaluated due to having to wait to accumulate the previous 15 year model climatology before the method can be applied. The IPO transition in the 2014–2016 time frame from negative to positive (predicted by Meehl et al. in in Nat Commun, 10.1038/NCOMMS11718, 2016) did indeed verify using all three methods, though each provides somewhat different skill values as a result of the respective limitations. There is no clear best method, as all are roughly comparable, and each has its own set of limitations and caveats. However, all three methods show generally higher overall skill in the AMO region compared to the IPO region.
      PubDate: 2022-12-01
       
  • Distinct influences of cold vortex over Northeast China on local
           precipitation in early summer and midsummer

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      Abstract: Abstract The station observations and reanalysis dataset are utilized to identify the cold vertex over Northeast China (NECV) in early summer (ES) and midsummer (MS) respectively. In this study, we focus on the characteristics of NECV and their distinct influences on local precipitations in ES and MS. The underpinning mechanisms are further inspected in terms of thermodynamic and dynamic processes. Results suggest that in ES (MS) the NECV is mainly located over Northern China-Southeastern Russia (Mongolia) and significantly correlated to the precipitations over the eastern (mid-western) regions of Northeast China. In the strong cases of precipitation, NECV displays northward shift and intensification in ES and MS respectively. Meanwhile, the upper-level wind anomalies suggest a northward displacement of polar front jet and a weakened subtropical jet in ES, and an enhanced polar front jet and a southward shift of subtropical jet in MS. The wind anomalies induced by meridional temperature gradients and the Rossby wave activities transported to Northeast China favor the development of atmospheric circulation vorticity and then promote the variations of NECV. Furthermore, the vorticity and temperature advections are favorable for the enhancement of ascending motion under quasigeostrophic approximation, which is combined with the sufficient water vapor transported from oceanic regions, triggering the regional precipitation. Intriguingly, the pathways of water vapor transport and disturbance energy propagation caused by the diverse external forcings are different in ES and MS, which sheds some fresh light on the insight into the subseasonal variations of NECV and the distinctive contributions to local precipitation.
      PubDate: 2022-12-01
       
  • Further probing the mechanisms driving projected decreases of extreme
           precipitation intensity over the subtropical Atlantic

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      Abstract: Abstract Regional projections of extreme precipitation intensity (EPI) are strongly influenced by changes of “extreme ascent”, i.e. ascending air during periods of extreme precipitation. Earlier studies have suggested that long-term changes in eddy length scale and vertical stability are key factors influencing extreme ascent projections, but these mechanisms have yet to be confirmed with controlled model experiments. In this study, we perform such controlled experiments using a cloud-resolving model (CRM). The selected CRM domains are three locations over the subtropical Atlantic Ocean where global climate models consistently project weakening of extreme ascent with accordingly decreased EPI. At each study location, four to ten pairs of 20-year-maximum precipitation events are simulated with the CRM, with each pair consisting of an event during the historical period (1981–2000) and an event during the future period (2081–2100). Large-scale forcings for these events are derived from members of an initial condition ensemble of the Canadian Earth System Model version 2 (CanESM2). These experiments reveal that, in all three study locations, weakening of differential cyclonic vorticity advection (dCVA) is a key driver of projected decreases in extreme ascent and EPI. Possible mechanisms responsible for weakening dCVA are discussed. Although there is evidence that EPI in the CRM has different sensitivity to large-scale forcings than CanESM2, the role of dCVA changes may nonetheless be important to consider for EPI changes in the real world.
      PubDate: 2022-12-01
       
  • July–September rainfall in the Greater Horn of Africa: the combined
           influence of the Mascarene and South Atlantic highs

