Variation Of Wind-driven Oceanic Gyre In The North Pacific And Its Feedback To Atmospheric Circulation

Adjustment of the North Pacific oceanic circulation and its impact on Pacific decadal climate variation has long been the scientific frontier issues in ocean dynamics and climate research.A key scientific question within is how the oceanic gyres adjust and generate Sea Surface Temperature (SST) anomalies,and further feedbacks to general circulation of atmosphere.A global ocean-atmosphere coupled model,named as FOAM1.5,is used to investigate this scientific issue.Based on atmospheric forcing modes picked up from NCEP/NCAR reanalysis data and using the "Partial Coupling" surgery of coupled GCM models,we systematically explore two issues as follows:(1)Adjustment of the North Pacific wind-driven circulation under the PNA-like and NPO-like anomalous wind stress forcing pattern and its impacts on atmospheric circulation.(2) Interaction of the subtropical North Pacific and the tropical Pacific under anomalous wind stress forcing.The innovative conclusions can be summarized as follow:(1)Under the positive PNA-like anomalous wind stress forcing, subpolar oceanic gyre in the North Pacific is strengthened and stretches southward,which leads to a southward shift of the boundary between subtropical and subpolar gyres, that is,the zonal jet of Kuroshio-Oyashio Extension(KOE).The response of SST presents the positive PDO-like anomalies.The feedbacks of the oceanic adjustment to the atmosphere of the North Pacific display significant seasonal ity.In winter,atmospheric response is dominated by the quasi-barotropic low-trough anomalies,which resembles positive NPO-like anomalous atmospheric circulation.In summer, baroclinic anomalies dominate the atmospheric response with high-ridge anomaly in the low troposphere and low-trough anomaly in the upper troposphere.(2)Under the positive NPO-like anomalous wind stress forcing,the North Pacific oceanic circulation system including the subtropical and the subpolar gyres are consistently strengthened,which induces acceleration of KOE zonal jet.The impact of the North Pacific oceanic adjustment to atmospheric circulation above is to generate the quasi-barotropic high-ridge anomalies,which resembles negative PNA-like anomalous atmospheric circulation in winter.(3)Local air-sea coupling is important for adjustment of oceanic gyres in the North Pacific.It is the role of air-sea coupling that makes oceanic circulation system shifts northward,which leads to KOE zonal jet shifts southward less about 30% driven by the positive PNA-like anomalous wind stress forcing,and shifts about 0.5-latitude northward under positive NPO-like anomalous wind stress forcing.(4) Under the positive PNA-like anomalous wind stress forcing,there are four primary physical mechanisms responsible for transmitting anomalous signal from the North Pacific to tropical Pacific:surface coupled Wind-Evaporation-SST(WES) positive feedback,enhanced STC, subduction process and the effect of coastal Kelvin wave.Feedback of air-sea coupling in the tropical Pacific to North Pacific atmospheric circulation generates negative PNA-like anomalies,which partially offsets the atmospheric forcing. (5)Under the PAN-like anomalous wind stress forcing,the North Pacific coupled adjustments triggers the triple-like structure SST anomalies in the downstream and positive NAO-like atmospheric circulation anomalies in the North Atlantic.In summary,there are three major findings on the roles of oceanic circulation in modulating Pacific decadal climate variation,which can be described as follows:(1) The PNA-like wind stress forcing may induce NPO-like atmospheric circulation anomalies,and the NPO-like wind stress forcing may trigger PNA-like atmospheric circulation anomalies,and there appears strong positive-negative asymmetry within.(2)The PNA-like wind stress forcing causes Kuroshio jet shifts northward or southward,and the NPO-like wind stress forcing leads to acceleration or deceleration of the Kuroshio jet,while local air-sea coupling in the North Pacific can modulate the shift of Kuroshio from one mode to another mode.(3) The North Pacific can influence tropical Pacific through four physical processes and then return the negative feedback from tropical Pacific to the North Pacific,which can weaken the extratropical forcing.