Attribution of Persistent Precipitation in the Yangtze–Huaihe River Basin during February 2019

In February 2019,a month-long persistent precipitation event occurred in the Yangtze–Huaihe River basin.The geopotential height field that affected the duration of this frontal rainfall was divided into a high-latitude part and a lowlatitude part for analysis.In the high-latitude part,a two-wave structure led to quasi-stationary circulation,and the change of both the blocking high pressure and Arctic Oscillation phase caused cold air to invade South China continuously and changed the frontal position.In mid-to-low latitudes,the persistent precipitation showed quasi-biweekly oscillation characteristics.The so-called“subtropical high–precipitation–anticyclone”(SHPA)feedback mechanism blocked the circulation systems in the mid-to-low latitudes and provided a continuous supply of water vapor for precipitation.As for the effect of sea surface temperature,the western North Pacific anomalous anticyclone stimulated by El Niño strengthened the intensity of the southerly wind and provided support for the redevelopment of the anticyclone system in the SHPA feedback mechanism.The sea surface temperature anomaly in the South China Sea provided sensible heating for precipitation,and convergent rising airflow was conducive to the occurrence of precipitation.Additionally,the SHPA mechanism provides a reliable basis for the prediction of persistent precipitation in winter in the mid-to-low latitudes.

Changes in Summer Persistent Precipitation over the Middle–Lower Reaches of the Yangtze River and Associated Atmospheric Circulation Patterns

Persistence is an important property of precipitation and its related impacts. However, changes in persistent precipitation and the possible underlying mechanisms in the context of global warming have not yet been discussed in sufficient depth. In this study, the changes in persistent precipitation in summer and related atmospheric circulation patterns over the middle–lower reaches of the Yangtze River(MLRYZR)—a typical monsoon region frequently hit by consecutive rainfall events—are analyzed based on observed daily precipitation and NCEP/NCAR reanalysis data from 1961 to 2019. The results reveal that persistent precipitation events(PPs) tend to happen in a more persistent way, with increased frequency and intensity in the MLRYZR region. Mechanism analyses show that persistent precipitation has happened along with simultaneous enhancement of some large-scale atmospheric circulation patterns,including the Lake Baikal blocking(BB), the Okhotsk Sea blocking(OB), and the western Pacific subtropical high(WPSH). Such enhanced anomalous circulation patterns could persistently reinforce the convergence and supply of water vapor in the MLRYZR region, contributing to the increase in PPs in this region. Based on the above results, we are able to offer some new insights into the long-term changes in precipitation structure and the possible causes. This study is also expected to support attribution studies on regional precipitation changes in the future.

A Comprehensive Classification of Anomalous Circulation Patterns Responsible for Persistent Precipitation Extremes in South China

Based on observational precipitation at 63 stations in South China and NCEP NCAR reanalysis data during 1951 2010,a cluster analysis is performed to classify large-scale circulation patterns responsible for persistent precipitation extremes（PPEs） that are independent of the influence of tropical cyclones（TCs）.Conceptual schematics depicting configurations among planetary-scale systems at different levels are established for each type.The PPEs free from TCs account for 38.6%of total events,and they tend to occur during April August and October,with the highest frequency observed in June.Corresponding circulation patterns during June August can be mainly categorized into two types,i.e.,summer-Ⅰ type and summer-Ⅱtype.In summer-Ⅰ type,the South Asian high takes the form of a zonal-belt type.The axis of upstream westerly jets is northwest-oriented.At the middle level,the westerly jets at midlatitudes extend zonally.Along the southern edge of the westerly jet,synoptic eddies steer cold air to penetrate southward;the Bay of Bengal（BOB） trough is located to the north;a shallow trough resides over coastal areas of western South China;and an intensified western Pacific subtropical high（WPSH） extends westward.The anomalous moisture is mainly contributed by horizontal advection via southwesterlies around 20°N and southeasterlies from the southern flange of the WPSH.Moisture convergence maximizes in coastal regions of eastern South China,which is the very place recording extreme precipitation.In summer-Ⅱ type,the South Asian high behaves as a western-center type.The BOB trough is much deeper,accompanied by a cyclone to its north;and a lower-level trough appears in northwestern parts of South China.Different to summer-Ⅰ type,moisture transport via southwesterlies is mostly responsible for the anomalous moisture in this type.The moisture convergence zones cover Guangdong,Guangxi,and Hainan,matching well with the areas of flooding.It is these set combinations among different systems at different levels that trigger PPEs in South China.

