A Recent Eastward Shift In The Summertime SST Anomaly Center In The Indo-Pacific Region Induced By The Pre-winter ENSO

The El Nino-Southern Oscillation(ENSO)is the strongest inter-annual variation signal in the tropical Pacific Ocean.The impact of the pre-winter ENSO decaying process on the following summer climate in East Asia is an important issue of long-term concern.We used the Sea Surface Temperature(SST)reanalysis data from ERSST and the wind field reanalysis data from NCEP/NCAR during 1970-2018 in the study.In this thesis,we focus on the phenomenon of the eastward shift in the summertime SST anomaly center in the Indo-Pacific region induced by the pre-winter ENSO and its climatic effects in recent years.The physical process and mechanism of this phenomenon will be examined and revealed in this study.Our results reveal that the increase of the pre-winter ENSO decaying rate in recent years may be the main reason for this phenomenon,and the increase of the ENSO decaying rate may be due to the fact that the evolution process of ENSO has been more influenced by the tropical Atlantic SST in recent years.The center of the SST anomaly in the Indo-Pacific during the following summer caused by ENSO in the pre-winter has been shifted eastward obviously during the recent decade.Before the 1990 s,the SST anomaly tended to be located in the western Indian Ocean;after the 1990 s,the SST anomaly center moved eastward to the Maritime Continent.The eastward shift of the SST anomaly is related to the change of ENSO teleconnection caused by the increase of the ENSO decaying rate.Before the1990 s,the ENSO decaying rate is much slower,which would lead to the presence of warm SST anomaly signals in the tropical eastern Pacific in the following summer caused by El Ni(?)o events in the pre-winter.The anomalous distribution characteristics of SST may lead to the anomalous structure of Walker circulation,which includes the rising over the tropical eastern Pacific and the sinking over the Maritime Continent.The abnormal atmospheric sinking motion in the Maritime Continent could suppress the convective activity and rainfall,thus causing the effect of an atmospheric cooling source.Then the easterly anomaly in the lower atmosphere in the equatorial Indian Ocean could be stimulated through the Gill type responses,being able to further deepen(shallower)the thermocline and increase(decrease)the SST in the western(eastern)equatorial Indian Ocean.Therefore,the warm SST anomaly observed during this period was more concentrated in the western Indian Ocean,while the SST warming in the Maritime Continental was not significant,resulting in the center of the SST anomaly in the Indo-Pacific Ocean in the summer of ENSO decaying years was more concentrated in the western Indian Ocean during this period.After the 1990 s,due to the increase of ENSO decaying rate in the pre-winter,there were nearly no warm SST anomalies in the tropical central Pacific in the summer after the pre-winter El Ni(?)o event,and even the SST anmalies in the tropical central Pacific have been converted to the cold anomalies.Such SST anomalous would lead to the anomalous structure of atmospheric Walker circulation,including the sinking over the equatorial central Pacific and the rising of the Maritime Continental.The abnormal rising of the atmosphere in the Maritime Continent may strengthen the atmospheric convective activity and rainfall,which could cause the effect of the atmospheric heat source.Then the westerly anomaly in the lower atmosphere in the equatorial Indian Ocean could be induced according to the Gill type responses,which could further promote the shallowness of thermocline and the decrease of SST in the western equatorial Indian Ocean.At the same time,the atmospheric convective activity in the Maritime Continent can also weaken the sea surface pressure(SLP)in the region.The SLP anomaly could intensify the spatial SLP distribution pattern with stronger SLP values in the north and weaker ones in the south,being able to enhance the northerly wind anomalies.The direction of the anomalous winds was opposite to the direction of the southerly background winds,resulting in the weakening of ocean surface evaporation and the reduction of heat loss through the ocean surface latent heat flux.These processes could promote the warming of SST in the Maritime Continent through wind-evaporation-SST(WES)mechanism.Therefore,the center of SST anomaly in the Indo-Pacific was shifted more eastward(more concentrated in the Maritime Continent)in the summer of ENSO decaying years during this period.The results of the heat budget diagnostic analysis for the oceanic mixed layer confirm the important role of the WES mechanism in the process of the summer SST variation in the Maritime Continent induced by the pre-winter ENSO.The phenomenon of the eastward shift of the Indo-Pacific SST anomalies,which were induced by the pre-winter ENSO,may have important climatic effects.On the one hand,this phenomenon will result in the significantly enhanced influence of the Maritime Continental SST anomaly on the western North Pacific Subtropical High(WPSH)in recent years.On the other hand,it will also result in a significantly enhanced correlation between rainfall in the vicinity of the Maritime Continent and the pre-winter ENSO.And this relationship may improve our ability to use the state of ENSO in the pre-winter to predict the rainfall anomalies in the vicinity of the Maritime Continent in the following boreal summer.Using the model results from the fifth coupled Model comparison Plan(CMIP5)model,the central locations of the summertime Indo-Pacific SST anomalies induced by the pre-winter ENSO were analyzed for 26 models.In the current climate period,most models could simulate the longitude and the trend of eastward movement of the SST anomaly center in the Indo-Pacific region in summer.In the future warming period,the longitude of the summertime SST anomalies in the Indo-Pacific region caused by the pre-winter ENSO would be more eastward than that of the current period in the model simulation results,with the increased correlation coefficient between the SST anomaly and the WPSH Index in the meantime.These results indicate that the influence of SST anomalies in Maritime Continent on the WPSH may continue to intensify under the future warming scenario.The research results in this thesis are helpful for us to better understand the process and mechanism of the interdecadal variability for ENSO teleconnections in the 1990 s,and deepen our understanding of inter-basin interactions among three oceans and their climatic impacts in recent years.