How to cite: Thompson, D.: Thermodynamic constraints on changes in temperature persistence and variance under climate change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10154, https://doi.org/10.5194/egusphere-egu23-10154, 2023.

Convective extreme El Niño (CEE) events, characterized by strong convective events in the eastern Pacific, are known to have a direct link to anomalous climate conditions worldwide, and it has been reported that CEE will occur more frequently under greenhouse warming. Here, using a set of CO₂ ramp-up and –down ensemble experiments, we show that frequency and maximum intensity of CEE events increase further in the ramp-down period from the ramp-up period. Such changes in CEE are associated with the southward shift of the Intertropical Convergence Zone and intensified nonlinear rainfall response to SST change in the ramp-down period. The increasing frequency of CEE has substantial impacts on regional abnormal events and contributed considerably to regional mean climate changes to the CO₂ forcings.

How to cite: Pathirana, G., Oh, J.-H., Cai, W., An, S.-I., Min, S.-K., Jo, S.-Y., Shin, J., and Kug, J.-S.: Explosive increase in convective Extreme El Niño events in the CO2 removal scenario, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1945, https://doi.org/10.5194/egusphere-egu23-1945, 2023.

Humid heat extremes, taking account of both temperature and humidity, have adverse impacts on society, particularly on human health. It has been suggested that quasi-stationary waves (QSWs) with anomalously high amplitudes contribute to the occurrence of near-surface precipitation extremes and temperature extremes in the mid-latitudes of Northern Hemisphere. While little attention is paid to the linkages between amplified QSWs and humid heat extremes. Using the ERA5 dataset, we identify amplified QSWs of zonal wavenumbers 5-7 (Wave 5-7) in summer months from 1979 to 2020. These amplified QSWs show clear circumglobal wave patterns horizontally and nearly barotropic structure vertically. Linking amplified Wave 5-7 to wet-bulb temperature (WBT) extremes, we find that amplified QSWs preferentially induce prominently prolonged WBT extremes in specific regions: north-central North America for amplified Wave 5; western United States, south-central Asia, as well as eastern Asia for amplified Wave 6; western Europe and the Caspian Sea region for amplified Wave 7. Analyses of physical processes indicate that accompanied by the amplification of Wave 5-7, the changes in horizontal temperature advection, adiabatic heating, downward solar radiation, moisture transport and moisture flux convergence, and surface latent heat fluxes largely account for the increase in persistence of WBT extremes.

How to cite: Lin, Q. and Yuan, J.: Linkages between Amplified Quasi-stationary Waves and Humid Heat Extremes in Northern Hemisphere Midlatitudes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-78, https://doi.org/10.5194/egusphere-egu23-78, 2023.

Rainfall is an essential climatic parameter for any region, and it can have a significant socioeconomic impact on society. In this study, the trend analysis of rainfall data of the Ajay River Basin was performed for daily rainfall data from the APHRODITE dataset. It is a gridded dataset with a resolution of 0.25*0.25 degree latitude and longitude with 1951 to 2007 long time series for Asia. The non-parametric Mann-Kendall test was used to detect the monotonic trend in the rainfall time series and the Theil-Sen estimator to look at the magnitude of the change. The quantile perturbation method is used for extreme rainfall analysis. The study reveals that total annual rainfall and the monsoon period (June, July, August, September) have increased over the basin's southern part at a 5% significance level. In the pre-monsoon period (March, April, May) rainfall has increased all over the basin area at the 5% significance level. Extreme rainfall anomalies were found in most of the basin region, but some periods had very high perturbation. In the 1950-1960s, the northern area of the basin showed statistically significant negative anomalies, while the southern region showed significant positive anomalies. The 1970-1980s was the period of the highest significant positive anomalies, with up to 110% change. Significant negative anomalies dominated most of the southern basin from 1980 to 2000. The study concluded that although total rainfall has increased, extremes have decreased in the region.

How to cite: Mandraha, S. and Ray, S.: Trend analysis of total, seasonal and extreme rainfall data for Ajay River Basin, West Bengal., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-628, https://doi.org/10.5194/egusphere-egu23-628, 2023.

The influence of moisture recycling and transport on major drought events is poorly understood, but essential to enhance our knowledge of the atmospheric water cycle. Here, we investigate this for two record-breaking droughts over the Mid-to-Lower Reaches of the Yangtze River (MLRYR), the winter-spring (WS) drought of 2011 and summer-autumn (SA) drought of 2019. Using a land–atmosphere water balance framework, we find the precipitation recycling ratio (the percentage of precipitation in a region derived from the same region’s evaporation) increased during both droughts, especially for the SA drought (from 14.5% to 22.9%). The WS drought was characterized by a 27.8% reduction in external advected moisture, originating principally from the northeast China and Bohai Sea (reduced by 22.3%) and from the northwest Pacific and South China Sea (25.7%). The SA drought was driven by a 43.8% reduction in external advected moisture, originating mainly from a southwesterly path, i.e. the Bay of Bengal and the South China Sea (reduced by 26.8%). From a regional viewpoint, moisture transportation from the Pacific Ocean (and South China Sea) decreased during the WS (SA) droughts, mainly resulting in moisture deficit over the MLRYR. Analyses reveal that this reduction was driven by strong negative convergence, which was unfavorable for precipitation formation and enhanced air flow out of the MLRYR. The weakened moisture transport was principally driven by seasonal mean flow rather than transient eddies. Changes in wind (i.e. dynamic processes), rather than specific humidity (i.e. thermodynamic processes) were dominant in regulating the seasonal mean moisture transport. Our study helps understand the atmospheric water cycle anomalies driving extreme drought events, and advances knowledge on moisture transportation and its controlling processes.

How to cite: Guan, Y. and Gu, X.: Tracing anomalies in moisture recycling and transport to two record-breaking droughts over the Mid-to-Lower Reaches of the Yangtze River, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2050, https://doi.org/10.5194/egusphere-egu23-2050, 2023.

Drought is the main natural hazard at the planetary scale and although this is a very complex phenomenon that involves many aspects of the hydrological cycle, there is always a deficit of precipitation compared to usual, understanding usual as climatological. This deficit can occur essentially for three reasons, either because there is less moisture available in the air column or because there is less atmospheric instability that forces air to rise, or for both reasons simultaneously. As the existing local humidity in the air column is mostly insufficient to justify precipitation, less humidity available for precipitation implies a deficit in the moisture which reaches the site in question. Therefore, on a global scale, moisture transport deficits lead to the occurrence of droughts.

In a first approximation, this humidity can have two origins, or comes directly from the ocean, or is subsequently recycled from the continents themselves. The processes that control the evaporation over oceans or the continents as well as the moisture transport are very different, and there is a variable relationship between the oceanic and terrestrial origin of precipitation both globally and regionally. In a second approximation, the main sources of humidity at a global level are those regions where evaporation greatly exceeds precipitation over them, which mainly occurs in the subtropical oceans, some inland seas, and the two continental areas known as green oceans, the Amazon and the Congo basins.

