Investigating The Key Atmospheric Circulations Of Winter Precipitation Anomalies Over The Tibetan Plateau And Its Response To Tropical Sea Surface Temperature

As one of the sensitive regions of climate change,the Tibetan Plateau(TP)has a complex topography with a special geographical location.TP is jointly influenced by the westerly climate,South Asian monsoon climate,and East Asian monsoon climate.The winter climate of TP is severely cold,mainly with solid precipitation.In recent years,the extreme snow events have become more frequently,which have brought many adverse influences on animal husbandry production,transportation,and herdsmen life.Therefore,investigating the precipitation variability over TP is of great significances for the climate change,ecological environment and social economy of both local and remote area.This paper focuses on the theme of the interannual variability of winter precipitation anomaly over TP.Firstly,statistical analysis and climate dynamics diagnostic methods are used to systematically analyze the main circulation characteristics that affect winter precipitation anomaly over TP from the perspective of atmospheric internal variability.Furthermore,it is found that the TP early winter precipitation anomaly with a strong interannual variability has a significant response to tropical sea temperature.The physical process of the anomalous response under different phases of the tropical Indian Ocean Dipole(IOD)and El Ni(?)o-Southern Oscillation(ENSO)is discussed,and numerical experiments are used for verification.Meanwhile,for the typical years of 2018,the characteristics of the anomalous circulation and water vapor transport are analyzed,and the evolutions of sea surface temperature in 2018 are combined to further verify the joint effect of IOD and ENSO on the early winter TP precipitation anomaly.Finally,the sea-air coupled models are used to evaluate the performances in predicting early winter precipitation anomaly over TP,analyze the model responses to sea surface temperature anomalies,and explore the sources of predictability.The major conclusions are summarized as follows:1.Wintertime precipitation characteristics of TP and its crucial effect factorsThe interannual variability of the wintertime precipitation over TP was strong.The snowfall is concentrated during late-December to mid-January,and the snowfall concentration showed a decreasing trend.The first mode of TP precipitation anomalies is regional uniformity,with an interannual cycle of 2-4 years and an interdecadal cycle of 14-20 years.In terms of the key circulation systems that affect TP precipitation,when the precipitation is uniformly high,the upper troposphere corresponds to the positive phase of the southern Eurasian(SEA)teleconnection,characterized as positive anomalies over the southwestern Europe,the Arabian Sea,and the northeast Asia,and negative anomalies over the Middle East and the Qinghai-Tibet Plateau,while the Middle East jet is stronger;the positive(negative)phase of North Atlantic Oscillation(NAO)is conducive to more(less)precipitation over TP,via modulating the SEA teleconnection and key circulation systems such as the Middle East jet stream.2.Response of interannual precipitation anomaly of TP to tropical SST in early winterThe interannual variability of TP precipitation in early winter(NovemberDecember)is strong.The above-normal TP precipitation in early winter is accompanied by a wave train spreading from Western Europe to East Asia.The ascending motions over TP and the enhanced water vapor transports over southwest China are significantly influenced by anomalous cyclone.The interannual variability of TP precipitation in early winter exhibits a significant response to the tropical Indian Ocean and Pacific Ocean.The years with above-normal precipitation are mostly concentrated in the years when the Ni(?)o 3.4 and the Indian Ocean Dipole(IOD)index are both positive,while the years with below-normal precipitation are mostly negative for both.When both IOD and ENSO are in positive phase,a Eurasian(EU)teleconnection wave train occurs in middle and high latitudes,and the cyclonic circulation anomaly and ascending motions are obvious over TP.The common and independent effects of IOD and ENSO show that IOD plays a dominant role in the early winter TP precipitation;the CAM5 numerical model is used to verify that the EU-like teleconnection wave train affects the physical process of the early winter TP precipitation under the background of the warm phase of IOD and ENSO.3.Causes of the Extreme Snowfall Anomaly over the Qinghai Plateau in Early Winter 2018An extremely heavy snowfall event occurred in early winter(November-December)of 2018 over TP broken the record,where the accumulated snowfall was three times more than normal,and the snowfall days were more than one times.During the period,the circulation anomalies showed that the polar vortex in the northern hemisphere was anomalously strong and expanded to the south,leading to the continuous development of the low trough from Lake Balkhash to Lake Baikal(EU index is the second lowest since 1961).The meridian of Eurasian circulation tended to be strong.With the phased increase of the Siberian high,cold air from the polar regions and high latitudes entered the northeastern part of TP along the north and northwest paths,and the upper and middle troposphere were respectively cold and warm.Controlled by advection,the difference in vertical structure increased the atmosphere baroclinicity and caused the unstable atmosphere stratification,providing better dynamical conditions for the extreme snow event.The positive phases of El Ni(?)o and IOD both had a significant impact on TP snowfall in early winter of 2018.Under their combined effect,the EU-like wave train presented extremely negative phase,which is conducive to the westward extension of the western Pacific subtropical high.Both water vapor transport and mid-latitude zonal water vapor transport increased significantly.The water vapor input on the western and southern borders of TP was significantly enhanced,and the net water vapor input ranked second in history.Specially,the ENSO warm phase was conducive to anomalous anticyclone over the Philippines,and the warm IOD phase exacerbated the southwesterly wind and enhanced the meridional water vapor transport.The combined effect of ENSO and IOD increased the water vapor input from the western and southern borders of TP,which provided abundant water vapor conditions in early winter.4.Evaluation of the model ability to simulate precipitation over the PlateauBased on the hindcasts of four datasets of seasonal prediction model systems,including BCC?CSM,ECMWF?System5,CFSv2,JMA?MRI?CGCM,the prediction performance was evaluated from the deterministic perspective.Overall,BCC?CSM had the best performance on predicting the spatial distribution and temporal variation of precipitation climatology in early winter.For the interannual variability of precipitation,JMA?MRI?CGCM performed best,followed by BCC?CSM.All the models had a good ability to simulate the regional-uniform precipitation,among which BCC?CSM had the highest time correlation coefficient with the observbation,and JMA?MRI?CGCM had the highest pattern correlation coefficient.Although all models could well reproduce the north-south reverse mode,BCC?CSM was the closest to the actual spatial pattern and interannual variation.The reasons for the better performance of BCC?CSM are the high prediction skills for the EU teleconnection and western Pacific subtropical high,and the accurate simulations of the physical processes of ENSO and IOD that affect early winter TP precipitation.