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Climate Dynamics
Journal Prestige (SJR): 2.445
Citation Impact (citeScore): 4
Number of Followers: 46  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1432-0894 - ISSN (Online) 0930-7575
Published by Springer-Verlag Homepage  [2468 journals]
  • The interannual impact of the North Atlantic sea surface temperature on
           the surface air temperature over southern China in February

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      Abstract: Prolonged low temperature significantly threatens transportation and planting in populous areas and is closely linked to slowly varying forcing such as sea surface temperature (SST). In this study, we reveal the impact of the north–south dipole pattern in extra-tropical North Atlantic SST (DNA) anomaly on surface air temperature (SAT) over southern China in February. The positive phase of the DNA is favorable for a positive North Atlantic Oscillation-like pattern at the troposphere via synoptic eddy vorticity forcing and triggers a southeastward-propagating Rossby wave train associated with the deepened East Asian trough and the strengthened Siberian high, which are conducive to enhancement of the East Asian winter monsoon. A stronger monsoon leads to lower-than-normal SAT over southern China by intensifying southward cold air outbreaks. Considerably, the extremely low temperature over southern China in February 2022 is explainable by this physical mechanism. Furthermore, experiments with a linear baroclinic model verify that the proposed downstream propagating Rossby wave train is mainly associated with the midlatitude North Atlantic SST anomaly. These results emphasize that the extra-tropical North Atlantic SST anomaly should be focused when interpreting and predicting the late winter SAT over southern China.
      PubDate: 2023-05-26
       
  • Impacts of land-surface heterogeneities and Amazonian deforestation on the
           wet season onset in southern Amazon

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      Abstract: Continued Amazonian deforestation perturbs the surface turbulent fluxes which are important for building the conditions for the wet season onset in the southern Amazon. This work evaluates the impacts of tropical deforestation on the onset and development of the Amazonian rainy season using a weather typing approach. We use 19-year simulations (2001–2019) with the Regional Earth System Model from the Institute Pierre Simone Laplace (RegIPSL) with twin control/deforestation experiments. RegIPSL represents the dominant modes or the atmospheric circulation patterns (CPs) of the daily-to-decadal circulation variability in tropical South America, and the evolution of atmospheric and surface conditions along the dry-to-wet transition period. According to RegIPSL, forests and crops contribute differently to the onset. During the dry-to-wet transition period, croplands/grasslands present a stronger shallow convection driven by a higher atmospheric temperature. Large-scale subsidence suppresses low-level convection in the region and deep convection only persists over forests where the atmosphere presents more convective potential energy. After the onset and the establishment of large-scale rainfall structures, both land covers behave similarly in terms of surface fluxes. Deforestation decreases the frequency of the CP typically linked to the onset. Changes in the spatial structure and frequency of the wet season CPs reinforce the hypothesis of a deforestation-induced dry season lengthening. Variations in the CP frequency and characteristic rainfall have opposite effects on accumulated rainfall during the dry-to-wet transition period. Whereas alterations in frequency are associated with a regional circulation response, changes in the CP characteristic rainfall correspond to a local response to deforestation.
      PubDate: 2023-05-26
       
  • Observed and projected changes in snow accumulation and snowline in
           California’s snowy mountains

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      Abstract: The Sierra Nevada and Southern Cascades—California’s snowy mountains—are primary freshwater sources and natural reservoirs for the states of California and Nevada. These mountains receive precipitation overwhelmingly from wintertime storms including atmospheric rivers (ARs), much of it falling as snow at the higher elevations. Using a seven-decade record of daily observed temperature and precipitation as well as a snow reanalysis and downscaled climate projections, we documented historical and future changes in snow accumulation and snowlines. In four key subregions of California’s snowy mountains, we quantified the progressing contribution of ARs and non-AR storms to the evolving and projected snow accumulation and snowlines (elevation of the snow-to-rain transition), exploring their climatology, variability and trends. Historically, snow makes up roughly a third of the precipitation affecting California’s mountains. While ARs make up only a quarter of all precipitating days and, due to their relative warmth, produce snowlines higher than do other storms, they contribute over 40% of the total seasonal snow. Under projected unabated warming, snow accumulation would decline to less than half of historical by the late twenty-first century, with the greatest snow loss at mid elevations (from 1500 to 3300 m by the mountain sub-regions) during fall and spring. Central and Southern Sierra Nevada peaks above 3400 m might see occasionally extreme snow accumulations in January–February resulting entirely from wetter ARs. AR-related snowlines are projected to increase by more than 700 m, compared to about 500 m for other storms. We discuss likely impacts of the changing climate for water resources as well as for winter recreation.
      PubDate: 2023-05-25
       