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      Abstract: Abstract July-September rainfall is a key component of Ethiopia’s annual rainfall and is a source of rainfall variability throughout inland Greater Horn of Africa. In this study we investigate the relative influences of the Mascarene (MH) and South Atlantic (AH) highs on July-September rainfall in a covarying region of the Greater Horn of Africa using CHIRPS observed rainfall and the ERA5 reanalysis. We show that a mixed metric using the circulation at 850 hPa of these two subtropical anticyclones (AH-MH), is better correlated with rainfall than individual high circulations. Variations in remote circulation are translated by changes in Central African westerlies and Turkana Jet wind speeds. We apply the AH-MH mixed metric to the CMIP5 and CMIP6 ensembles and show that it is a good indicator of mean July-September rainfall across both ensembles. Biases in circulation are shown to be related to the Hadley circulation in CMIP5 atmosphere-only simulations, while causes of biases in CMIP6 are more varied. Coupled model biases are related to southern ocean warm biases in CMIP5 and western Indian Ocean warm biases in CMIP6. CMIP6 shows an improved relationship between rainfall and Turkana Jet winds and Central African westerlies across the ensemble.
      PubDate: 2022-12-01
       
  • Features of climatological intraseasonal oscillation during Asian summer
           monsoon onset and their simulations in CMIP6 models

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      Abstract: Abstract Climatological intraseasonal oscillation (CISO), as phase-locking of transient intraseasonal oscillation (ISO) to the annual cycle, is an important seasonal evolution of the Asian summer monsoon (ASM). We identified six CISO centers related to the ASM onset: the wet CISO begins from the central-to-eastern equatorial Indian Ocean (CIO) around pentad 25, which propagates eastward to the western Pacific (WP) around pentad 26, northward to the Bay of Bengal (BoB) around pentad 28, northeastward to the South China Sea (SCS) around pentad 29, and then northward to East Asia (EA) around pentad 34, while the wet CISO over the Indian summer monsoon (ISM) region peaking around pentad 32 originates from the northwestern Indian Ocean. The CISO over the CIO at pentad 25 is related to the phase-locking of Madden-Julian Oscillation (MJO), caused by the seasonal jump of the Intertropical Convergence Zone (ITCZ) from the Southern Hemisphere to the Northern Hemisphere over the Indian Ocean, signaling the beginning of the ASM CISO. The multi-model ensemble mean (MME) of the Coupled Model Intercomparison Project Phase 6 (CMIP6) models can well reproduce these strong CISOs over the ISM, BoB, SCS, and EA regions, although the simulated intensity is weaker and the peak wet phase is 2–3 pentads later than that in the observation. The very weak CISOs over the CIO and WP in the simulations challenge our subseasonal prediction of the ASM since its intraseasonal variability is usually forced by the tropical ISO. These CISOs would be significantly enhanced over the BoB and EA under the future global warming in a high emission scenario of the CMIP6 models, while the CISOs over the CIO and WP would be still absent.
      PubDate: 2022-12-01
       
  • Moisture sources of heavy precipitation in Central Europe in synoptic
           situations with Vb-cyclones

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      Abstract: Abstract During the past century, several extreme summer floods in Central Europe were associated with so-called Vb-cyclones propagating from the Mediterranean Sea north-eastward to Central Europe. The processes intensifying the precipitation in synoptic situations with Vb-cyclones in the Danube, Elbe, and Odra catchments are only partially understood. Our study aims to investigate these processes with Lagrangian moisture-source diagnostics for 16 selected Vb-events. Moreover, we analyse the characteristics of typical moisture source regions during 1107 Vb-events from 1901 to 2010 based on ERA-20C reanalysis dynamically downscaled with COSMO-CLM+NEMO. We observe moisture contributions by various source regions highlighting the complex dynamical interplay of different air masses leading to moisture convergence in synoptic situations with Vb-cyclones. Overall, up to 80% of the precipitation originates from the European continent, indicating the importance of continental moisture recycling, especially within the respective river catchment. Other major moisture uptake regions are the North Sea, the Baltic Sea, the North Atlantic, and for a few events the Black Sea. Remarkably, anomalies in these oceanic source regions show no connection to precipitation amounts in synoptic situations with Vb-cyclones. In contrast, the Vb-cyclones with the highest precipitation are associated with anomalously high evaporation in the Mediterranean Sea, even though the Mediterranean Sea is only a minor moisture source region on average. Interestingly, the evaporation anomalies are not connected with sea-surface temperature but with wind-speed anomalies (Spearman’s rank correlation coefficient \(R\approx\)  0.7, significant with \(p<0.01\) ) indicating mainly dynamically driven evaporation. The particular role of the Mediterranean Sea hints towards possible importance of Mediterranean moisture for the early-stage intensification of Vb-cyclones and the pre-moistening of the continental uptake regions upstream of the target catchments.
      PubDate: 2022-12-01
       
