In the first half of winter 2020/21, China has experienced an extremely cold period across both northern and southern regions, with record-breaking low temperatures set in many stations of China. Meanwhile, a moderate La Niña event which exceeded both oceanic and atmospheric thresholds began in August 2020 and in a few months developed into its mature phase, just prior to the 2020/21 winter. In this report, the mid−high-latitude large-scale atmospheric circulation anomalies in the Northern Hemisphere, which were forced by the negative phase of Arctic Oscillation, a strengthened Siberian High, an intensified Ural High and a deepened East Asian Trough, are considered to be the direct reason for the frequent cold surges in winter 2020/21. At the same time, the synergistic effect of the warm Arctic and the cold tropical Pacific (La Niña) provided an indispensable background, at a hemispheric scale, to intensify the atmospheric circulation anomalies in middleto-high latitudes. In the end, a most recent La Niña prediction is provided and the oncoming evolution of climate is discussed for the remaining part of the 2020/21 winter for the purpose of future decision-making and early warning.
The prediction skill and source of the predictability of the East Asian summer monsoon (EASM) system are examined in this work based on four state‐of‐the‐art seasonal climate forecast models including BCC_CSM1.1, ECMWF_SYS4, NCEP_CFS2 and TCC_CPS2. The prediction of the climatology and interannual EASM pattern and the impact on the prediction are further investigated. It is noted that the four models have some skill in predicting summer rainfall in the East Asia, however, the skill is low on average and also largely regional dependence. The interannual variation of EASM measured by monsoon circulation index is well reproduced, implying that the broad‐scale feature/pattern of EASM has higher predictability than the detailed spatial variation of EASM rainfall. The possible sources of predictability of the interannual variability of EASM are associated with the El Niño‐Southern Oscillation (ENSO) and the north Indian Ocean (NIO) sea surface temperature (SST) anomalies. The correlation pattern of rainfall with the NIO SST is characterized by a tripole pattern from south to north of East Asia, which is different from the correlation distribution of the southern‐northern dipole with ENSO, suggesting that NIO SST may exert influence on the EASM independently. The major biases in climatology of EASM in the models are the northward shift of the western Pacific subtropical high (WPSH) and weak monsoonal southerly over the coast of East Asia, which leads to the prediction bias of the Meiyu/Baiu/Changma (MBC) rainfall belt. The prediction of the interannual EASM pattern presents two deficiencies: too weak rainfall variability and northward shift of the dipole rainfall pattern (opposite variation between MBC and the northwestern Pacific), that may be caused by the biases of WPSH in the models.
A record-breaking heat wave hit the Yangtze River valley during the boreal summer of 2022, and caused severe social and economic losses. One prominent feature of this long-lived heat event was its persistence and abnormal intensification in August. This study investigated the physical mechanisms be responsible for the intensification of this heat event in late summer under the background of a La Niña event. The prolonged heat event was directly related to the intensification and westward extension of the western North Pacific subtropical high, which can be attributed to the synergistic effects of an anomalous western North Pacific anticyclone and the eastward extension of the South Asian high in the upper troposphere. The anomalous anticyclone in the western North Pacific, which was induced by negative sea surface temperature anomalies in the central tropical Pacific, strengthened in August. The positive sea surface temperature anomaly in the western Pacific warm pool and enhanced in-situ convection led to anomalous high pressure over the Yangtze River valley via the local meridional circulation. Atmospheric convergence and descending motion over the Yangtze River valley was amplified in August as a result of the zonal shift in the South Asian high from the Iranian Plateau to the Tibetan Plateau. The Silk Road pattern index of August 2022 was the lowest since the 1990s. The abnormal negative phase of the Silk Road pattern contributed to both the zonal shift in the South Asian high and the westward extension of the western North Pacific subtropical high, which led to the abnormal intensification of the heat event over the Yangtze River valley in August 2022.
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