  • Moisture sources for precipitation variability over the Arabian Peninsula

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      Abstract: We apply the Lagrangian-based moisture back trajectory method to two reanalysis datasets to determine the moisture sources for wet season precipitation over the Arabian Peninsula, defined as land on the Asian continent to the south of the Turkish border and west of Iran. To accomplish this, we make use of the evaporative source region between 65°W–120°E and 30°S–60°N, which is divided into twelve sub-regions. Our comparison of reanalyses and multiple observations allows us to validate datasets and highlight broad-scale similarities in characteristics, notwithstanding some inconsistencies in the southwest AP. The results indicate north-to-south spatiotemporal heterogeneity in the characteristics of dominant moisture sources. In the north, moisture for precipitation is mainly sourced from midlatitude land and water bodies, such as the Mediterranean and Caspian Seas. Areas further south are dependent on moisture transport from the Western Indian Ocean and parts of the African continent. The El Niño-Southern Oscillation (ENSO) exhibits an overall positive but sub-seasonally varying influence on the precipitation variability over the region, with noticeable moisture anomalies from all major source regions. A significant drying trend exists over parts of the Peninsula, which both reanalyses partially attribute to anomalies in the moisture advection from the Congo Basin and South Atlantic Ocean. However, considerable uncertainty in evaporation trends over the terrestrial evaporative sources in observations warrants additional modeling studies to further our understanding of key processes contributing to the negative trends.
      PubDate: 2023-05-25
       
  • Western disturbances vs Non-western disturbances days winter precipitation

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      Abstract: While studying the Western Disturbances (WDs) and associated precipitation forming mechanism in changing time, we found that there is significant amount of precipitation received during Non-WDs days too. In addition, it is also highlighted that all the WDs don’t precipitate. And hence cumulative winter precipitation is kind of summation of all the available precipitation and NOT only of WDs days’ precipitation. Thus here understanding of WDs vs Non-WDs days precipitation dynamics, linkages and changes in last three decades are deliberated. Threshold for evaluating non-precipitating and precipitating day(s) is chosen as < 0.1 mm/d and = or > greater than 0.1 mm/d, respectively. The present finding shows decreasing trend (p-value 0.039) of WDs days precipitation and intriguingly increasing trend (p-value 0.57) of Non-WDs days precipitation. These trends, though statistical not significant, are based on Mann–Kendall test. Further, Non-WDs days precipitation over the western Himalayas (WH) is less studied and researched. Thus dynamics of WDs vs Non-WDs precipitating days precipitation is evaluated and corresponding precipitation and moisture sources days are assessed. As knowingly, WDs precipitation is mainly from western sources and Arabian Sea (AS); whereas Non-WDs days precipitation derive moisture from Bay of Bengal (BoB). Present work presents preliminary investigation for Non-WDs days precipitation.
      PubDate: 2023-05-25
       
  • The onset and cessation of rainy season over the Hengduan Mountains

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      Abstract: In this study, the onset and cessation of rainy season over the Hengduan Mountains (HM) are determined by an objective method. The onset and cessation of climatological rainy season are largely controlled by large-scale circulation systems, such as the Indian summer monsoon (ISM), 200 hPa subtropical jet, and the South Asian high. Both onset and cessation dates exhibit strong interannual variability. For late onset, the lower-level northeasterly anomalies over the north Indian Ocean (IO) delay the advance of moisture transported by ISM, and are excited by the positive SST anomalies over the southwest IO. In accord with late cessation, the equatorial west IO cooling adjusts the zonal overturn circulation and causes the west side wet anomalies, which trigger the north IO cyclonic wind anomalies and the northwest Pacific anticyclonic wind anomalies following Gill response. These two anomalous flows merge, turn southerly, and continuously transport moisture to the HM, prolonging the rainy season. At upper levels, the late onset and cessation related circulation anomalies are almost mirror images and highly resemble the Silk Road pattern. The anomalous upper and lower-level circulation cooperate and favor the dynamical and thermodynamical processes, which contribute to the vertical motion anomalies and finally affect the onset and cessation dates. Especially, the 200 hPa geopotential height anomaly center on the north side of the Arabian Sea (AS) is stronger than the other centers both for late onset and cessation, and coincides well with the nearby Rossby wave source, which are reinforced via the planetary vorticity stretching term due to the upper-level wind anomalies induced by the IO SST anomalies.
      PubDate: 2023-05-23
       
