| El Ni(?)o-Southern Oscillation(ENSO),as the strongest interannual signal in the Earth’s climate system,influences global climate through atmospheric teleconnections;therefore,simulation and prediction of ENSO are of great societal importance.As more salinity observations have become available for analysis,recent studies have shown that salinity plays an important role in ENSO evolution and its predictability.In fact,sea surface salinity(SST)and sea surface temperature(SSS)jointly determine sea surface density,which affects surface currents through pressure gradients,and further modulates ENSO via advection processes.However,salinity effects are not considered in the scheme to calculate the widely used Ocean Surface Current Analysis Real-time(OSCAR)product and in the Intermediate Coupled Model(ICM)for ENSO research and prediction.In this thesis,the physical processes associated with salinity effects are included in these dynamical frameworks of different complexity to investigate the salinity effect on the surface currents in the equatorial Pacific and ENSO.The main results are as follows:(1)Investigating the salinity effect on the equatorial Pacific surface currents based on an improved OSCAR-based scheme.The thermohaline effects on surface currents in the equatorial Pacific are systematically evaluated based on the analytical scheme used to calculate surface currents in producing the widely used OSCAR product.Several calculations are performed with the original OSCAR-based scheme and an improved scheme including the salinity effect.It is demonstrated that the thermohaline effects are generally dominated by the temperature role,but the salinity effect plays a non-negligible role near the dateline and in the far eastern Pacific.The thermohaline effects are quantified and highlighted on annual,seasonal and interannual time scales.For annual mean surface currents,temperature and salinity effects work together to reduce the amplitudes of zonal currents north of the equator by about half,while their effects tend to be compensated by for each other south of the equator.Seasonally,the thermohaline effects are dominant in spring due to the weakened wind effect;the salinity effect on zonal currents is comparable to the temperature effect in the eastern Pacific in spring,and plays a more important role than the temperature effect in the western Pacific most of the year.Interannually,in the far eastern Pacific,the salinity effect explains up to 25% of the total variance;in the eastern and central Pacific,the contribution of temperature is negative and accounts for-10%;in the western Pacific,the salinity effect plays a more important role than the temperature effect,with contributions up to-10% west of 165°E and 10% east of 165°E during La Ni(?)a events,respectively.(2)Investigating the impacts of diabatic part on ENSO based on an improved Intermediate Ocean Model(IOM).In the original framework of the IOM,the ocean dynamical fields are only driven by wind stress,while the impact of diabatic part related to freshwater and heat fluxes on currents and ENSO are not involved.Thus,we improve the IOM by introducing the impacts of diabatic part of mixed layer density anomalies on currents and ENSO.After considering the impacts of the diabatic part,the interannual variability of simulated meridional surface currents in the equatorial central Pacific is significantly enhanced,exhibiting higher correlation(from 0.3 to 0.5)and more comparable amplitude with observations.For surface zonal currents and SST,the adiabatic part plays dominant roles,while the diabatic part slightly enhance their variabilities near the dateline.With the diabatic part involved,the interannual variability of SST in Ni(?)o 3.4 and Ni(?)o 4 regions increased by 4% and 12%,respectively.The obvious impacts of diabatic part are seen during the developing and decay phases of the2015-2016 super El Ni(?)o event,both with the SST near the dateline increased by about0.5°C.In the central equatorial Pacific,anomalies of warm SST and negative SSS work together to contribute to negative density anomalies and anomalous northward cross equatorial flow,which gives rise to anomalous upwelling through meridional divergence.The mixed layer heat budget analysis shows that the vertical advection induced by diabatic part brings the anomalous warm signal from the subsurface layer to the surface mixed layer,and the meridional advection process further spreads the warm water outward.During the development phase the temperature effect dominates the diabatic part,while during the decay phase the salinity effect is more important.(3)Investigating the impacts of diabatic part on ENSO based on an improved ICM.The improved IOM with diabatic part is coupled with a statistical atmospheric model to analyze the impacts of diabatic part on ENSO under the influence of air-sea interactions.In the improved ICM,the diabatic part not only directly modulates ENSO through advection processes,but also changes the wind field and indirectly affect ENSO through the Bjerknes feedback.In the ICM,diabatic part exhibits a positive feedback effect on ENSO,increasing its amplitude by about 40%.With diabatic part involved,the interannual variabilities of SST,zonal wind stress,surface currents,sea level,and the temperature of subsurface water entrained in the mixed layer(!)are all enhanced.The impact of diabatic part on ENSO often persists into the second year after the event,which is favorable for second-year warming or cooling events.A case study is conducted for a second-year cooling event induced by the diabatic part in the ICM.It is demonstrated that during the decay phase of the first La Ni(?)a event,strong positive density anomalies due primarily to salinity changes are seen in the southern tropical central and western Pacific,which directly generate anomalous southward crossequatorial surface currents through pressure gradients.As a result,diabatic part directly lead to cold SST anomalies in the tropical central Pacific via meridional and vertical advection processes.Through the Bjerknes feedback,the cold SST anomalies trigger easterly wind anomalies in the western Pacific,which further induce westerly current anomalies in the central basin and cold !anomalies in the eastern basin.The mixed layer heat budget analysis shows that the second-year cooling event develops mainly through the vertical diffusion process related to the ! anomalies indirectly induced by diabatic part through air-sea interactions.In this thesis,the simplified analytical surface current framework,intermediate ocean model,and intermediate coupled model are improved by further considering physical processes associated with salinity effects.Based on these improved tools,we illustrate the important effects of salinity on surface currents near the dateline and in the far eastern Pacific,reveal the important role of diabatic part on the simulation of meridional surface currents in the central Pacific,clarify the physical mechanism of diabatic effects on ENSO through advection process,and find that salinity effects working together with the Bjerknes positive feedback could trigger second ENSO events.Therefore,this thesis has important significance for the error analysis of current products,ocean model simulations,and ENSO predictions. |
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