Changes in Persistent Precipitation in Northwest China and Related Large-Scale Circulation Features

Based on China's daily precipitation data of 2415 stations and ERA5 hourly reanalysis data from 1961 to 2019, the station-based and regional precipitation events over Northwest China(NWC) are identified and sorted into persistent precipitation(PP, duration ≥ 2 days) events and non-persistent precipitation(NPP, duration = 1 day) events;and then changes in the persistence structure of the PP and NPP events over NWC and the long-term mean adjustment of the related large-scale circulation configuration are analyzed. The results show that PP and NPP both witness an increasing trend over most parts of NWC. In terms of the total precipitation at most stations and the regional mean, contributions from PP have been increasing, while those from NPP have been decreasing. This demonstrates that the wetting trend in NWC is mainly caused by the increase in PP. Through analyzing the large-scale circulation corresponding to regional PP events at several representative levels, we found that the westerly jet at 200 hPa, the ridge/trough systems at 500 hPa, and the Mongolian low at sea level are the key circulation systems responsible for regional PP events over NWC. As for long-term mean changes after and before 1990(a shifting point recognized by previous studies), it is found that the extent of the South Asian high becomes larger and the westerly jet shifts northward by approximately 1.5 degrees in the upper troposphere. The ridge near the Ural Mountains and the ridge downstream of NWC strengthen by approximately 10–30 hPa at 500 hPa. Furthermore, the difference between the Mongolian low trough and its surrounding high pressure increases by approximately 2 hPa at the sea level. The combinations of circulation changes from upper to lower levels facilitate the strengthening of ascending motions. These adjustments in circulations create more favorable conditions for PP to occur over NWC in the last three decades.

Changes in Persistent and Non-Persistent Flood SeasoPrecipitation over South China During 1961–2010

The characteristics and possible causes of changes in persistent precipitation（PP） and non-persistent precipitation（NPP） over South China during flood season are investigated using daily precipitation data from 63 stations in South China and NCEP/NCAR reanalysis data from 1961 to 2010. This investigation is performed using the Kendall＇s tau linear trend analysis, correlation analysis, abrupt climate change analysis, wavelet analysis, and composite analysis techniques. The results indicate that PP dominates total precipitation over South China throughout the year. The amounts of PP and NPP during flood season vary primarily on a 2–5-yr oscillation. This oscillation is more prominent during the early flood season（EFS; April–June）. NPP has increased significantly over the past 50 years while PP has increased slightly during the whole flood season. These trends are mainly due to a significant increase in NPP during the EFS and a weak increase in PP during the late flood season（LFS; July–September）. The contribution of EFS NPP to total flood season precipitation has increased significantly while the contribution of EFS PP has declined. The relative contributions of both types of precipitation during LFS have not changed significantly. The increase in EFS NPP over South China is likely related to the combined efects of a stronger supply of cold air from the north and a weaker supply of warm, moist air from the south. The increase in NPP amount may also be partially attributable to a reduction in the stability of the atmosphere over South China.

Low-Frequency Oscillations of the East Asia-Pacific Teleconnection Pattern and Their Impacts on Persistent Heavy Precipitation in the Yangtze-Huai River Valley

Based on the daily reanalysis data from NCEP NCAR and daily precipitation data from the China National Meteorological Information Center,an ensemble empirical mode decomposition method is employed to extract the predominant oscillation modes of the East Asia Pacific（EAP） teleconnection pattern.The influences of these low-frequency modes on persistent heavy precipitation in the Yangtze Huai River（YHR）valley are investigated.The results indicate that the EAP pattern and rainfall in YHR valley both exhibit remarkable 10 30- and 30 60-day oscillations.The impacts of the EAP pattern on the YHR persistent heavy precipitation can be found on both the 10 30- and 30 60-day timescales the 10 30-day scale for most cases.Composite analysis indicates that,on the 10 30-day timescale,formation of the EAP pattern in the lower and middle troposphere is determined by convective systems near the tropical western Pacific;whereas in the middle troposphere,the phase transition is jointly contributed by both the dispersion of zonal wave energies at higher latitudes and convective systems over the South China Sea.In the context of the10 30-day EAP pattern,the anomalously abundant moisture is transported by an anomalous subtropical anticyclone system,and strong moisture convergence results from that anomalous anticyclone system and a cyclonic system in the midlatitude East Asia.Such a combination of systems persists for at least three days,contributing to the formation of persistent heavy precipitation in the YHR valley.