Sub-seasonal Evolution Of EAP Pattern And Its Relationship With East Asian Atmospheric Circulation

The East Asian Pacific(EAP)pattern is one of significant factors affecting East Asian summer atmospheric circulation and climate anomalies.It is important to understanding the sub-seasonal evolution characteristics of EAP pattern and revealing East Asian atmospheric circulation changes corresponding to EAP pattern evolution.Based on daily NCEP/NCAR reanalysis dataset and the surface temperature and precipitation data from 756 China stations,this paper discusses the sub-seasonal evolution characteristics of EAP pattern during its development stage,peak time and weakening stage.Futhermore,this paper also discusses the impact of polar j et and subtropical jet in the relationship between EAP and climate anomalies over China.The main conclusions are as follows:1.The sub-seasonal evolution characteristics of EAP patternBased on daily reanalysis of the geopotential height data,the EAP pattern typical events were selected according to the EAP pattern index and it reveals the sub-seasonal evolution characteristics of EAP pattern.During the EAP pattern development stage,anomalous signals appear in Philippines.2-3 days latter,anomalous centers appear in the east China and Japan region and the Okhotsk Sea region,showing negative-positive-negative or positive-negative-positive tripolar distribution.A typical meridional structure occurs at the peak time,and the three anomalous centers weaken almost simultaneously during the weakening stage and disappear 3-4 days latter The strongest center is at 500hPa heigh and there are quasi-barotropic systems.In the thermal structure there is significant positive and negative inverse distribution among three centers during the evaluation of EAP pattern related to the anomalies of Philippines.Based on daily TN wave activity flux,it is found that the development and maintenance of EAP is related to the dispersion of the Rossby wave from Philippines to the mid-high latitudes.2.The evolution of East Asian atmospheric circulation corresponding to EAP patternBased on daily reanalysis of wind and geopotential height dataset,it is analyised that the evolution of the East Asian atmospheric circulation system corresponding to the sub-seasonal evolution of EAP pattern.From the developing stage to the peak time during EAP positive events,the low-level westerly winds over Philippines enhances,the easterly wind in the tropopause weakens.The west wind maximum wind speed center over the east China and Japan region weakens and its position moves northward,while the maximum wind speed center of the westerly wind over the Okhotsk Sea region weakens and the zonal wind weakens at the upper troposphere in the south of 15N.The position of the ascending motion center moves to the north over the low latitude area,and the ascending center moves to the Okhotsk Sea region and the position of the descending center unchanges.There is an opposite trend from the peak time to the weakening stage during uring EAP positive events.As for EAP negative events,the zonal wind weaken in the upper troposphere over Philippines,and the meridional wind weakens in the upper troposphere of 15-30N over the east China and Japan region.During the EAP positive events,the South Asian high ehances in the east,the western Pacific subtropical high weaken and its position retreated eastward,and the ridge over 125E develops.During the EAP negative events,the South Asian high weakens in the east,the western Pacific subtropical high enhances and its position moves weatward,and the trough over 125E develops.Through lead-lag synthetic analysis,it is found that the development of EAP positive events is related to the eastward movement of the ridge-trough systems.3 Connection between EAP pattern and concurrent variation of the upper-level jests over East AsiaThe variation of East Asian streams connected with EAP pattern is investiged based on daily NCEP/NCAR reanalysis dataset.The EAP positive event is accompanied by a weaker polar j et with migrating northward and a weaker subtropical jet.It is obivious that the subtropical j et is separated into two parts over land and ocean.The subtropical jet over the land moves northward near the 135E.The EAP negtive event is accompanied by a strong polar jet with migrating southward and a strong subtropical jet.The subtropical jet over the land enhances with its position migrating southward whihc merges with the subtropical jet over the ocean into a strong jet stream.The center of baroclinic growth rate coincides with the center of the j et stream.There is a negative correlation between the EAP pattern and the subtropical j et with advancing the jet over land 2 days and lagging the jet over ocean 2 days.EAP pattern has a significant impact on the temperature anomalies in northeast China and precipitation anomalies in east China.The concurrent variation between the polar j et and the subtropical jet has an important impact in linking the EAP events to climate anomiles in China.With a strong subtropical jet and a strong or weak polar front jet there is significant impact in linking the EAP pattern with temperature and recipitation anomalies in different regions of China.With a weak subtropical jet and a strong or weak polar front j et,the impact is limited and has significant regional differences.