Southeast Australia (SEA) experienced a wet February as well as an extremely wet March accompanied by devastating floods during 2021. Regional water vapor balance analysis at different levels indicates the leading role of water vapor inflow through zonal boundaries during February, and the dominant contribution of water vapor input through meridional boundaries during March, providing adequate anomalous moisture for abnormal precipitation. The horizontal distribution of vertically integrated water vapor flux is characterized as an anomalous cyclonic circulation pattern around the Tasman Sea and SEA, responsible for the intensified water vapor transport along northwesterlies from the tropical Indian Ocean and along anomalous onshore easterlies from the Tasman Sea during both months. Partition of the contributions of dynamic and thermodynamic processes to the anomalous atmospheric water vapor flux reveals the dominant role of the anomalous wind field, but the anomalous variation in the moisture field also plays a part in the water vapor convergence for SEA. The presence of upper and lower large-scale atmospheric circulations ascertains that cyclonic water vapor flux is attributed to a dominant equivalent-barotropic cyclone system over SEA. The plausible joint impacts of internal forcing from the positive Southern Annular Mode (SAM) oscillation, and external forcing from La Niña, are further confirmed by composite analysis; a La Niña-induced low-pressure system dominates the lower level over the Australian continent, and the SAM-caused anomalous cyclonic disturbance propagating from higher latitudes governs the higher level above southern Australia, leading to the important equivalent-barotropic cyclonic circulation just above the region of interest.
This study analyzes the change characteristics of compound extreme events (CEEs) of temperature and precipitation (including warm‐wet, warm‐dry, cold‐wet and cold‐dry) in China on interannual and interdecadal scales between 1901 and 2019. The results demonstrate a long‐term increasing trend and interdecadal oscillations in CEEs total frequency. However, the frequency of each type of CEEs changes in a different manner compared with total CEEs frequency. There are fewer CEEs but increasing warm‐dry during 1901–1950. The period 1951–1995 are characterized by frequent cold CEEs (cold‐wet and cold‐dry), cold‐wet are largely distributed in most areas except for northeast and coastal areas of China, while cold‐dry are distributed in most areas except for the northwest regions of China. There are frequent warm CEEs (warm‐wet and warm‐dry) and fewer cold CEEs during 1996–2019. Warm‐wet frequently occurs in the Tibetan Plateau and northwest China, and warm‐dry mainly concentrates in southwest and northern China during this period. The frequency of warm‐dry and cold‐wet were higher than that of warm‐wet and cold‐dry over the past 119 years, whereas warm‐wet increased fastest in the northwest region after 1996, consistent with the warming and wetting characteristics in the northwest region of China. Further study show that long‐term change and low frequency oscillations have the greatest impact on CEEs among different time scale factors. Furthermore, the temperature rise caused by climate change affects the interdecadal characteristics of CEEs in China through the changes of circulation fields such as East Asian trough and subtropical high and the configuration between them.
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