  • Process-based analysis of the impacts of sea surface temperature on
           climate in CORDEX-SEA simulations

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      Abstract: This paper investigates the process behind climate model biases, especially in rainfall, during the southwest (June to August; JJA) and northeast monsoon (December to February; DJF) seasons over the Philippines using the downscaled Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations of the Southeast Asia Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment-Southeast Asia (SEACLID/CORDEX-SEA). Biases in the surface and near-surface climate are assessed over land in eastern (EastPH) and western (WestPH) Philippines using observation and reanalysis datasets. Results show that models with well-represented sea surface temperature (SST) provided better added values after downscaling. Although the CMIP5-driven models tend to overestimate both surface energy and water partitioning, simulations with well-represented SST have better surface energy partitions. In contrast to the reference simulation driven by ERA-Interim reanalysis, the CMIP5-driven simulations tend to underestimate (overestimate) moisture flux convergence during the DJF (JJA) season, which might have contributed to the overall underestimation (overestimation) of rainfall in this season. Further investigation along a vertical transect shows the differences in governing mechanisms for each season. During DJF (JJA) season, the simulation with underestimated (overestimated) SST, near-surface air temperature, and mixing ratio results in underestimated (overestimated) rainfall over EastPH (WestPH). In addition, the CMIP5-driven models were found to overestimate low-level winds, which may have induced the deep convection regardless of season, suggesting the role of low-level winds in the convection scheme used in RegCM4 aside from topography and land cover.
      PubDate: 2023-05-23
       
  • Impacts of Pacific Ocean SST on the interdecadal variations of tropical
           Asian summer monsoon onset: new eastward-propagating mechanisms

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      Abstract: Understanding the variability and mechanisms of monsoon onset is extremely prominent for water management and rain-fed agriculture. Previous studies have shown a significant interdecadal advance in Asian summer monsoon (ASM) onset after the late-1990s and attributed it to the sea surface temperature anomalies (SSTA) in the tropical Pacific. However, the westward-propagating mechanisms revealed by previous studies (Walker circulation, equatorial Rossby wave response) are gradually decaying westward, which cannot explain the observational facts of stronger low-level winds over the Arabian Sea than the South China Sea. Based on longer datasets and multiple methods, this study reveals the influences of Pacific SST on the interdecadal changes of ASM onset through two eastward-propagating mechanisms: the equatorial Kelvin wave response to the SSTA in the equatorial central Pacific, and the extratropical Rossby wave train associated with SSTA in the subtropical North Pacific. These two eastward-propagating mechanisms mainly modulate the ASM onset via altering the meridional temperature gradient, which is more evident over the Arabian Sea and is more consistent with the observations. Special attention has been paid to the generation and maintenance of the extratropical Rossby wave train, which is less understood compared to the other mechanisms. This Rossby wave train can be excited by the upper-level divergence associated with the warm SSTA in the subtropical North Pacific. In addition, it can effectively gain available potential energy and kinetic energy from the basic flow, and exhibits strong positive interactions with the synoptic-scale eddies. This Rossby wave train is a newly recognized mechanism by which the extratropical Pacific SSTA influences the tropical ASM.
      PubDate: 2023-05-22
       
  • Atlantic decadal-to-bidecadal variability in a version of the Kiel Climate
           Model

