MODELING ON THE EFFECTS ON ENSO DUE TO INTERANNUAL VARIABILITY OF FRESHWATER FLUXES IN THE TROPICAL PACIFIC
Freshwater flux(FWF;defined as precipitation minus evaporation)at the air-sea interface is one of the important atmospheric forcings to the ocean in the tropical Pacific climate system,additional to wind stress and heat flux that influence the El Nino-Southern Oscillation(ENSO).Although these atmospheric forcing fields act together on the ocean,the individual forcing role of FWF and the related processes and mechanisms still need to be deeply explored.We examined the net effect of FWF interannual variability on the upper ocean by using an ocean general circulation model(OGCM)and further quantified them using heat and salt budget analyses as well as diagnostic analyses that distinguish between the relative contributions of temperature and salinity.Results indicate that during El Nino events,the FWF positive anomaly forcing directly leads to a decrease in sea surface salinity(SSS)in the central equatorial Pacific and enhances the stability of the upper ocean stratification.Furthermore,the FWF positive anomaly forcing also increases buoyancy flux(BF)in the central equatorial Pacific,which reduces vertical mixing and entrainment processes,resulting in a local shoaling of the mixed layer.In terms of climate effects,the oceanic processes induced by the interannual FWF variability contribute to the maintenance and enhancement of sea surface temperature(SST)warm anomalies in the eastern equatorial Pacific,with vertical mixing and entrainment being the dominant contributing terms in the warming process.Quantitative analysis showed that SSS changes are closely related to the forcing intensity of the FWF,with an increase in FWF anomalies in the central equatorial Pacific by 1.0 mm/d corresponding to a decrease in SSS by about 0.02 and to an increase in SST in the eastern equatorial Pacific by about 0.12 ℃.In the case of the 1997～1998 El Nino event,for example,the incremental SST warm anomalies due to positive FWF effects contribute 11.8％of the total SST anomalies.The separate role of the FWF forcing field in ENSO revealed in this paper helps to refine our understanding of the entire coupled sea-air system in the tropical Pacific Ocean and provides scientific guidance for further improvement of ENSO representations and predictions.
freshwater flux at the air-sea interfaceENSOsea surface salinity and temperaturethe tropical Pacifica layer ocean modelheat and salt budget analysesrelative contribution of temperature and salinity
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