Intraseasonal Variation Of Winter Meridional Wind In East Asian Monsoon Region And Its Possible Mechanism

Based on the ERA-interm reanalysis data from 1979 to 2013 year,analyze systematically the intraseasonal variation and its possible mechanism of winter meridional wind in East Asian monsoon region.The results have shown that winter meridional wind anomaly in East Asian monsoon area in Southern China area changes significantly,the oscillation period is 10-20 days(quasi-biweekly oscillation QBWO).On a quasi-biweekly scale,850 hPa anomalous north wind shows the characteristics of the spreading from the high latitude to the low latitude,anomalous meridional wind near 60�N spreads in the direction of southeast,the weak anomalous meridional wind near 30�N in subtropical zone spreads eastward,they converge in South China.There exists conversion from basic air flow to quasi-biweekly scale fluctuation energy,therefore,anomalous meridional wind will enhance significantly in Southern China.There are two large-value centers with long-term QBWO at 300 hPa over the eastern part of China.The volatility in the subtropical region is more pronounced than that at 850 hPa.The two centers merge in the eastern part of China,and the convergence of the fluctuations leads to an increase in fluctuation energy.There are obvious interannual oscillations in South China,and the number of occurrences of intraseasonal oscillations in South China tends to increase in the past 34 years.The main causes of this interannual variation in the meridional quasi-biweekly oscillation in winter mainly include the following:the main cyclical variations of meridional winds in the strong years are significantly different in 10-20 days,while there are two cycles in the weak years of typical oscillations.They are 30-60 days and 10-20 days,respectively.There are significant energy conversion centers in the typical strong oscillations in South China,providing energy to the quasi-biweekly wind field for the basic air flow.There is a significant positive correlation between SST in the Indian Ocean and the extension zone of Kuroshio in the strong years.The anomalous warming of SST results in high altitude being affected by high pressure.The 500 hPa circulation field in South China is located between the abnormal low pressure trough and the abnormal high pressure ridge.At this time,the average wind field is an abnormally northerly wind.Under such a guiding air flow in the average wind field,it will be easier to guide more anti-cyclones to South China.The intraseasonal variation of meridional wind in winter has a significant effect on the period of extreme rain and snow events.In the early period,southern China was mainly affected by the meridional wind that oscillated from an abnormal southerly to an abnormally northerly wind during the 10-20-day oscillation period,and was mainly dominated by severe cooling weather;in the later Period,it was mainly affected by the 30-60-day and 10-20-day.30-60 days abnormal northerly wind in north of 3 0oN,gradually shifts southwards and controls most of southern China.Abnormal southerly winds maintain south of 30�N,which is conducive to the continuous transport of water vapor to southern China,producing strong precipitation.The two are superimposed on top of each other,which together provide favorable conditions for low-frequency circulation in areas where extreme precipitation events occur in South China,resulting in extreme rain and snow in southern China.