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      Abstract: Atlantic decadal-to-bidecadal variability (ADV) is described from a multimillennial control integration of a version of the Kiel Climate Model (KCM). The KCM’s ADV is the second most energetic mode of long-term North Atlantic variability in that simulation, whereas the Atlantic multidecadal variability (AMV) is the leading mode that has been described in a previous study. The KCM’s ADV can be regarded as a mixed oceanic gyre-overturning circulation mode that is forced by the North Atlantic Oscillation. The extratropical North Atlantic sea surface temperature (SST) anomalies associated with the model’s ADV initially exhibit a tripolar structure in the meridional direction, which is linked to the gyre circulation. After some years, the SST-anomaly pattern turns into a monopolar pattern located in the subpolar North Atlantic. This transition is related to the overturning circulation. The AMV and the ADV co-exist and share some similarities. Both modes of variability rely on the upper-ocean heat transport into the subpolar North Atlantic. They differ in the importance of the gyre and overturning circulations. In the ADV, gyre and overturning-heat transports into the subpolar North Atlantic are equally important in contrast to the AMV where the overturning contribution dominates.
      PubDate: 2023-05-20
       
  • Performance evaluation of CMIP6 GCMs for the projections of precipitation
           extremes in Pakistan

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      Abstract: Extreme weather events are more detrimental to human culture and ecosystems than typical weather patterns. A multimodel ensemble (MME) of the top-performing global climate models (GCMs) to simulate 11 precipitation extremes was selected using a hybrid method to project their changes in Pakistan. It also compared the benefits of using all GCMs compared to using only selected GCMs when projecting precipitation extremes for two future periods (2020–2059) and (2060–2099) for four shared socioeconomic pathways (SSPs), SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5. Results showed that EC-Earth3-Veg, MRI-ESM2-0 and NorESM2-MM performed best among GCMs in simulating historical and projecting future precipitation extremes. Compared to the MME of all GCMs, the uncertainty in future projections of all precipitation indices of the selected GCMs were significantly smaller. The MME median of the selected GCMs showed increased precipitation extremes over most of Pakistan. The greater increases were in RX1day by 6–12 mm, RX5day by 12–20 mm, Prcptot by 40–50 mm, R95ptot by greater than 30 mm, R99ptot by more than 9 mm, R4mm ≥ 4 mm by 0–4 days, R10mm by 2–6 days, R20mm by 1–3 days, and SDII by 1 mm/day, CWD by one day, CDD by 0–4 days in the northern high elevated areas for SSP5-8.5 in the late future. These results emphasize the greater influence of climate change on precipitation extremes in the northern, high-elevation areas, which provide the majority of the country’s water. This emphasizes the necessity to adopt suitable climate change mitigation strategies for sustainable development, particularly in the country's northern regions.
      PubDate: 2023-05-20
       
  • Analysis of synoptic weather patterns of heatwave events

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      Abstract: Heatwaves (HWs) are expected to increase both in duration and intensity in the next decades, but little is known about their synoptic and mesoscalar behavior, which is especially important in mid-latitude regions. Most climate research has focused on temperature analysis to characterize HWs. We propose that a combination of temperature and synoptic patterns is a better way to define and understand HWs because including atmospheric circulation patterns provides information about different HW structures that can irregularly affect the territory, and illustrate this approach at the regional and urban scales using the Iberian Peninsula and the Metropolitan Area of Barcelona as case studies. We first select HW events from 1950 to 2020 and apply a multivariate analysis to identify synoptic patterns based on mean sea level pressure, geopotential height at 500 hPa, and maximum daily 2 m temperature. The results indicate that four synoptic patterns reproduce at least 50% of the variance in HWs, namely, “stationary and stable”, “dynamic and advective”, “stationary and advective”, and “dynamic, advective and undulated”. Next, we apply the analysis to the Representative Concentration Pathway future scenarios (RCPs) 4.5 and 8.5 from the Coordinated Regional Climate Downscaling Experiment (CORDEX) to determine how these synoptic trends can change in the future. The analysis shows that the four synoptic patterns continue to explain 55 to 60% of the variance in HWs. Future HW events will be characterized by an increase in geopotential height at 500 hPa due to the northward shift of the anticyclonic ridge. This is especially true for RCP8.5, which simulates business as usual incrementing fossil fuel use and additionally shows an increase in atmospheric dynamism in north advections from all directions in comparison with RCP4.5. These findings point to the importance of considering the geopotential height in HW prediction, as well as the direction of advections.
      PubDate: 2023-05-19
       
  • Effects of Indian Ocean Dipole initialization on the forecasting of La
           Niña 1 year in advance