It is known where the humidity coming from the whole ocean or the whole continent precipitates, as well as the sinks of the humidity that comes from these large individual sources. It has also been studied how anomalous moisture transport affects droughts in specific regions, but the probability of occurrence of droughts at a planetary scale on continental areas given a deficit of the moisture transported from the global oceanic area, the global continental area, and each of these major sources has not been fully evaluated. Here we make use of a Lagrangian approach widely used and checked, which consists of estimating how much precipitation comes from the humidity arriving from a specific moisture source and enables to reveal the origin of the atmospheric moisture deficit underlying the occurrence of droughts. 

How to cite: Gimeno-Sotelo, L., Sorí, R., Vázquez, M., Nieto, R., Vicente-Serrano, S. M., and Gimeno, L.: Unravelling the origin of the atmospheric moisture deficit that leads to droughts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2866, https://doi.org/10.5194/egusphere-egu23-2866, 2023.

Concurrent heatwaves and droughts that occur over northeastern China (NEC) bring severe threats to human lives and crop productions. In the present study, a probability-based index that simultaneously considers precipitation deficiency and high temperature is calculated to represent concurrent heatwaves and droughts over NEC. Based on this index, the characteristics of concurrent heatwaves and droughts over NEC in summer are investigated using the year-to-year increment approach. The results indicate that the occurrence of concurrent heatwave and drought over NEC is closely related to the Polar-Eurasian teleconnection pattern and the Pacific-Japan teleconnection pattern. Further analyses indicate that the sea ice content in the Barents Sea in March (SICBS), the La Niña-like sea surface temperature (SST) in February (SST-PC1), and the northwestern Siberia soil moisture in April (SM) are coincidently linked to the two teleconnection patterns mentioned above. Based on their corresponding physical mechanisms, these three independent predictors are chosen to construct a physical-empirical prediction model for the prediction of concurrent heatwaves and droughts over NEC. Results suggest that this physical-empirical prediction model performs well with a high correlation coefficient and a low root mean squared error between the observed and predicted concurrent heatwaves and droughts over NEC for the period 1979–2018. Moreover, the cross-validation test and independent hindcasts both suggest that the physical-empirical model proposed in the present study with the three independent predictors has good prediction skills.

How to cite: Li, H.: Mechanisms and prediction of concurrent heatwaves and droughts in July–August over northeastern China, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7631, https://doi.org/10.5194/egusphere-egu23-7631, 2023.

In the context of global warming, droughts occur more frequently and have caused great losses to human society. Therefore, understanding the potential changes in future droughts under climate change is of great scientific importance. In this paper, combining with climate models from CMIP6, the emergent constraint and the Model Goodness Index (MGI) are used to analyze the characteristics of meteorological, agricultural and hydrological droughts in China under four socioeconomic scenarios in the mid- and late 21st century. The results show that in the mid-21st century, there will be more frequent meteorological, agricultural and hydrological droughts in northern China. In the late 21st century, longer and more intense droughts are more likely to occur in China than in the mid-21st century. This indicates that drought events in China will gradually become more continuous and serious from the middle to the late 21st century. Additionally, northwestern and central China will be the main areas where the three types of drought areas and extreme droughts will increase in the future. In the mid-21st century, a higher socioeconomic scenario will suppress drought, which will enhance drought conversely in the late 21st century. These findings are of great significance for drought monitoring under climate change and can provide a basis for making a drought response plan.

How to cite: Xue, R., Li, W., Li, H., Luo, X., and Ai, W.: Future projections of meteorological, agricultural and hydrological droughts in China using the emergent constraint, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1580, https://doi.org/10.5194/egusphere-egu23-1580, 2023.

Summer hot and dry extremes (defined as high air temperature and low atmospheric humidity) in monsoon (climatologically high-humidity) region, may cause severe disasters, such as flash droughts. However, it remains unclear whether hot (dry) extremes are amplified on dry (hot) days to warming temperature. Here, taking eastern monsoon China (EMC) as a typical monsoon region, we find a fastest positive (negative) response of air temperature (atmospheric humidity) on driest (hottest) days to per unit warming, indicating amplified warming (drying) of hot (dry) extremes on dry (hot) days (i.e. coupling hotter and drier extremes) especially in southern EMC.  The southern EMC is also a hotspot where the coupling of hot and dry extremes has become significantly stronger during the past six decades. The increasing hot-dry extremes in southern EMC is associated with anomalies in large-scale environmental conditions, such as reduced total cloud cover, abnormal anticyclone in upper atmosphere, intense descending motion, and strong moisture divergence over this region. Land-atmosphere feedbacks play another important role in enhancing the hot-dry coupling via increasing land surface dryness (described as decreasing evaporation fraction). The decreasing evaporation fraction is associated with drying surface soil moisture which is controlled by decreases in pre-summer 1-m soil moisture and summer-mean precipitation. Given hot extremes (atmospheric humidity) are (is) projected to increase (decrease) in the future, it is very likely to witness more hot-dry days in monsoon regions and associated disasters, which should be mitigated by adopting adaptive measures. 

How to cite: zhang, X. and Gu, X.: Coupling hotter and drier extremes under elevating air temperature over eastern monsoon China, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1982, https://doi.org/10.5194/egusphere-egu23-1982, 2023.

Extreme events seriously affect human health and natural environment. In the present study, several indexes that can describe the severity of compound extreme high temperature and drought/rainy events (CHTDE/CHTRE) are constructed based on copulas. According to observations, CHTDE and CHTRE have intensified in most areas of China during 1961–2014. The significant increase trend in the severity of CHTDE and CHTRE is basically consistent with simulations under historical anthropogenic forcing. This result proves that changes in CHTDE can be largely attributed to anthropogenic climate change. The historical greenhouse gas forcing is identified to be the dominant factor that affects the severity of CHTDE in China, particularly in the Tibetan Plateau and Northwest China. Moreover, the contribution of anthropogenic forcing to the linear change of the CHTRE severity in China is more than 90%. In addition, the ozone and land use signals also can be detected on change of CHTDE and CHTRE.

How to cite: Li, W., Wang, H., Xue, R., Li, H., Duan, M., Luo, X., and Ai, W.: Anthropogenic impact on the severity of compound extreme high temperature and drought/rainy events in China, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1579, https://doi.org/10.5194/egusphere-egu23-1579, 2023.

