Influence Of The Subtropical Northeast Pacific Sea Surface Temperature Anomaly On El-ni?o And Southern Oscillation

Recently studies show that signals form extratropical north Pacific have significant influences on the tropical air-sea systerm,as well as the ENSO phenomenons,and can help improve the pridiction of ENSO.Subtropical northeast Pacific(180 to 90 W,10N to 30N),as a key region connects the extropical and the tropical Pacific,plays an important role in the process that extropical sigals transport to the tropics.This study,based on the ERSST v4 dataset and NCEP/NCAR Reanalysis dataset during 1948 to 2016,diagnozes the impact of Subtropical SSTA signal on the air-sea systerm and on ENSO and its physical mechanism using the methods of composite analysis,regression anaylsis,lead/lag correlation analysis,and principal component analysis,and gets following conclusions:(1)There is a significant positive correlation between the subtropical northeast Pacific and equatorial central Pacific.The warm events of subtropical northeast Pacific in boreal spring many times correspond the ENSO warm events in the following winter.(2)The warm SSTA signals in the subtropical northeast Pacific exhibit a northeast to southwest structure that lead to the equatorial central Pacific.(3)The results of heat budget analysis show that,at the inter-annual time scale,the main element that influences the SSTA variability is the latent heat flux at the air-sea interface.In the boreal winter,the positive(downward)latent heat flux heats the ocean,and eventually leads to the positive SSTA in the following spring.It is the sea surface wind speed anomaly that controls the latent heat flux anomaly.(4)The extratropical north Pacific atmospheric inter-annual variability,especially the North Pacific Oscillation(NPO),has a significant correlation with the generation of the SSTA signals.The south-low-and-north-high NPO phase correponds the weakening of trade wind and positive downward latent heat flux anomaly.The NPO also has an influence on the structure of the SSTA signal.(5)The thermodynamic air-sea interactive process,also known as wind speedevaporation-SST feedback,plays an important role in the trasition of subtropical northeast Pacific SSTA signals to the equatorial central Pacific.The SSTA firstly leads to a SLPA above it.Then the variation of SLPA grandient leads to the variation of sea surface wind speed anomaly,as well as the latent heat flux anomaly,and finally makes the SSTA signal transports to the southwest.A series of numerical experiments are designed in this study by the CAM3-GRGO coupled model to examine the results of the analysis before.In the SSTA experiments,an SSTA signal is set in the subtropical northeast Pacific of model ocean,and then the model goes on evolving freely.The difference between the SSTA experiments and the control run(without any external influences)can partly represents the response of the model air-sea systerm to the SSTA signal.And a composite ensemble of differences shows the common form of the responses.The results of the numerical experiments confirm that,in the model air-sea systerm,SSTA signals generating in the subtropical northears Pacific will influence the SLPA field,and then change the sea surface wind speed and latent heat flux,which then impact the SST field in turn,and finally leads the SSTA signal transpoting to the southwest.The dominating process that control the signal transition and its structure is the thermodynamic air-sea interaction,the wind speed-evaporation-SST feedback.