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      Abstract: The influence of strong Indian Ocean Dipole (IOD) events on the evolution of the following year’s La Niña events is investigated using the National Marine Environmental Forecasting Center (NMEFC) operational seasonal forecasting system. The observation results show that when the strong IOD occurs, the tropical Pacific can be in different sea surface temperature states. As such prediction system can well reproduce the air-sea evolution of the 1998/1999 and 2020/2021 La Niña events, the ocean temperature initializations in December during above events were perturbed with the system to assess the role of the oceanic channel and atmospheric bridge across the maritime continent in the forecasting of the La Niña events 1 year later. In the case of the neutral state of the tropical Pacific at the peak of the 2019 positive IOD, pacemaker experiments have demonstrated that the Indian Ocean subsurface temperature initialization in December 2019 is critically important for the successful prediction of the 2020/2021 La Niña. Experiments of sea surface temperature initialization in December 2019, with only the Indo-Pacific atmospheric bridge at work, failed to predict the 2020/2021 La Niña 1 year in advance. The comparison underlined the dominant role of the oceanic channel dynamics in the evolution of the 2020/2021 La Niña. Forecasting experiments beginning from the 1997/98 El Niño with and without the IOD initializations suggest that the delayed feedback of the tropical Pacific coupled system is more important in the forecasting of the 1998/1999 La Niña, with the Indo-Pacific oceanic channel dynamics playing a secondary yet very important role. Our study has underscored the importance of the Indo-Pacific oceanic channel dynamics in ENSO prediction and suggested the effectiveness of IOD subsurface temperature initialization in La Niña predictions at the 1-year lead time.
      PubDate: 2023-05-19
       
  • Evaluation and correction of sub-seasonal dynamic model forecast of
           precipitation in eastern China

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      Abstract: Assessing the capability of sub-seasonal rainfall forecast of dynamic model and proposing correction method is quite an important topic in current climate research field. From the perspective of rainfall amount, rainy days and rainfall-belt evolution, the sub-seasonal forecast ability of the European Centre for Medium-Range Weather Forecasts (ECMWF) model for the main rainy-season rainfall in eastern China is evaluated. Evaluation results show that the forecast biases increase gradually with the increase of forecast lead time, characterized by the predicted rainfall amount being obviously higher and the rainy days being much longer than observation. In order to reduce the forecast biases of sub-seasonal rainfall forecast of ECMWF model, the rainy day-based correction (RDC) method is proposed in this study. Cross validation results indicate that RDC method can modify the number of rainy days forecast of ECMWF model with the SCC of rainy days increasing by 12.96% ~ 18.62%, and the RMSE decreasing by 56.49% ~ 63.78%. The problem of maximum continuous rainy days being too long in the model forecast can be also improved. Meanwhile, the spatial correlation coefficient (SCC) of rainfall amount forecast of the ECMWF model with the observation weakly increases by 0.61% ~ 1.56% and the root mean square error (RMSE) decreases by 3.50% ~ 7.60% after the RDC treatment. Therefore, RDC method presents a good performance on improving the sub-seasonal forecast of rainy days, and maximum continuous rainy days, which can be further applied in other models’ sub-seasonal forecast error correction.
      PubDate: 2023-05-19
       
  • Modulations in the Indian Summer Monsoon–ENSO teleconnections by the
           North Tropical Atlantic

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      Abstract: North Tropical Atlantic Sea Surface Temperature (i.e., NTA SST) anomalies emerge as a key-driver of the whole El Niño Southern Oscillation–Indian Summer Monsoon (ENSO–ISM) system. However, as the underlying physical mechanisms are not yet well understood, this study made an attempt to have deeper insights on the role of the NTA SST variability on the ISM, ENSO and their mutual relationships. The evidences from observations and a Pi-Control coupled simulation demonstrate the pronounced biennial nature of the NTA–ENSO–ISM system and suggest the precursory role of the NTA SSTs in this biennial ENSO–ISM system. As the cause-and-effect relationships are difficult to disentangle, ensembles of short coupled sensitivity experiments are conducted by imposing observed warm (cold) SST anomalies over NTA. These 1-year simulations start from various January initial conditions corresponding to strong El Niño (La Niña) events as identified from Pi-Control and subsequently impose warm (cold) SST anomalies over the NTA region after the El Niño (La Niña) peak in January. The sensitivity experiments support the hypothesis of a key role of NTA SSTs in the reversal of the ENSO conditions through their capacitor effect. They further illustrate the nonlinear characteristics of this system as cold NTA SST perturbations are more influential than warm NTA SSTs. This non-linearity brings up new perspectives on the NTA–ENSO–ISM system, as it is further reflected in the asymmetric response in the simulated ENSO–ISM, with the cold NTA perturbations initiated from the La Niña conditions showing a stronger anomalous ISM response during boreal summer, which is in contrast with the feeble ISM response in the warm NTA perturbations experiment using the El Niño initial conditions. This non-linearity of NTA–ENSO–ISM has larger implications in a global warming scenario, as the climate variability over NTA region is projected to intensify in the future.
      PubDate: 2023-05-17
       