  • Modulation of the Tibetan Plateau snow cover on the interannual variations
           of the MJO-Related winter surface air temperature anomalies over East Asia
           

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      Abstract: Abstract Previous studies have revealed that the Madden–Julian oscillation (MJO) can exert a profound impact on the surface air temperature (SAT) anomalies over East Asia during winter (December–February). In this study, it is found that such MJO-related winter SAT anomalies can be modulated by the interannual variability of the Tibetan Plateau snow cover (TPSC). During the excessive TPSC (ETPSC) winters, the MJO-related East Asian SAT anomalies in phase 3 are significantly colder than normal. However, during the reduced TPSC (RTPSC) winters, such SAT anomalies are close to normal. A linear baroclinic model is used to examine the possible physical mechanisms. During the ETPSC winters, the more energetic MJO can excite stronger poleward Rossby waves and intensify the upper level cyclonic anomalies over East Asia and lead to the significantly colder SAT anomalies. While during the RTPSC winters, the suppressed MJO convection excites weaker poleward Rossby waves and cannot make colder SAT anomalies over East Asia. The numerical evidences also show that the variation of the mean state could affect the teleconnections but it does not benefit a stronger Rossby wave train over East Asia in ETPSC winters. These results confirm that the TPSC can exert a profound modulation effect on the MJO teleconnection and further impact on the winter SAT anomalies over East Asia.
      PubDate: 2022-12-01
       
  • The impact of air–sea coupling on the simulation of the hydroclimatic
           change over Peninsular Florida

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      Abstract: Abstract This study analyzes from a pair of downscaled climate projections over Peninsular Florida (PF) at 10 km grid spacing. One of the downscaled projections corresponds to atmospheric downscaling only with a regional atmospheric model (called the Regional Spectral Model [RSM]). The other projection is related to the coupled ocean–atmosphere Regional Spectral Model-Regional Ocean Model (RSM-ROMS), which downscales both the atmospheric and the oceanic components of the global model simultaneously. The RSM-ROMS shows a better verification of the current climate than the corresponding RSM simulation for some atmospheric variables (precipitation and precipitable water) both over PF and the surrounding oceans. The moisture budget differences between the RSM-ROMS and the RSM simulations for both the current and the future climate show that the differences are larger over the surrounding oceans than over PF. However, RSM-ROMS shows a stronger projected drying over PF than RSM in the mid-twenty-first century. The RSM-ROMS displays a smaller deficit of freshwater over the oceans than RSM because of differences between the simulations in the advection of moisture, divergence of moisture, and moisture flux divergence from transient eddies. The differences in the moisture budget between the simulations over PF are small because of compensatory differences between the divergence of moisture from changes in divergent circulation and the divergence of fluxes from the transient eddies. Our analysis indicates that the air–sea coupling in RSM-ROMS affects the mean gradient of the moisture, the mean divergence, and the transients, which then modulate the advection of moisture, the divergence of moisture, and the convergence of moisture flux, respectively, setting it apart from the RSM simulation.
      PubDate: 2022-12-01
       
  • Diurnal variability of lower and middle atmospheric water vapour over the
           Asian summer monsoon region: first results from COSMIC-1 and TIMED-SABER
           measurements