This study finds a significant negative correlation between the December–February mean surface temperature (Ts_DJF) and the following June–August mean surface temperature (Ts_JJA) in South Korea for the period 1991–2017. This indicates that colder winters tend to precede hotter summers with extreme seasonality, while mild winters generally precede mild summers. This winter-to-summer association can be attributed to persistent atmospheric circulation anomalies on the Eurasian continent during the preceding winter and spring characterized by cyclonic circulations in Europe and East Asia and anti-cyclonic circulation in the Arctic regions. Resembling a negative Arctic Oscillation (AO) pattern combined with a negative Polar/Eurasia (PE) pattern, these atmospheric patterns tend to cause colder winters in South Korea and to increase the springtime sea surface temperatures in the western tropical Pacific (WTP) and in the North Atlantic (tripole pattern, NATRI). High WTP and NATRI values induce summertime anti-cyclonic circulations and then hotter summers in Korea with different pathways, the former via northward Rossby wave propagation in response to strong convection over a warm Philippine Sea and the latter via both extratropical Rossby wave propagation from the North Atlantic to East Asia and tropical connections from the tropical Atlantic to the Indian Ocean and then increased summer precipitation in South Asia. Under the opposite conditions (e.g., positive AO and PE phases in winter and negative WTP and NATRI), mild summers are preceded by mild winters. Since the early 1990s, the aforementioned atmospheric circulation anomalies during winter have shown greater persistence, creating the negative correlation between Ts_DJF and Ts_JJA. These findings provide useful information for the long-lead prediction of summer temperatures and heat waves in South Korea.

How to cite: Myoung, B.: Recent Trend of Cold Winters Followed by Hot Summers in South Korea due to the Combined Effects of the Warm Western Tropical Pacific and North Atlantic in Spring, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2985, https://doi.org/10.5194/egusphere-egu23-2985, 2023.

Climate change can be reflected in terms of  shift in mean climatology as well as shift in the distribution of rainfall and temperature extremes over time. Southern Africa has distinct climate regimes that includes arid and semi-arid climates as well as relatively humid climate. This results in distinct spatio-temporal response of the region to climate change. In this study, major extreme climate indices for the region are derived from daily CHIRPS rainfall, ERA-Interim minimum, maximum and average temperatures to understand the spatio-temporal variability. The long term mean of the coldest annual day-time maximum temperature (90^thpercentile) is observed over Lesotho highlands and adjoining areas in South Africa (21.0^oC) whereas the warmest day-time temperature (37.4^oC) is observed over areas bordering South Africa, southern Botswana and Namibia during the recent four decades. The trend in this indices shows warming (up to 1^oC/decade) over southwestern South Africa, along coastal strips of South Africa, much of Mozambique, northwestern Zimbabwe, northern and western Zambia, eastern Angola and cooling over central Botswana. The annual night-time minimum temperature (10^th percentile) is increasing northward in contrast to day-time maximum temperature which, in addition, exhibits zonal gradient. In terms of frequency, 10 to 12% of  the days in a year experienced maximum temperature above 90^th percentile whereas 9 to 11 % of the days in a year observe night-time minimum temperature below 10^th percentile. The annual heat wave duration indices show longest duration (8 days) over Southern Angola, northern Namibia, southern  Zambia, northern Zimbabwe and Botswana and decrease from here northward and southward. Annual number of days with rainfall  above 10 mm is about 10 days over western South Africa, Botswana, southern Zimbabwe, Namibia and southern Angola. In contrast,  it is in the range of 32 to 74 days over northern Angola, Zambia and Mozambique with increasing trend over Botswana, eastern parts of Zambia and Angola.  Similar trend in maximum 5 to 10 day total rainfall is observed over the same areas. Central part of the Southern Africa region exhibits the highest annual continuous dry days (147 to 254 days) whereas the southern and northern parts of the region has the lowest annual continuous dry days ( about 76 days). The long term mean of maximum annual continuous wet days increases northward from 3 days over western parts of South Africa and Namibia to 21 days over  Angola and northern Malawi. Besides secular trend, the rainfall extreme indices have coherent cyclic modes of variability with a period of 3.6 to 3.8 years accounting for 9 to 12 % of the total variance whereas the temperature extreme indices show periodicity of about 2.5 years accounting for 18 to 19% of the total variance. The periodicities are possibly associated with ENSO events that modulate interannual variability.

How to cite: Mengistu Tsidu, G.: Spatio-temporal variability of rainfall and temperature extremes over Southern Africa, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3037, https://doi.org/10.5194/egusphere-egu23-3037, 2023.

Observations have shown sub-seasonal reversal of temperature anomalies between early and late winter over Eurasia, which is distinct from the seasonal mean condition. Based on the reanalysis data, the 1800-year control simulation and the 40-member ensemble simulations in 1920–2100 from the Community Earth System Model (CESM) Large Ensemble (CESM-LE), this study reveals that the reversal of surface air temperature (SAT) anomalies between early and late winter is one of the dominant and intrinsic features of the Arctic-Eurasian winter climate. Such a reversal is characterized by “colder Arctic, warmer Eurasia” in December (January–February) and ‘warmer Arctic, colder Eurasia’ in January–February (December). Robust climate dynamic processes associated with the reversal of SAT anomalies, including sub-seasonal reversals of anomalies in the Ural blocking, mid-latitude westerlies and stratospheric polar vortex, are found in both reanalysis data and CESM simulations, indicating the important role of internal atmospheric variability. Further analysis reveals that the reversal of Ural blocking anomalies in late December can be a potential precursor for the reversal of SAT anomalies in late winter. The reversal of mid-latitude westerly wind anomalies associated with the Ural blocking can affect upward propagation of planetary-scale waves especially with wavenumber 1, subsequently promoting the contribution of stratospheric polar vortex to the reversal of SAT anomalies in late winter over the Arctic-Eurasian regions. Such a troposphere-stratosphere pathway triggered by the perturbation of tropospheric circulations is confirmed by the CESM-LE simulations, and it may be useful for the prediction of sub-seasonal reversal of SAT anomalies.

How to cite: Xu, X.: Atmospheric contributions to the reversal of surface temperature anomalies between early and late winter over Eurasia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4395, https://doi.org/10.5194/egusphere-egu23-4395, 2023.

Extreme events are on the rise. The 2022 compound heatwave and drought event caused significant vegetation mortality and serious ecosystem destruction in Europe that urgently need to be investigated. In this study, we used climate data (ERA5-Land air temperature at 2 m and precipitation) and remote sensing products (kNDVI derived from MODIS and ESA CCI Land Cover product) to investigate the dynamics of the 2022 extreme events and vegetation responses. Furthermore, we compared the effects of this year to other normal as well as abnormal years in Europe. We propose a ranking-based approach that compares cumulative sums of climate variables and kNDVI over the growing season to determine extreme areas and compound intensity over the last 23 years.

The results show that the 2022 event is a widespread compound heatwave and drought event, with a similar spatial pattern to the 2018 extreme event, but less severe. Vegetational response differed among land cover classes, grassland was more affected while deciduous trees were barely affected in the 2022 event. In general, vegetation recovered relatively quickly after the 2022 event.

Our ranking-based approach enables an effective comparison and characterization of climate extremes and their effects on vegetation over different years. A more in-depth analysis of spatial and temporal patterns can contribute to the development of targeted measures and support decision-makers in responding to climate extremes.

How to cite: Ji, C., Montero, D., Grupp, V., Mora, K., and D. Mahecha, M.: Comparison Analysis of the Climate Extreme in 2022, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6127, https://doi.org/10.5194/egusphere-egu23-6127, 2023.