  • A broadscale shift in antarctic temperature trends

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      Abstract: During the second half of the twentieth century, the Antarctic Surface Air Temperature (SAT) trends are characterized by fast warming over West Antarctica but mild cooling over East Antarctica. However, after 2000, the warming over several stations in the Antarctic Peninsula slowed down, whereas the South Pole experienced fast warming. These reversed SAT trends show strong regionality and seasonality, together with large uncertainty and disagreement among different observational and reanalysis datasets, which makes it difficult to achieve a comprehensive understanding of the multi-decadal Antarctic SAT trend and its reversal. In this study, we use the Combined Maximum Covariance Analysis (CMCA) method to extract the most coherent modes of the Antarctic SAT trends among six reanalysis datasets and 26 station-based observations. Further analysis shows that the reversals of the SAT trends before and after 2000, especially for austral spring and summer, are mainly attributed to the reversed trends of the leading CMCA modes and their related atmospheric circulation and thermal advection patterns over Antarctica. For austral spring, the reversal of the west-warming-east-cooling pattern over Antarctica is closely related to the changes of thermal advection induced by the anomalous circulation center over the Antarctic Peninsula–Weddell Sea region. For summer, the post-2000 reversal of the Antarctic Peninsula-warming-East Antarctic-cooling is attributed to the stratospheric ozone recovery over the Antarctic, and the associated adjustment of the southern annular mode. The CMCA decomposition better combines the information from different measurements, clarifying the long-term SAT trend and its reversal for different seasons.
      PubDate: 2023-05-17
       
  • Decadal variability of extreme high temperature in mid- and high-latitude
           Asia and its associated North Atlantic air–sea interaction

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      Abstract: The decadal variability of extreme high temperature (EHT) in the mid and highlatitudes of continental Asia (MHLCA) and associated mechanisms were investigated. Observational analysis indicated that, after removing global warming, the first leading mode of the EHT events showed a meridional dipole pattern and had significant decadal variability. During the periods 1980–1994 and 2012–2019 (1995–2011), EHT events were more frequent and stronger (less frequent and weaker) in western-central Siberia than normal, whereas they were less frequent and weaker (more frequent and stronger) in the wide area to the south of Lake Baikal. Further Observational and CESM (Community Earth System Model) results based analysis suggested that decadal change in air–sea interaction in the North Atlantic play an important role in shaping the decadal variability of EHT events in MHLCA. On decadal timescales, meridional negative–positive–negative sea surface temperature (SST) anomalies in the North Atlantic and their coupled positive North Atlantic Oscillation can trigger stronger wave activity flux and cause a significant anticyclonic–cyclonic teleconnection wave train in the troposphere over the mid and high latitudes of the Eurasian continent. As a result, an anomalous high-pressure center is evident in western-central Siberia, and thereby reducing total cloud cover and causing stronger solar heating. Thus, EHT events become more frequent and stronger. On the contrary, for the areas south of Lake Baikal, EHT events are weakened by corresponding cyclonic circulation anomalies. Additionally, the CAM5 (Community Atmosphere Model version 5) experiments suggested more important driving role of the decadal North Atlantic tripole SST anomalies in the abovementioned processes.
      PubDate: 2023-05-16
       
  • Correction to: Near-term regional climate change in East Africa

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      PubDate: 2023-05-15
       
  • Evaluation and application analysis of kilometer-scale convective
           parameters derived from a statistical downscaling method over Central
           China

