SST Variations In SPCZ Region And Their Possible Impacts In Boreal Summer

Based on the monthly mean data including NCEP/NCAR reanalysis,Sea Surface Temperature(SST)data from Hadley Centre,CMAP precipitation data from NOAA,GOD AS oceanic current data,SODA wind stress data,and surface temperature data from CRU,a key region of South Pacific Convergence Zone(SPCZ)is identified.We have investigated the characteristics of seasonal changes of South Pacific Convergence Zone and the possible mechanisms of SPCZ formation and maintenance.Besides,by using the methord of synthetic analysis,we have analyzed the interannual variation of SST anomalies in the South Pacific Convergence Zone region and its relationship with the regional climate of the Maritime Continent(MC)region during the nearly 35 years in the boreal summer.And at the same time,on the basis of the former,the method of linear regression analysis is used to diagnose the long-term trend of SST and its influence on the change of South Pacific Convergence Zone.The results are summaried as follows:In the South Pacific Convergence Zone region,there is a transition from convergence zone caused by east-west wind to that caused by east wind in northern hemisphere in April,and an opposite trend of transition in northern hemisphere in December.In the lower troposphere,the upward extension of the South Pacific Convergence Zone and convergence are significantly higher and stronger in winter than those in the spring,summer and autumn.Two mechanisms explain the formation and maintenance of the SPCZ.First,topography plays a certain role.When blocked by topography,the equatorial vortex travels along the Australian terrain.Such condition is conducive to the formation and maintenance of the anticyclonic circulation in the Australian region and that of the cyclonic circulation in the SPCZ region.Meanwhile,with the effect of left Coriolis force in southern hemisphere,warm sea water tends to converge in the South Pacific Convergence Zone and form an area with high SST which heats the atmosphere above and contributes to the formation and maintenance of SPCZ.Second,diabatic heating essentially impacts on the SPCZ.Heat source in the SPCZ region warms its upper atmosphere and produces the zonal gradient forcing which drives convergence in the lower air.It will also contribute to the formation and maintenance of SPCZ.In boreal summer,the interannual variation of SST anomalies in the SPCZ region is closely related to the climate variations in the Maritime Continent region.After filtering the long term-trend of SST,when the SST anomalies is wanner in the SPCZ region,there has a positive anomaly of the heating in the southwest of MC region,which heats the atmosphere above it.And in this region,with the updraft,the anomalous convergence in the lower troposphere and anomalous divergence in the upper troposphere are observed,resulting in more precipitation there.Meanwhile,in the eastern part of the MC region,there has a negative anomaly of the heating,which cools down the atmosphere above it.With the downdraft,the anomalous divergence in the lower troposphere and anomalous convergence in the upper troposphere are observed,resulting in less precipitation to occur.Moreover,on the basis of the previous analysis,after filtering the signal of ENSO,when the SST anomalies is wanner in the SPCZ region,in the MC region,both the heating and the precipitation are corresponding to positive anomalies in the west and negative anomalies in the east.Similarly,for circulation,it corresponds to an anomalous convergence in the lower troposphere and an anomalous divergence in the upper troposphere in the west of MC region while an anomalous divergence in the lower troposphere and an anomalous convergence in the upper troposphere in the east of MC region.Compared with the former,the intensity of positive and negative anomalies or the convergence and divergence regions of each variable will be reduced.In boreal summer,there is an obvious upward trend of SST in the South Pacific Convergence Zone region,which has a certain influence on the change of the SPCZ.When the long-term trend of SST in the SPCZ region is significantly increased,in the west and southeast of SPCZ,it has a positive anomalies of the heating that will heat the atmosphere above it.And with the updraft,there also has an anomalous convergence in the lower troposphere and an anomalous divergence in the upper troposphere,making more precipitation.Besides,in the northeast,it has a negative anomalies of the heating,which will cool down the atmosphere above it.In this case,with the downdraft,an anomalous divergence in the lower troposphere and an anomalous convergence in the upper troposphere are induced,making less precipitation there.All of these are conducive of the increased intensity of the SPCZ in the west and southeast and of the reduced intensity in the northeast of the SPCZ.Moreover,there is likely to have a certain link between the global warming and the apparent upward trend of SST in the SPCZ region.