Analyses of the standardized precipitation evaporation index (SPEI), using the season-reliant empirical orthogonal function (S-EOF) method, indicate that the second leading mode of drought over Northeast China features an in-phase variation from spring to summer. Such an in-phase change is closely connected to the persistence of geopotential height anomalies around Lake Baikal. The positive height anomalies around Lake Baikal, with an equivalent barotropic structure in the troposphere, can decrease water vapor transport into Northeast China and induce anomalous descending over Northeast China during both seasons, favoring precipitation deficit and high temperature in situ and hence resulting in the synchronous variations of spring and summer droughts. Further investigation reveals that the spring North Atlantic Oscillation (NAO) plays a notable role in the in-phase change of spring-summer droughts over Northeast China. The positive phase of spring NAO could induce spring drought over Northeast China directly through its influence on the above atmospheric circulations via a zonal wave train emanating from the North Atlantic. Meanwhile, it can also increase the soil moisture in Central Siberia by enhancing the local snow depth. The wetter soil moisture in the following summer, in turn, increases the meridional temperature gradient between the middle and high latitudes and then forces westerly anomalies around 60°N, consequently yielding positive height anomalies around Lake Baikal which favor the occurrence of summer drought over Northeast China. Therefore, the spring NAO is hypothesized to contribute to the in-phase variations of spring-summer droughts over Northeast China through the combined roles of zonal wave train and Central Siberian soil moisture.

How to cite: Hu, Y.: In-phase Variations of Spring and Summer Droughts over Northeast China and Their Relationship with the North Atlantic Oscillation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6270, https://doi.org/10.5194/egusphere-egu23-6270, 2023.

Three extreme cold events successively occurred across East Asia and North America in the 2020/21 winter. This study investigates the underlying mechanisms of these record-breaking persistent cold events from the isentropic mass circulation (IMC) perspective. Results show that the midlatitude cold surface temperature anomalies always co-occurred with the high-latitude warm anomalies, and this was closely related to the strengthening of the low-level equatorward cold air branch of the IMC, particularly along the climatological cold air routes over East Asia and North America. Specifically, the two cold surges over East Asia in early winter were results of intensification of cold air transport there, influenced by the Arctic sea ice loss in autumn. The weakened cold air transport over North America associated with warmer northeastern Pacific sea surface temperatures (SSTs) explained the concurrent anomalous warmth there. This enhanced a wavenumber-1 pattern and upward wave propagation, inducing a simultaneous and long-lasting stronger poleward warm air branch (WB) of the IMC in the stratosphere and hence a displacement-type Stratospheric Sudden Warming (SSW) event on 4 January. The WB-induced increase in the air mass transported into the polar stratosphere was followed by intensification of the equatorward cold branch, hence promoting the occurrence of two extreme cold events respectively over East Asia in the beginning of January and over North America in February. Results do not yield a robust direct linkage from La Niña to the SSW event, IMC changes, and cold events, though the extratropical warm SSTs are found to contribute to the February cold surge in North America.

How to cite: Yu, Y.: An Isentropic Mass Circulation View on the Extreme Cold Events in the 2020/21 Winter, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1192, https://doi.org/10.5194/egusphere-egu23-1192, 2023.

Drylands play an essential role in Earth’s environment and human systems. Although dryland expansion has been widely investigated in previous studies, there is a lack of quantitative evidence supporting human-induced changes in dryland extent. Here, using multiple observational datasets and model simulations from phase 6 of the Coupled Model Intercomparison Project, we employ both correlation-based and optimal fingerprinting approaches to conduct quantitative detection and attribution of dryland expansion. Our results show that spatial changes in atmospheric aridity (i.e., the aridity index defined by the ratio of precipitation to potential evapotranspiration) between the recent period 1990–2014 and the past period 1950–74 are unlikely to have been caused by greenhouse gas (GHG) emissions. However, it is very likely (at least 95% confidence level) that dryland expansion at the global scale was driven principally by GHG emissions. Over the period 1950–2014, global drylands expanded by 3.67% according to observations, and the dryland expansion attributed to GHG emissions is estimated as ∼4.5%. Drylands are projected to continue expanding, and their populations to increase until global warming reaches ∼3.5℃ above preindustrial temperature under the middle- and high emission scenarios. If warming exceeds ∼3.5℃, a reduction in population density would drive a decrease in dryland population. Our results for the first time provide quantitative evidence for the dominant effects of GHG emissions on global dryland expansion, which is helpful for anthropogenic climate change adaptation in drylands.

How to cite: Shuyun, F. and Xihui, G.: Greenhouse gas emissions drive global dryland expansion but not spatial patterns of change in aridification, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2063, https://doi.org/10.5194/egusphere-egu23-2063, 2023.

High temperatures and droughts pose a great threat to the human health, social economy and ecosystems. A large number of previous studies have focused on meteorological hot-dry events (based on temperature and precipitation), but there is a lack of comprehensive studies about hydrological hot-dry events (based on temperature and runoff). Here, using the ensemble empirical mode decomposition method and Copula function, we assess spatio-temporal evolution of global compound hot-dry events from temperature and runoff, and quantify their drivers based on monthly temperature and runoff data during 1902-2019. We find there is a significant warming at an unprecedented pace over the past 118 years, especially in the mid-latitudes of the Northern Hemisphere. However, changes in accumulated trends in precipitation and runoff show complex patterns globally. Probabilities of meteorological and hydrological hot-dry events both have been increasing significantly, but hydrological events are more likely to occur with higher spatial homogeneity, wider coverage and more severe damage. To analyze its underlying driving mechanism, we estimate quantitatively the contribution of high temperature, low runoff and the dependence between high temperature and low runoff to the compound event. High temperature plays a dominant role in the driving mechanism. In several regions, such as Australia, Europe and South America, hot-dry events could be considered as a potential hazard caused by increasing temperatures. Runoff deficit and dependence between the two, together with high temperature, exacerbate the occurrence of compound hot-dry events. Our findings provide a promising direction to predict joint probability of hot-dry events. Hydrological hot-dry events have seldom been considered, so far, in strategic policy formulation and risk assessment. Our results offer a powerful tool to improve planning and strategies to adapt to climate change.

How to cite: Min, R. and Gu, X.: Increasing likelihood of global compound hot-dry extremes from temperature and runoff during the past 120 years, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2299, https://doi.org/10.5194/egusphere-egu23-2299, 2023.

Generally, it is well known that the East Asian heatwaves are strongly affected by the Pacific-Japan pattern and circum-global teleconnection pattern. However, recent studies suggest that various teleconnection patterns also can contribute to the East Asian heat waves (e.g., Scandinavian pattern, Arctic Oscillation, El Nino-Southern Oscillation, etc.). However, the teleconnection between the Arctic-Siberian Plain (ASP) warming and East Asian heat waves has been unexplored. This study investigates the teleconnection mechanism between East Asian heatwaves and the warming over the ASP for the last 42 years (1979-2020). The results show that the enhanced surface radiative heating by the anticyclonic anomalies over the ASP region increases the air temperature and surface evaporation, amplifying the thermal high pressure via positive water vapor feedback. The Rossby wave, amplified by land-atmosphere interaction in the ASP, propagates to East Asia through the upper troposphere, causing favorable atmospheric patterns for the occurrence of the East Asian heatwaves.