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      Abstract: Thunderstorms are usually accompanied by heavy precipitation and other disasters. Therefore, it is very important to study the thermodynamic and dynamic mechanisms that control the occurrence and development of storms. However, the relatively coarse resolution of the reanalysis may limit the evaluation and application of its derived convective parameters at regional and local scales, especially in high mountains. Given that the effect of grid averaging on ERA5 can be well-reflected in the monthly mean, the paper proposes a methodological framework to achieve the statistical downscaling of convective parameters (SDCP). After the convection-related variables are decomposed, we introduce a multi-scale geographically weighted regression (MGWR) model and take the Normalized Difference Vegetation Index (NDVI) and the Digital Elevation Model (DEM) as explanatory variables to improve the spatial resolution of monthly-scale convective parameters. The convective parameters chosen for the downscaling experiments are closely related to thunderstorm occurrence, involving instability, moisture, and wind shear. ERA5 and SDCP datasets’ quality is evaluated based on measurements available from 11 radiosonde stations over 10 years. We also explore the climatology of thunderstorm environments and the corresponding trends. The results show that the accuracy of the SDCP data obtains additional added value in complex-terrain areas. Compared with ERA5, the downscaled results have a finer spatial structure. A strong but spatially and scale-varying relationship exists between the monthly mean convective parameter and the resampled NDVI and DEM. Considering the very high predictive ability of the MGWR model, the performance of the SDCP algorithm mainly depends on the accuracy of the ERA5 data. Our study demonstrates the possibility of high spatial resolution land surface features observed by remote sensing satellites in improving the spatial resolution of thermodynamic parameters.
      PubDate: 2023-05-15
       
  • Changes in SST regulate hydroclimatic patterns in the monsoon marginal
           zone, northern China

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      Abstract: The influence of recent global warming on the intensity, timing, and extent of the East Asian summer monsoon (EASM) remains not fully understood. Here we reconstruct an EASM precipitation history of the past ~ 500 years based on sedimentary multi-proxy indices from Lake Daihai, northern China. We find low EASM precipitation between ~ AD 1517 and ~ 1850, with three sharply weakened intervals, which were broadly concurrent with the Chinese dynastic and cultural transitions. The EASM intensified after ~ AD 1850, changing from cold-dry to warm-wet conditions, with greater multidecadal variability. These features match well with the coeval sea surface temperature (SST) records over the North Pacific and North Atlantic Oceans, suggesting that changes in global SSTs could have forced changes in EASM intensity and modulated regional hydroclimate on these timescales. We propose that the increased interhemispheric temperature gradients associated with the recent global warming might push northward the western Pacific Subtropical High (WPSH), the westerlies, and the monsoon front. Meanwhile, the multi-decadal warming in the North Atlantic may also strengthen the westerlies and the EASM. These multiple factors could have collectively led to higher precipitation in the converging zone of the westerlies and the WPSH since ~ AD 1850.
      PubDate: 2023-05-14
       
  • Significant weakening effects of Arctic sea ice loss on the summer western
           hemisphere polar jet stream and troposphere vertical wind shear

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      Abstract: The westerly wind on the poleward side of the summer polar jet stream (PJS) over the Western Hemisphere has significantly weakened since the 1980s. A weak summer PJS causes warming surface temperatures and deficient precipitation over Alaska and western North America, favoring extreme wildfire events. This study investigates influences of Arctic sea ice loss on the summer PJS variability over the Western Hemisphere. Regression analysis first provides observational evidence that Arctic sea ice reduction is related to a weakening summer Western Hemisphere PJS at interannual time scales. Atmospheric model ensemble simulations are then used to demonstrate that Arctic sea ice loss significantly contributes to observed Western Hemisphere Arctic warming and reduced meridional temperature gradient between midlatitudes and the pole in the lower and middle troposphere, acting to weaken the troposphere zonal wind and vertical wind shear from 55° to 75°N, and about 20–30% of observed weakened summer PJS trend during 1979–2014. Observational analysis and the model-based results also indicate that a significant portion of the observed trends of the PJS and vertical wind shear during 1979–2014 might be attributed to the decadal variability of the summer North Atlantic Oscillation (NAO). In the future climate, as more and more ice melts in the summer, the weakening effect of sea ice on the PJS will continue and will be superimposed onto the natural decadal variability of the PJS.
      PubDate: 2023-05-13
       
 
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