Mechanisms Of Sea Surface Temperature And Salinity Variability Of Ningaloo Ni(?)o And Effect Of Mesoscale Eddies

Ningaloo Ni(?)o(NN),as a dominant climate mode of interannual variability in the southeast Indian Ocean(SEIO),has a strong influence on the regional whether and climate change,and also significantly affects the coral ecological system and fishery resources.It is of great scientific value and practical importance to study the mechanisms of temperature and salinity variation of NN.Firstly,this study utilized a high-resolution(3～12 km)regional oceanic modeling system(ROMS)to explore the sea surface temperature(SST)variability of NN.With the high-quality boundary conditions and atmospheric forcing,the model simulates well eight NN events and their major spatialtemporal characteristics with a high fidelity during 1993–2016.A series of hierarchical model experiments are carried out to quantitatively evaluate the effects of key processes on SST warming.The results reveal that the predominant process controlling SST evolution is the surface latent heat flux,with a contribution of >60%;while the roles of both Indonesian Throughflow(ITF)and local wind forcing range from 10% to 20%,thus playing a secondary effect.Reduce of surface latent heat is essential in driving the growth of SST warming,which is mainly caused by the warming of air temperature from the low-latitude warm and moist air carried by the northerly wind anomaly(accounting for the majority of latent heat flux).The established SST warming in the mature stage of NN prompts the increase of latent heat loss,which further initiates the begin of decay stage.These analyses clearly explain the key physical processes that regulate the SST evolution of NN.This study shows that NN can also induce profound variability in ocean salinity.At the mature stage of NN,the SEIO region displays large-scale sea surface salinity(SSS)freshening of 0.15–0.20 psu.ROMS sensitivity experiments show that this SSS freshening is mutually caused by the increased local precipitation(>50%)and enhanced fresh-water transport of the ITF(>40%)during NN events;the effects of other processes,such as local winds and evaporation,are secondary(<10%).In addition,the ITF feeds the southward fresh-water advection near the eastern boundary,which is crucial in causing the strong freshening(> 0.20 psu)near the Western Australian coast.Owing to the strong modulation effect of the ITF,SSS of the SEIO bears a higher correlation with the El Ni(?)o-Southern Oscillation(ENSO),such as 0.57,0.77,and 0.70 with Ni(?)o-3,Ni(?)o-4,and Ni(?)o-3.4 indices,respectively,than that of SST(-0.27,-0.42,and-0.35).This reflects the difference between mechanisms of temperature and salinity variation of NN.In order to explore whether the salinity variability has a feedback effect on the warming of NN,an idealized model experiment with artificial damping for salinity anomaly was conducted in this study.The results indicate that ocean salinity has limited impact on ocean near-surface stratification in the SEIO and thus minimal feedback effect on the warming of NN.ROMS simulation also displays prominent mesoscale noises in the warming signature of NN,which is confirmed by satellite microwave SST data.Therefore,the diagnostic analysis of high and low resolution model experiments is carried out in this study to explore the importance of oceanic mesoscale eddy on the NN.The results show that in the satellite observation and high-resolution(～3 km)simulations,the warming signatures originated largely from the ITF are transmitted westward from the eastern boundary by mesoscale eddies.Few eddies propagate long distances offshore due to strong dissipation and slow propagation,and as a result NN signatures are predominantly confined near the coast.On the contrary,in coarse-resolution(～100 km)simulations that cannot resolve eddies,anomalous energy propagates westward swiftly as long Rossby waves with much weaker dissipation,and more energy spreads to the ocean interior with the suppression of coastally trapped warming signatures.Eddies also induce local SST warming “hotspots”,promoting surface latent heat release and mesoscale air-sea interactions.These processes are not resolved by coarse-resolution models.However,a multi-member ensemble simulation confirms the strong influence of mesoscale eddies arising from ocean internal instability in the spatial redistribution of SST and SSS anomalies and evaluates their impacts on the predictability of NN.Through computing “signal-to-noise” ratio,the results indicate high predictability over the west coast of Australia and degraded predictability in the ocean interior.These results highlight the importance of resolving mesoscale oceanic processes in the simulation of NN.In conclusion,this study clarifies systematically the physical mechanisms of temperature and salinity variability of NN in the SEIO and reveals the importance of mesoscale eddies on the NN.The research findings can not only deepen our full understanding of NN variability but also strengthen the model ability to predict the NN.