How to cite: Kim, J.-H., Kim, S.-J., Kim, J.-H., Hayashi, M., and Kim, M.-K.: The Arctic-Siberian Plain warming drives the heat waves in East Asia., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6335, https://doi.org/10.5194/egusphere-egu23-6335, 2023.

The South Asian High (SAH) experienced a decadal weakening in the late 1970s under global warming. Based on an evaluation of the historical runs from CMIP6 models, we quantitatively assessed the contributions of different external forcing using “good” models that reasonably simulated the decadal decline of the SAH. All-forcing runs yielded the weakened SAH after the late 1970s, albeit the decadal decline was underestimated by most models. Compared to the insignificant contributions of greenhouse gas and natural forcing, anthropogenic aerosol played a dominant role in the decadal decline of the SAH. The increased aerosol likely drove a cooling surface over the Tibetan Plateau and East China via its effect on radiation. Consequently, the weakened heat source over the Tibetan Plateau and associated thermodynamic effects over East China would have driven a cooling of eddy temperature and cyclonic anomalies in the upper troposphere, respectively, thereby causing the decline of the SAH.

How to cite: Zhang, D.: Contributions of External Forcing to the Decadal Decline of the South Asian High, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6331, https://doi.org/10.5194/egusphere-egu23-6331, 2023.

In the last decade, tropical rainforests, e.g. the Amazon basin, have experienced events of extreme droughts. Such events have huge impacts on the forest, as trees are damaged. Therefore, it is relevant to gain deeper understanding on the main mechanisms causing such events, and further clarify the role of moisture recycling over the continent vs. moisture transport from oceanic regions.

We study the role of moisture- and heat transport for the Amazon basin, with special focus on drought events. We show how these extreme events differ from normal conditions, with special focus on the changes in atmospheric transport. We analyse the atmospheric transport with the particle dispersion model FLEXPART using meteorological input data from the ERA5 reanalysis. In this Lagrangian model, the atmosphere was filled homogeneously with particles, which were traced forward in time and represent the global atmospheric mass transport. From this Lagrangian reanalysis dataset, covering the years 1979-2021, air masses over the Amazon basin are selected and traced backward in time.

Based on that, we investigate the role of continental and oceanic moisture source areas, incorporating also information on soil moisture and burned areas. Thereby, we highlight the relevance of moisture recycling over continental - vs.  moisture transport from oceanic areas. For example, we found that for the northern parts of the Amazon basin the most important moisture source is the Atlantic Ocean, thus this area is less affected by deforestation in the southern areas.

How to cite: Baier, K. and Stohl, A.: The Role of Moisture and Heat Transport for Extreme Droughts in the Amazon Basin - a Lagrangian Perspective, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7643, https://doi.org/10.5194/egusphere-egu23-7643, 2023.

The frequency, intensity and duration of weather and climate extremes have increased globally over the past several decades, and the trend is projected to continue. It is important to understand the changing nature of these extremes as it contributes to better monitoring and prediction, thereby reducing the risk to society. Changes in climate and associated weather extremes may be caused both by natural factors such as internal variability, volcanic eruptions, and solar variability as well as anthropogenic factors such as GHGs, aerosols and land use changes. It is essential to differentiate between the contributions of these drivers in order to take suitable measures to mitigate and adapt. Though the changes in temperature extremes are well-documented, rainfall extremes are significantly heterogeneous around the world.

This study analyses extreme precipitation indices over India (developed by the Expert Team on Climate Change Detection and Indices; ETCCDI) and their relationship with different modes of climate variability. The study examines the mean climatology, long-term trends and variability in extreme precipitation indices over India using the daily gridded rainfall data from the India Meteorological Department (IMD). The analysis is carried out over different homogeneous zones as well as custom-defined areas over India in different seasons. The study finds significant variability of extreme indices in the 2 to 4 year time scales and highlights the role of Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO) in modulating extreme precipitation. 

How to cite: Sajeevan Thankamani, C. and AchutaRao, K.: Trends and Variability in Extreme Precipitation over India, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8502, https://doi.org/10.5194/egusphere-egu23-8502, 2023.

Marine Heat Waves (MHWs), persistent and anomalously sea water temperature warm events, are known to have significant impacts on marine ecosystems as well as on air-sea exchanges. As global ocean temperatures continue to rise, MHWs have become more widespread, threatening marine ecosystems and their services for food-provision, livelihoods and recreation. Detecting and predicting the occurrence, intensity and duration of these extreme events, and understanding their impacts on marine ecosystems is a key step towards developing science-based solutions for sustainable development.

The project “deteCtion and threAts of maRinE Heat waves – CAREHeat”, funded by ESA in the framework of the Ocean Health initiative, aims at improving the current MHW detection and characterization methodology, as well as advancing the understanding of the physical processes involved, and the corresponding ecological and biogeochemical changes.

This is being to be achieved following a multidisciplinary approach capitalizing on the large potential offered by satellite Earth observations, complemented with in situ field measurements, physical and biogeochemical ocean reanalyses, biogeochemical modelling and emerging machine learning technologies. In this presentation an overview of the CAREHeat Project activities and its preliminary results will be provided. In particular the assessment of the major gaps in scientific knowledge, existing products and tools in MHW detection will be discussed. A first version of the CAREHeat MHW Global Atlas covering the entire satellite era (1981-today) will be presented and analysed to investigate the year-to year MWH variability in spatial extension, intensity, duration and rate of evolution.

Specific work has also been done to investigate the impact of sea temperature trends and prominent climate modes, as El Nino Southern Oscillation (ENSO), in order to disentangle the slow-varying SST component and quasi-periodic oscillations from the abrupt changes that are characteristics of these extreme events.

A first preliminary analysis of the impact of MWHs on marine ecosystem will be presented.  

Up to date about the project research and results can be found visiting the  CAREHeat website (www.careheat.org) or on Twitter (@ careheat_)

How to cite: Santoleri, R. and the CAREheat team: Detection, characterization and trends of Marine Heat waves in the global worming scenario: the CAREHeat Project, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8961, https://doi.org/10.5194/egusphere-egu23-8961, 2023.

Southwest China (SWC) is vulnerable to disasters caused by extreme precipitation. This study investigates the mechanisms of low-latitude intraseasonal oscillations affecting regional persistent extreme precipitation events (RPEPEs) over SWC during rainy seasons. Most of the RPEPEs over SWC are dominated by 7–20-day variability. The RPEPEs over SWC are preconditioned by two different types of 7–20-day Rossby waves with almost opposite phases over the western North Pacific (WNP). The two types of 7–20-day Rossby waves have direct (indirect) effects on Type 1 (2) RPEPEs, respectively. For Type 1, a coupled 7–20-day low-level anticyclone and suppressed convection originating from the tropical WNP propagate northwestward and cover the region from the South China Sea (SCS) to the Bay of Bengal before the RPEPEs. The anticyclone triggers ascending motion over SWC and transports more moisture to SWC, favoring the SWC RPEPEs. Before the Type 2 RPEPEs, a coupled 7–20-day low-level cyclone and enhanced convection propagates from the tropical WNP to the SCS. The enhanced convection over the SCS leads to the westward extension of the western Pacific subtropical high (WPSH) and the eastward shift of the South Asian high (SAH). The variations in the WPSH and the SAH directly cause SWC RPEPEs by inducing ascending motion and transporting moisture. The mechanisms for Type 2 RPEPEs tend to work under the background with a strong WPSH. Using a Lagrangian model, we found that both the 7–20-day oscillations and their background atmospheric circulations result in significant differences in moisture sources for the two types of RPEPEs. These findings benefit a better understanding of SWC extreme precipitation events.

