| Based on the temperature and salinity data of 2004 to 2017 from the gridded Argo(BOA_Argo),features,evolution mechanism of the barrier layer in the Tropical Pacific and its relationship with the El Nino-Southern Oscillation(ENSO)events were analyzed in this study.The results show that the tropical Pacific barrier layer presents three significant latitudinal bands that extends from the western Pacific warm pool to the eastern Pacific,centered at approximately 12°N,5°N,10°S,respectively;there are permanent and thick barrier layer in the warm pool,and its thickness more than about 20 m,the barrier layer on the 12°N latitude belt mainly occurs in winter,and on the 10°S latitude belt mainly exists in summer.The tropical Pacific barrier layer has obvious interannual variability in the warm pool,and its thickness varies by more than 15 m.The main mode of MV-EOF indicates that the distribution pattern of precipitation anomaly in the western Pacific is in good agreement with the anomaly in the barrier layer,both of which show obvious anti-phase distribution pattern of east(positive)and west(negative),the positive(negative)anomaly of precipitation causes the positive(negative)anomaly in the barrier layer,there is a significant 2?4 year cycle in the variation of its time coefficient and its correlation with Nino3.4 index is 0.87.Composite analysis shows that most of the barrier layer variations during ENSO are mainly confined in the western central equatorial Pacific,west of 160°W.During El Nino(La Nina),the barrier layer is anomalously thick(thin)east of about 160°E,and anomalously thin(thick)west of 160°E.For the El Nino events,MLD anomalies caused by heavy precipitation,low salinity water transport,and downwelling Kelvin waves dominated the anomalously thick barrier layer east of the equatorial 160°E,the variation of the barrier layer thickness in this region lags behind the Nino3.4 index for one month.;while during La Nina,the ILD anomaly can be attributed to the seawater convergence,subduction is the main reason for the thicker barrier layer appearing west of the equatorial 160°E,the variation of the barrier layer thickness in the region leads the Nino3.4 index by one month.Through comparative analysis the evolution characteristics of the barrier layer during the eastern Pacific El Nino(EPEN)and the central Pacific El Nino(CPEN),it is found that during the peak period of the CPEN event,the abnormally thick barrier layer in the equatorial Pacific mainly appears west of the dateline,with a thickness variation of over 10m;However,during the peak period of the EPEN event,the abnormally thick barrier layer in the equatorial Pacific mainly appeared east of the dateline,and the thickness change was more significant than that of CPEN,which could reach more than 15 mAnalysis of zonal migration of the barrier layer in the equator Pacific Ocean shows the thick barrier layer moves eastward during El Nino and westward during La Nina,and is roughly in the same phase as the zonal migration of the eastern edge of the warm pool and the sea surface salinity(SSS)front.In most cases,the thicker barrier layer is confined to the eastern edge of the warm pool(west of 1700 W),near or west of the SSS front,and the barrier layer near the SSS front may be due to horizontal advection,heavy precipitation,and Kelvin wave.It is worth noting that during the period from September 2015 to February 2016,the SSS front of the equatorial central Pacific disappeared,corresponding to the disappearance of the thick barrier layer,which indicates the position of the thick barrier layer region can be closely related to the zonal position of the SSS front at the eastern edge of the western Pacific warm pool,and its specific mechanism needs further numerical simulation.The salinity budget analysis of the SSS front gap area shows that the long-term mix layer salinity budget balance in this area is mainly the balance between the external force of freshwater flux and the adjustment of the marine process itself.During the period when the SSS front disappeared,the strong negative contribution of the freshwater flux term due to strong precipitation caused the salinity of this region become abnormally fresher,and the weak positive contribution of the oceanic advection to the salinity change was not enough to provide favorable conditions for the formation of the SSS front in this period.In addition,the change of freshwater flux term in this area is obviously ahead of the change of advection transport term by one month,indicating that ocean advection is the passive response of the ocean to the forcing of freshwater fluxFinally,the regression analysis of precipitation,sea surface current,wind stress and barrier layer shows that precipitation and wind stress mainly affect the thickness of the barrier layer by affecting the MLD,while meridional and zonal currents may affect the ILD through the convergence and subduction of sea water.Thick barrier layer tends to be formed under the conditions of weak winds,heavy precipitation,strong westward zonal currents,and meridional flow converging on both sides of the equator toward the equator.For the change of the barrier layer in the western equatorial Pacific,precipitation can explain 30%?3 5%,the contribution of wind stress can reach about 20%,and the effect of zonal flow can explain 15%?20%... |
PDF Full Text Request