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      Abstract: Abstract First observations on the vertical structure of diurnal variability of water vapour in the lower and middle atmosphere using 13 years of COSMIC-1 and 18 years of SABER observations over the Asian summer monsoon region are presented in this paper. The most significant and new observation is that the middle stratospheric water vapour (SWV) enhancement is observed between 9 and 18 LT, whereas it is between 6 and 15 LT near tropopause in all the seasons. The diurnal amplitude of water vapour near tropopause is between 0.3 and 0.4 ppmv. Bimodal peaks are found in the diurnal amplitude of SWV, maximizing between 25 and 30 km (~ 0.4 ppmv), and between 45 and 50 km (~ 0.6 ppmv). The analysis reveals that the diurnal variability in the lower SWV is controlled by the tropical tropopause temperature, whereas the middle and upper SWV is primarily controlled by methane oxidation. The results are presented and discussed in the light of present understanding.
      PubDate: 2022-12-01
       
  • Linkage between cross-equatorial potential vorticity flux and surface air
           

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      Abstract: Abstract The source of potential vorticity (PV) for the global domain is located at the Earth’s surface. PV in one hemisphere can exchange with the other through cross-equatorial PV flux (CEPVF). This study investigates the features of the climatic mean CEPVF, the connection in interannual CEPVF with the surface thermal characteristics, and the associated mechanism. Results indicate that the process of positive (negative) PV carried by a northerly (southerly) wind leads to the climatologically overwhelming negative CEPVF over almost the entire equatorial cross-section, while the change of the zonal circulation over the equator is predominately responsible for CEPVF variation. By introducing the concept of “PV circulation” (PVC), it is demonstrated that the interannual CEPVF over the equator is closely linked to the notable uniform anomalies of spring cold surface air temperature (SAT) over the mid–high latitudes of Eurasia by virtue of the PVC, the PV-θ mechanism, and the surface positive feedback. Further analysis reveals that equatorial sea surface temperature (SST) forcing, such as the El Niño–Southern Oscillation and tropical South Atlantic uniform SST, can directly drive anomalous CEPVF by changing the zonal circulation over the equator, thereby influencing SAT in the Northern Hemisphere. All results indicate that the equilibrium linkage between CEPVF and extratropical SAT is mainly a manifestation of the response of extratropical SAT to tropical forcing by virtue of PVC, and that the perspective of PVC can provide a reasonably direct and simple connection of the circulation and climate between the tropics and the mid–high latitudes.
      PubDate: 2022-12-01
       
  • ENSO and PDO-related interannual and interdecadal variations in the
           wintertime sea surface temperature in a typical subtropical strait

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      Abstract: Abstract The Taiwan Strait has the largest temporal variability in the wintertime sea surface temperature (SST) along the China coast. The warming and cooling trends reach about ± 1 °C per decade in winter during 1982–1999 and 1999–2014, respectively, which are about 4 times larger than neighboring coastal area and open ocean. Previous studies have noted these opposite trends, but the cause remains unclear due to insufficient study on teleconnections of this local signals to large-scale climate signals (e.g., Pacific Decadal Oscillation, PDO; El Niño–Southern Oscillation, ENSO). Using different period filters, wintertime SST anomaly in the Taiwan Strait (TS-SSTa) of different timescales were separated and connected to the large-scale climate signals. Besides the impact of global warming, we also found that the interdecadal signal of PDO contribute significantly to the warming and cooling trends of the wintertime TS-SSTa during 1982–2014 (mostly the positive PDO phase). During the positive PDO phase, a sea level pressure (SLP) dipole develops at the North Pacific, leading to a northeasterly wind jet along the Eurasian eastern coast, and affecting the interdecadal wintertime TS-SSTa through the East Asia Winter Monsoon (EAWM) and North Pacific Subtropical Gyre. During the negative PDO phase, the influence from North Pacific weakens, and the tropical Pacific has a greater influence on the interannual wintertime TS-SSTa through the northward movement of the ENSO-related cyclone/anticyclone anomaly over East Asia. Thus, the influence from North Pacific (PDO) and tropical Pacific (ENSO) alternately control the TS-SSTa variation during the positive and negative PDO phases.
      PubDate: 2022-12-01
       
 
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