How to cite: Nie, Y.: Regional Persistent Extreme Precipitation Events over Southwest China under Different Low-Latitude Intraseasonal Oscillations during Rainy Season, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6377, https://doi.org/10.5194/egusphere-egu23-6377, 2023.

Heatwaves are expected to increase not only in intensity but also in frequency and duration in the next decades. Most of the studies are focused on the temperature variable, but little is known about their synoptic structure, which is especially important in mid-latitude regions.

In this study, we propose a Principal Sequence Pattern Analysis (PSPA) to classify the main synoptic patterns that define heatwaves in the northeast of the Iberian Peninsula. This is done by finding the most correlated input variables that represent the highest number of variance possible. The database used for this analysis comes from ERA5 reanalysis data covering the 1951-2020 period, in which we have selected three variables: mean sea level pressure (MSLP), geopotential height at 500 hPa (Z500) and maximum daily temperature at 2 meters (TMAX). Once the historical analysis is prepared, the same steps are repeated for CORDEX models (1951-2000) to discuss the performance of these models simulating heatwave periods.

The multivariate analysis has resulted in four synoptic patterns that explain more than 50% of the total variance. The four patterns are divided into two groups, stationary and dynamical. The HWs with highest temperatures in the Metropolitan Area of Barcelona are the prefrontal patters, which are dynamical and undulated at Z500 and undetermined at MSLP, with mean maximum temperatures around 35°C. However, the warmest pattern in inland areas is stationary and stable, generated at Z500 by intense anticyclonic ridges covering the Iberian Peninsula and at MSLP by deep thermal lows. The CORDEX models simulate similar patterns but less defined due to the lack of resolution. Z500 results in an overestimation of the anticyclonic ridges and MSLP shows an underestimation of pressure gradients, simulating more undetermined patterns. However, there are discrepancies between the models, which will result in different future projections in the climate change scenarios.

How to cite: Ventura, S., Villalba, G., Miro, J. R., and Peña, J. C.: Evaluation of the main heatwave patterns in the northeast of the Iberian Peninsula using ERA5 and CORDEX models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7451, https://doi.org/10.5194/egusphere-egu23-7451, 2023.

The Yangtze River Valley (YRV) is the main rice-growing region in China and agriculture production in YRV plays a vital role in ensuring food security. Spring is the key season of plowing preparation and sowing, and drought during this period could cause serious threats to agricultural activity in YRV. As a basic feature of drought, consecutive dry days (CDDs), especially the extreme-CDDs with long duration, could directly reflect the drying degree and serve as a good indicator of drought. Therefore, knowledge of variations and mechanisms of spring extreme-CDDs has significant implications for a comprehensive view of spring drought in YRV. Based on daily station precipitation data, the variability of spring extreme-CDDs in YRV is investigated. It is found that the extreme-CDDs in YRV experienced a significant decadal increase around the early 2000s. Associated with this decadal change, the Mongolian high (MH) and western North Pacific anticyclone (WNPA) are significantly intensified and weakened, respectively. The intensified MH and weakened WNPA lead to anomalous northerlies and water vapor divergence over YRV, providing favorable atmospheric conditions for more extreme-CDDs over the region. Further mechanism analyses suggest that the transition of mega-El Niño/Southern Oscillation (mega-ENSO) from the negative-phase to positive-phase contributes to the decadal weakening of WNPA. And the phase transition of Atlantic Multidecadal Oscillation (AMO) and decadal decrease of sea ice over the Barents Sea lead to intensified MH through exciting atmospheric wave train. Multiple linear regression shows that there could be a synergistic role of mega-ENSO, AMO, and sea ice over the Barents Sea in the decadal change in YRV extreme-CDDs around the early 2000s. Analysis on the simulation of 14 models in the Atmospheric Model Intercomparison Project (AMIP) experiment from phase 6 of the Coupled Model Intercomparison Project (CMIP6) shows that the models can reproduce the observed decadal intensification of MH and weakening of WNPA around the early 2000s, indicating the contribution of mega-ENSO, AMO, and sea ice over the Barents Sea to the decadal changes in MH, WNPA and extreme-CDDs in YRV.

How to cite: Zeng, Z.: Decadal change of spring extreme consecutive dry days in the Yangtze River Valley around the early 2000s: Synergistic effect of mega-El Niño/Southern Oscillation, Atlantic Multidecadal Oscillation, and Arctic sea ice, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11219, https://doi.org/10.5194/egusphere-egu23-11219, 2023.

In this study, the variations in the extreme high-temperature event (EHE) over Northern Asia (NA) and the associated possible mechanisms are explored. On an interdecadal timescale, NA EHE frequency experienced a significant interdecadal increase around the mid-1990s, which could be associated with the phase shift of the Atlantic Multidecadal Oscillation. On an interannual timescale, the first two empirical orthogonal function modes of the NA EHE frequency exhibit meridional dipole pattern (EOF1) and diagonal tripolar pattern (EOF2), respectively. Further analysis reveals that the EOF1 mode is related to the Polar-Eurasian teleconnection pattern (POL), while the EOF2 mode is associated with North Atlantic Oscillation (NAO) and Pacific-Japan/East Asia-Pacific pattern (PJ/EAP). The fitted EHE frequency based on the atmospheric factors (POL, NAO and PJ/EAP) can explain the interannual variation in the regionally averaged EHE frequency by 33.8%. Furthermore, three anomalous sea surface temperature (SST) patterns over the North Atlantic-Mediterranean Sea region and around the Maritime Continent are associated with the two EHE modes by intensifying the pronounced atmospheric teleconnections. Analysis on the simulation of five models in the Atmospheric Model Intercomparison Project experiment further confirms the impact of the pronounced SST patterns on the POL, NAO and PJ/EAP. From a synoptic perspective, the atmospheric patterns responsible for the NA EHE are investigated. By applying a hybrid regionalization approach to the daily maximum temperature, three subregions of NA can be identified: western NA, central NA, and southeastern NA. To better understand the mechanism for the formation of EHE in each subregion of NA, the EHE-related synoptic circulation patterns over each subregion are further categorized into two types. These six synoptic circulation patterns influence the NA EHE occurrence through different radiation and advection processes. From the forecasting perspective, six wave train patterns are explored as the precursors of the six synoptic circulation patterns, separately. These wave train patterns appear over the upstream regions of NA subregions with at least three-day lead, and provide potential forecasting information for the NA EHEs. The results may deepen our understanding of the NA EHE formation and provide information for the prediction and forecast of NA EHE.

