Simulation And Prediction Of The North Jump Of The West Pacific Subtropical High In Summer

We calculated ridge index since 1979 to 2014,area index and west ridge point index from NCEP/NCAR Reanalysis ? and HadlSST data set,giving analysis about abnormal activity of subtropical high.We blamed variation of subtropical high on three kinds of field and discuss them separately,which contain wind,geopotential height and vertical speed.By doing regression with time of first northern jump and temperature of sea water,building prediction equations in key area of oceanic temperature,we got such conclusions from oceanic and atmospheric analyses:Subtropical high has larger area on year of positive phase than in negative phase,whose west ridge point is much more western,ridge is much more southern.This is same as what happens when tropical Eastern Pacific and Indian Ocean are abnormally warmer.The whole flux of vapor can only reach Middle and lower reaches of the Yangtze River in this time,leads subtropical high to south.On negative phase year,it can be delivered to North China,where latent heat releases,south wind builds up,and leads subtropical high to north.On positive phase year,summer anomaly field of vector wind exists anti-cyclonic circulation in low latitudes area of East Asia,weakens the subtropical monsoon circulation in East Asia;while in middle latitude area cyclonic circulation,strengthen circulation in frontal zone of Meiyu.The distribution of anomaly wave train in 500 hPa potential height anomaly field from south to north acts as “+”,“-”,“+”,and high is stopped by Okhotsk Sea,leads subtropical high to south.Characters in negative phase year are opposite with on positive phase year.200 hPa high in South Asia goes east along 30oN,causing divergence and rising in north of 30oN,and also convergence and subsidence in south of 30oN;convergence and subsidence airflow in high altitude of Jiangnan diverges in middle and low altitude,which strengthens locally negative vorticity,inducing the subtropical high goes west.Besides,positive anomaly area of vertical speed exists in East Asia topical area and in middle latitude area negative area exists,which claims convergence and rising motion becoming weaker in slot region of tropical monsoon in East Asia,stronger in frontal zone of Meiyu.At this time,summer monsoon in East Asia is much more weaker,which benefits subtropical high going west.In July,August and September of last year,the relationship between time of first north jump of subtropical high with temperature of seawater are globally less.Temperature in west of Indian Ocean influences most in February,while in EquatorPacific influences most in July.Thus it is reasonable that we choose 95o~115oE,10o~60oS as temperature in west of Indian Ocean,120o~170oW,5oS~5oN also known as Nino 3.4 index to predict time of first north jump of subtropical high.After disposing the tendency that the time of first north jump of subtropical high comes advanced,the regression prediction equation satisfies original time sequences very well.There are anomalies in distribution of seawater temperature in 1991 and 1998.In1991,positive anomaly of seawater temperature in middle and high latitude area was in Okhotsk Sea,and the low latitude one was in middle of Equator Pacific;in Indian Ocean it was continually higher than average while lower than in Pacific Ocean.In1998,temperatures in Pacific Ocean of middle and high latitude showed negative anomaly,positive anomaly in West Pacific Ocean and apparently in Equator East Pacific;in Indian Ocean it kept positive anomaly from February to June.Strong El Nino influenced fist north jump of subtropical high intensively.It was affected by200 hPa South Asia high pressure in high layer,strong westerly zone,latent heat release of 500 hPa precipitation area in middle layer and southerly component on west side of WPSH.In May,year of 1998,east ridge point of South Asia high pressure in high layer went eastern with much more area;center of strong westerly zone preferred north with larger coverage;the distribution of precipitation benefited north jump of subtropical high;south component of western of WPSH in low layer was stronger.In May,year of1991,high pressure in South Asia didn't come up,strong westerly zone preferred north with larger coverage;latent heat released in north of subtropical high in middle layer,which had benefits of subtropical high moving to north;latent heat release got suppression in the places that WPSH existed,which influenced north jump of WPSH.From so far,we can get the conclusion that the variation of atmosphere activity influences time of north jump of WPSH in different aspect.