The Role Of Ocean Front And Synoptic Transient Forcing In The Formation Of The North Pacific Gyre Oscillation

Based on atmospheric reanalysis data provided by NCEP,sea surface temperature(SST)data provided by NOAA and the air-sea heat fluxes data.etc from 1960 to 2016,this paper uses climate dynamics diagnostic methods to analyze the spatial and temporal characteristics of the North Pacific Gyre Oscillation(NPGO)and associated ocean-atmosphere anomaly patterns.Furthermore,the relationship between the two oceanic fronts and NPGO is studied.The relative contribution of the diabatic heating forcing,transient eddy heating forcing and vorticity forcing associated with NPGO to the North Pacific geopotential height tendencies in the winter is analyzed.Finally,this paper proposes a possible positive feedback mechanism for the formation of a midlatitude ocean-atmosphere interaction with NPGO.The results show that NPGO is the North Pacific interdecadal SST anomaly,which is second only to Pacific Decadal Oscillation,and its SST anomaly mainly presents a north-south dipole pattern.The corresponding atmospheric anomaly is represented by the North Pacific Oscillation(NPO)in the lower layer and Western Pacific(WP)teleconnection wave train in the upper layer.NPGO has multiple time scales,mainly with quasi-10-year and quasi-24-year oscillation period.Since the 1980 s,the intensity of NPGO has increased significantly.NPGO is closely related to the changes of the two oceanic fronts in the North Pacific in winter.When NPGO is in warm phase,a stronger subarctic frontal zones(SAFZ)occurs with a northward shifted subtropical frontal zones(STFZ),which makes the atmospheric baroclinicity over the oceanic front increase.The activity of atmospheric transient eddy is very vigorous and the corresponding transient eddy heat transport and vorticity transport are strengthened,of which transient vorticity forcing is dominant.The potential height caused by transient vorticity forcing tends to be a barotropic structure in the vertical direction and a dipole structure in the horizontal direction,corresponding to positive NPO and WP phase and increase of the lower Aleutian Low(AL).Besides,the westerly jet in the middle latitude strengthens and moves northward.the loss of surface heat fluxes,and the upper oceanic dynamic processes such as the Ekman current,further maintain the SST dipole anomaly of the NPGO warm phase that the SST is cold anomaly northern 40°N and warm anomaly southern it,thus forming a midlatitude air-sea interaction positive feedback mechanism associated with the NPGO.