How to cite: Hong, H.: Variations in Summer Extreme High-temperature Events over Northern Asia and the Possible Mechanisms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11276, https://doi.org/10.5194/egusphere-egu23-11276, 2023.

In the summer (July and August) of 2022, unprecedented heat wave occurred along the Yangtze River Valley (YRV) over East Asia while unprecedented flood occurred over western South Asia (WSA), which are located on the eastern and western sides of Tibetan Plateau (TP). By analyzing the interannual variability based on observational and reanalysis data, we show evidences that these two extreme events are mutually connected, and the anomalous zonal flow over subtropical Tibetan Plateau (TP) explains a major fraction the extreme events occurred in 2022. In summer, there is a warm center in the atmosphere over TP, and the isentropic surfaces incline eastward (westward) with altitude on the eastern (western) side of the warm center over TP. As adiabatic flow move along isentropic surfaces, anomalous easterly (westerly) flow generates anomalous descent (ascent) on the eastern side of TP and anomalous ascent (descent) on the western side of TP via isentropic gliding. The anomalous easterly flow is extremely strong to reverse the climatological westerly flow over subtropical TP in 1994, 2006, 2013 and 2022. The easterly flow in 2022 is the strongest since 1979, and it generates unprecedented descent (ascent) anomaly on the eastern (western) side of TP, leading to extreme heat wave over YRV and extreme flood over WSA in 2022. The anomalously strong easterly flow over subtropical TP in 2022 is dominated by atmospheric internal variability related to mid-latitude wave train, while the cold sea surface temperature anomaly over the tropical Indian Ocean increases the probability of a reversed zonal flow over TP by reducing the meridional gradient of tropospheric temperature.

How to cite: He, C., Zhou, T., Zhang, L., Chen, X., and Zhang, W.: Extremely hot East Asia and flooding western South Asia in the summer of 2022 tied to reversed flow over Tibetan Plateau, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15195, https://doi.org/10.5194/egusphere-egu23-15195, 2023.

Devastating floods in July-August 2022 led to one-third of Pakistan being under water. The rainfall over Pakistan in these months was extreme—four to six times the 30-year average. We investigate the cause of this historically unprecedented flooding and extreme rainfall using station measurements and reanalysis datasets. In July-August 2022 there was an abnormal distribution of south Asian summer monsoon (SASM) precipitation characterized by more precipitation in Southern Pakistan and Central India but less precipitation in the south of the western Tibetan Plateau. The abnormal distribution of monsoon rainfall was dominated by the weakening of the 200-hPa northern westerly winds and SASM, and associated with anomalous westward moisture transport in the south of the Tibetan Plateau. Moreover, the temperature of the western Tibetan Plateau reaches its peak in 2022. The “heat pump effect” of the Tibetan Plateau led to positive geopotential height anomalies over Pakistan and the western Tibetan Plateau in the mid-to-upper troposphere. This blocked the 200-hPa northern westerly winds and shifted them northward. At the lower troposphere, the easterly winds are enhanced, and the SASM is suppressed. Furthermore, the Tibetan Plateau warming caused increased glaciers melt and large amounts of meltwater that feed the upper Indus River and worsened the floods. In the context of global warming, summers temperatures and melting will increase in the Tibetan Plateau. Our results indicate that this will cause massive flooding over Pakistan, such as in 2022, to become a common occurrence.

How to cite: Li, H.: Tibetan Plateau warming-induced abnormal distribution of south Asian monsoon precipitation contributes to Pakistan's fatal flood in 2022, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10722, https://doi.org/10.5194/egusphere-egu23-10722, 2023.

Spatiotemporal features of summertime extreme precipitation occurrences (EPEs) over eastern China subregions were captured by using observations from the CN05.1 daily data set and K-means clustering from 1961 to 2018. Five subregions including South China (SC), the Yangtze River Basin (YRB), the HeTao Area (HTA), North China (NC), and Northeast China (NEC) are identified, which is distinct from previous studies. The accompanying evolution of synoptic development are discussed, including the high rates of EPEs transfer from the YRB-type to the SC-type (18%), the HTA-type to the YRB-type (16%), the HTA-type to the NC-type (22%), and the NC-type to the NEC-type (25%). Intrinsic relationships that exist within these types of regional EPEs have not been recorded by previous studies. The intraseasonal evolution of summer EPEs shows a northward migration of the rainbelt influenced by the East Asian summer monsoon system. Moreover, temporal variations of regional EPEs from interannual timescales to long-term trends are examined.

How to cite: Hao, X., He, L., Li, H., and Han, T.: How Do Extreme Summer Precipitation Events Over Eastern China Subregions Change?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10759, https://doi.org/10.5194/egusphere-egu23-10759, 2023.

Although northern Asia's temperature was the fourth highest on record, Northeast Asia was severely damaged agricultural and marine products due to the cold condition in April 2020. Previous studies have shown that the dipole atmospheric circulation over Siberia and the East Sea (also referred to as the Japan Sea) rendered this cold environment. Here we show that the atmospheric structure affecting the cold condition over northeast Asia was a mixed result of the East Atlantic/Western Russia (EAWR) pattern and blocking. The wave train was originated from the vorticity forcing of northwest/central Russia and propagated toward the southeast via the climatologically westerly and northerly flows. Furthermore, the blocking days over Siberia increased approximately ten times in April 2020 than climatology along with the easterly anomaly over Mongolia–northeast China. The blocking occurrence might be connected to wavy westerly at the high latitudes. The strong blocking and EAWR pattern led to the robust dipole atmospheric structure with the prevailing northerly wind in April 2020, thereby causing the cold over northeast Asia. Our results help to understand the cause of the cold condition in April over northeast Asia and its impact on the land and ocean ecosystems.

How to cite: Kim, G.-U., Oh, H., Kim, Y. S., Son, J.-H., Jeong, J., and Jeong, J.-Y.: Cause of the cold condition over northeast Asia in April 2020, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10834, https://doi.org/10.5194/egusphere-egu23-10834, 2023.

