Extreme persistent rainfall poses serious impacts on human and natural systems, predominately through its related hydrogeological disasters. Due to sustained heavy downpours, the summer of 2020 was the second wettest on record over Northeast Indian subcontinent since 1901. Here, we find that this orographically anchored extreme rainfall event was largely associated with the anomalous anticyclone (AAC) over the Indo‐Northwest Pacific region and La Niña‐induced Walker circulation intensification. The overall effect of anthropogenic forcings contributed little to the occurrence probability of this event, because the warming and wetting effects of greenhouse gases were almost negated by anthropogenic aerosols. Climate models project a prominent increasing trend of such extreme event under future greenhouse‐induced warming due to increase in atmospheric water vapor and 2020‐like AAC frequency. Our findings thus call for scaling up climate change adaptation efforts for increasingly extreme persistent rainfall in highly populated but low‐resilience South Asian developing countries.
Eastern China (EC) suffered an extreme drought with long-lasting duration and record breaking intensity in late summer-autumn 2019. Our diagnosed results show that the central Pacific (CP) El Niño, in tandem with warm sea surface temperature anomalies (SSTAs) over the Kuroshio extension (KE) region, induces the meridionally elongated cyclonic circulation anomalies stretching from the western North Pacific (WNP) to the Yellow Sea. Its western flank corresponds to overwhelming low-level northerly wind anomalies over EC, which result in deficient moisture and anomalous descent over EC and hence cause the extreme drought in 2019. To investigate the relative contributions of SSTAs over different regions, we performed sensitivity experiments, and analyzed the relationship between the extreme drought like 2019 (2019Drought-like event) and the SSTAs in CMIP6 historical simulations. Modelling evidences reveal that both warm SSTAs over the central-equatorial Pacific and the KE region are indispensable for shaping the meridionally elongated cyclone anomaly. Specifically, the cyclone anomaly over the WNP induced by CP El Niño aligns with the cyclone anomaly over the Yellow Sea induced by the warm SSTAs over the KE region, merging into a meridionally stretched cyclone anomaly to the east of EC. Consequently, the northerly anomalies stretch across EC, leading to unfavorable atmospheric conditions and the rainfall deficit there. Projection results show the occurrence probability of 2019Drought-like event will increase by 20% (decrease by 40∼50%) under high (medium-low) emission scenario compared to present day climate, indicating the nonlinear response of extreme drought to different emission scenarios and the urgency of carbon emission reduction.
Long-term winter and summer Madden-Julian Oscillation (MJO) trends in the past 138 years (1871-2008) were examined using National Oceanic and Atmospheric Administration (NOAA) 20th Century Reanalysis V2c dataset. It is found that MJO shows a distinctive different trend between boreal winter and summer. While the MJO intensity in both boreal winter and summer has a rising trend, the winter trend is much greater than the summer trend. As a result, the winter-summer difference shows a significant increasing trend. The distinctive winter and summer trends are attributed to the difference of atmospheric background circulation (such as vertical velocity) and static stability responses to the global warming between boreal winter and summer over equatorial eastern Indian Ocean. In boreal winter, both the surface moistening and strengthened inter-tropical convergence zone convection contribute to an increase of MJO activity. This is in contrast to boreal summer when a greater static stability and anomalous subsidence tend to offset the moistening effect, leading to a relatively weaker increase of the MJO activity.
With the recurrence of high-impact extreme weather and climate events and the growing demands by the public to know the causes after the events, event attribution has emerged as a frontier of climate change research. Typically, an event attribution study focuses on one individual extreme event that has just occurred. Studies rarely examine human influence on multiple extreme events in different times of the past. Here we conduct a comprehensive attribution analysis on a number of the heaviest precipitation events in the Yangtze River Valley (YRV) during the past 100 years. We start by defining extreme precipitation events as the heaviest precipitation over a fixed area size that is of direct relevance to flood preparedness and management. When examining the events over the historical time, we allow the precise locations of the area to change in different years. Four extremely strong events are identified, and they happened in the summer of 1931, 1954, 1998 and 2020. A clear difference in the impacts of greenhouse gases (GHGs) and anthropogenic aerosols (AAs) on the events was found. Their impacts were negligible in the early period and became more and more discernible since the late 20th century. The temporal change of human influence on the events shows gradual strengthening impacts of GHGs and AAs with time. Their competing effects led to a slight human influence and then gradually increasing influence on extreme precipitation after the 21st century. GHGs have exerted a larger influence on short-duration precipitation events while AAs have had a larger influence on monthly mean precipitation. The more extreme the precipitation event, the clearer the anthropogenic influence.
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