Structure,Variability,and Dynamics Of The Pacific North Equatorial Subsurface Current

The subthermocline circulation in the tropical Pacific Ocean has an important impact on the global ocean conveyor belt,warm pool heat redistribution and climate change.Historically,due to the lack of subsurface observations,the global ocean subthermocline circulation research is sparse and far behind the upper circulation.Over the past decades,with the implementation of the international Argo float program,the subthermocline circulation has become an international research hotspot.In this paper,based on the temperature and salinity data and parking depth veloctity of Argo profling floats,combined with the historical hydrological observations along130°E and 137°E sections and the five-year moving average temperature and salt data,the P-vector method and Argo Parking depth velocity are used to calculate the absolute geostrophic current.Using the linear continuously stratified ocean circulation model to study the mean state and seasonal to interdecadal variability of the North Equatorial Subsurface Current(NESC)in the Pacific Ocean,and unraval the essential processes of its formation and variation.The results show that NESC is a subthermocline flow located between 3°N-7°N and 200 m to 800 m from east to west across the entire Pacific Basin,with an average velocity of 2 cm s^-1 to the west.The subsurface current originates in the eastern Pacific,with its core descending to deeper isopycnal surfaces and moving to the equator as it flows westward.The water mass of the NESC is mainly North Pacific Intermediate Water,and mixes with the high salinity water of the South Pacific carried by north part of the North Subsurface Countercurrent(NSCC,the so-called Tsuchiya jet).The origion of the NESC is the westward flow formed by the turning around the Costa Rica Dome(CRD)in the eastern Pacific Ocean.The NESC reaches its peak from July to September every year,and transports westward to the western boundary current,and has shown signs of entering the subsurface of Indonesia sea.The linear continuously stratified model shows that the formation mechanism of NESC is the superposition of the lower baroclinic modes responses of the ocean circulation to the wind curl forcing.The NESC has strong seasonal variation and seasonal flow reversal occurs in winter.The seasonal variation of the NESC in the western Pacific is controlled by the vertically propagating Rossby wave which driven by the wind stress curl on the surface in the central-eastern Pacific Ocean,and the wind stress curl variations is mainly caused by the meridional movement of the Intertropical Convergence Zone.The study also found that the effects of the equatorial Kelvin wave and the equatorial Rossby wave on the seasonal changes of the NESC are much smaller than the effect of the Rossby wave off the equator,and thus the roles are secondary.NESC became weaker during El Ni?o and significantly strengthened during La Ni?a.Over the past 60 years,NESC has exhibited pronounced interdecadal variability.Its transport increased in 1957-1976 and 2000-2016,and weakened in 1977-1999.These changes are closely related to the variations of wind field during different periods.