The summer extreme high temperature days (EHTDs) in the Northern Hemisphere have been frequently detected, posing a serious threat to the safety of human life, agricultural production, and the ecological environment of many countries. This study investigates the decadal variation of summer EHTDs in northern Eurasia (30°–70°N, 10°–130°E) during 1960–2018, using the EHTD index provided by Hadley Center and the atmospheric circulation and sea surface temperature (SST) data provided by NOAA. Statistical analysis shows that the first principal component of the EHTD index fluctuates slightly over a relatively low level during 1960–1994, while it increases significantly during 1995–2018. Moreover, Z-test and sliding t-test confirm that the decadal variations of the EHTD index in terms of trends and the climatological mean values change significantly around 1994/1995. Therefore, the total period is divided into two phases, i.e., fewer EHTDs and an insignificant trend during the period from 1960 to 1994, and more EHTDs with a significant increasing trend during the late period from 1995 to 2018. During 1960–1994 (1995–2018), low pressure and cyclonic (high pressure and anticyclonic) anomalies controlled Lake Baikal and the Caspian Sea, favoring more (less) cloud cover and precipitation, absent (sufficient) solar radiation and increased (decreased) EHTDs over there. Global warming and internal variability of the North Atlantic are both responsible for the decadal variations of EHTDs. On one hand, regression analysis shows that the global warming trend shows a significant influence on the positive pressure anomalies over the areas to the south of Lake Baikal. On the other hand, during 1995–2018, the anomalous Rossby wave activities induced by warmer than normal North Atlantic leads to high-pressure anomalies over the Caspian Sea, resulting in the significant anticyclonic anomaly over the area, which favors the more frequent occurrence of EHTDs than those during 1960–1994. Meanwhile, the Atlantic jet is located northward. The area around the Caspian Sea is to the right side of the jet stream exit. Under such a background, the negative vorticity advection at the upper-level troposphere would lead to the divergence anomaly and strengthen the sinking motion between lower- and higher- levels. Thus, the summer EHTD tends to be maintained over the Caspian Sea.

How to cite: Fan, Y.: Variations of summer extreme high temperatures in northern Eurasia during the recent decades, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10945, https://doi.org/10.5194/egusphere-egu23-10945, 2023.

In recent years, heatwaves around the globe have eclipsed long–term maximum temperature records. While a stronger warming over land than sea is both expected and observed, regional hot extremes are warming at an even faster pace. Modeling studies have suggested that soil moisture–temperature feedbacks drive this amplification in climate projections of the ongoing century, and there is solid observational evidence of a link between high temperatures and desiccating soils for individual events: dry soils invoke a shift in the surface energy partitioning toward sensible heating, thereby promoting higher air temperatures. A particularly notable heatwave unfolded in late June 2021 in the Pacific Northwest, baffling the scientific community with its high intensity. Using a combination of reanalysis data and factorial Earth System Model simulations, we show that heat released from condensation over the North Pacific and local soil moisture deficits strongly contributed to the extreme heat where the temperatures were most anomalous. Mediated by desiccating soils, our analysis also points to complex land–atmosphere interactions beyond intensified surface sensible heating. Since it remains unclear to what extent an enhanced thermodynamic potential — such as epitomized by this remarkable “black swan event” — is responsible for the observed exacerbation of heatwaves in recent decades, we we also investigate the link between summertime soil drought and hot extremes in our changing climate.

How to cite: Schumacher, D. L., Hauser, M., and Seneviratne, S. I.: Assessing the role of thermodynamic drivers underlying extreme heat in a warming climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11753, https://doi.org/10.5194/egusphere-egu23-11753, 2023.

Extreme events are rare atmospheric phenomena that cause significant damage to humans and natural systems, but detecting extreme events in the future in a changing climate can be challenging. Traditionally, temperature distributions were assumed to follow a normal distribution and certain thresholds were used to define extreme events. However, the mean and the variance of temperatures are expected to change in a future climate, which might limit the application of traditional methods for detecting extreme events.

We found that daily maximum surface temperature data can be described accurately using a multimodal distribution. In this study, we therefore used a statistical method called Gaussian Mixture Models (GMM) to fit a multimodal distribution to daily near-surface maximum air temperature data from simulations participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) for 46 Intergovernmental Panel on Climate Change (IPCC) land regions. GMM allowed us to use the parameters from the Gaussian distribution fitted to the higher temperatures to define the thresholds for the return period of extreme events. We analysed the change in the return periods of extreme temperature events in study regions compared to the historical period (1980-2010) under future Global Warming Levels (GWL) of 1.5°C, 2°C, 3°C and 4°C for each Shared Socioeconomic Pathways (SSP) scenarios. 

How to cite: Paçal, A., Hassler, B., Weigel, K., Kurnaz, M. L., and Eyring, V.: Detecting Extreme Temperature Events Using Gaussian Mixture Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13740, https://doi.org/10.5194/egusphere-egu23-13740, 2023.

Atmospheric blockings have widespread, long-lasting, and severe consequences in a variety of regions, causing climate extremes such as drought, heavy rainfall, cold spells, and heatwaves. Depending on where the blocking occurs, climate anomalies caused by high-latitude Euro-Atlantic winter blockings, in particular, have a significant impact on Arctic sea ice export in the North Atlantic, and thus on the freshwater budget in deepwater formation regions, and also impact Greenland ice cap and Arctic sea ice recovery during the cold polar season. Understanding the evolution of future winter Euro-Atlantic blockings in the context of climate change is thus critical for accurately predicting future changes in cryosphere systems and ocean circulation. The future evolution of these blockings, however, remains highly uncertain because coupled climate models generally fail to reproduce their frequencies of occurrence and spatial locations in historical runs. Meanwhile, recent research has shown that historical atmospheric blockings are much better simulated in climate models with eddy-permitting ocean resolutions due to more accurately represented mean climate states and air-sea interactions. Here, we show that eddy-permitting climate models provide blocking projections with much lower uncertainties in terms of frequency and spatial extent by using an ensemble of more than a hundred of CMIP6 climate model simulations, both ran with and without eddying ocean models. Finally, we show from the set of model simulations with eddying ocean models that the frequency of blocking types leading to dryer and warmer winter conditions in North Atlantic-Arctic regions for the next three decades is likely to increase under strong warming scenarios. Such an evolution in blocking activity would trigger large sea ice export events in the North Atlantic and a low rate of recovery of Artic sea ice and the Greenland ice cap during winter, leading to quicker ice loss in general than for climate models with standard ocean grids resolutions.

How to cite: Michel, S. L. L., von der Heydt, A. S., and Dijkstra, H. A.: New projections of winter Euro-Atlantic atmospheric blocking activity under strong warming scenario and consequences for Arctic land and sea ice, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15205, https://doi.org/10.5194/egusphere-egu23-15205, 2023.

Abstract

The empirical probability distribution of extreme precipitation in the eastern United States comprises heavy rainfall events stemming from the moisture held by the Atmospheric Rivers (ARs). In many sites, ARs trajectories can have varying impacts on the extreme precipitation seasonality based on the moisture source and tracks. Consequently, a characterization of location specific and regional patterns of timing of extreme precipitation caused by ARs and their non-stationarity has salience for both scientific and engineering concerns. To this end, analysis of annual maximum daily precipitation (AMP) at 581 long-term stations across the eastern United States was pursued in this study to evaluate the role of moisture sources and tracks in the seasonality of extreme rainfall-AR related events (AMP-AR) and their temporal changes over the 1950–2015 period. The key results from this study include: (a) spatio-temporal variation in the fraction of annual maximum precipitation events linked to ARs, and (b) a marked influence of moisture sources on the seasonality of AMP-AR related events. Results from this study have important bearing on the flood risk management and preparedness.

How to cite: Dhakal, N. and Aljoda, A.: Role of atmospheric moisture sources and pathways in the seasonality of extreme precipitation over the eastern United States, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16634, https://doi.org/10.5194/egusphere-egu23-16634, 2023.