A New Hybrid Coupled Model Of Atmosphere-Ocean-Biogeochemistry And Modelling Study Of ENSO Modulation

As the most prominent interannual climate variability,El Ni?o and Southern Oscillation?ENSO?phenomenon originates from atmosphere-ocean interactions in the tropical Pacific,and can exert significant influence on the global climate and biogeochemistry.So far,large uncertainty and intermodel differences still exist in ENSO prediction and simulations.Multi-scale biogeochemical processes interact with the ocean and atmosphere,which further affects the diversity of ENSO.In this dissertation,a new hybrid coupled model of the atmosphere and ocean physics-biogeochemistry?HCM-AOPB?is developed to study the feedback of multi-scale biogeochemical processes onto the tropical Pacific climate,including interannually varying chlorophyll;tropical instability waves?TIWs?-induced chlorophyll perturbations,and combined effect of freshwater flux and chlorophyll.This HCM-AOPB consists of a simplified coupled ocean-atmosphere system and a biogeochemical model.For ocean-atmosphere coupled system,an ocean general circulation model?OGCM?is coupled with a statistical atmospheric model for interannual anomalies of wind stress and freshwater flux?FWF?.Model simulations are shown to illustrate the model's ability in simulating the physical and biogechemical processes of the tropical Pacific climate.The HCM-AOPB serves as a simplified earth system model,and provides a cost-effective model tool for studying the interactions between the atmosphere,ocean,and biogeochemistry in the tropical Pacific.Using this HCM-AOPB and observed data,three aspects are investigated,including modulation effects of interannually varying chlorophyll and tropical instability waves?TIWs?-induced chlorophyll perturbations,and combined effect of chlorophyll and FWF onto ENSO.???For the modulation of interannually varying chlorophyll onto ENSO,ocean chlorophyll absorbs solar radiation and modulates the penetrative radiation in the upper ocean,which induces biological heating effects and further affects climate system.However,this effect has not been well understood and controversy still exists about how chlorophyll affects the tropical Pacific climate?i.e.increases or decreases the amplitude of ENSO?.A series of simulations are conducted using the HCM-AOPB,and results show that interannually varying chlorophyll tends to weaken the ENSO amplitude by 22%.The penetrative solar radiation(Q_pen)out of the mixed layer?ML?is highly dependent on interannual variability of chlorophyll,which alters the thermal structure,stratification,and vertical mixing process,serving as a negative feedback onto ENSO.During El Ni?o,in the western-central Pacific,a decrease in chlorophyll(0.05-0.1mg m^-3)leads to an increase in Q_pen and a decrease in absorbed solar radiation within the mixed layer.Thus,differential heating between the mixed layer and the subsurface tends to destabilize the stratification and enhance the vertical mixing,and consequently leads to a cooling effect on sea surface temperature?SST?.This effect is converse during La Ni?a.This new mechanism is different from classical therory,i.e.chlorophyll absorbs solar radiation within the mixed layer and directly affects SST.???Intraseasonal varability of biological process can also exert influence on climate;we focus on the feedback of TIWs-induced chlorophyll perturbations onto the ocean and ENSO.TIWs-induced chlorophyll perturbations tend to weaken the intensity of TIWs itself?7%-9%?,and increase the ENSO amplitude by 27%.High and low chlorophyll concentrations are associated with cold and warm phases of TIWs-induced perturbations in SST.The perturbations of chlorophyll further change Q_pen,which affects the temperature and density structure,and consequently weakens the baroclinic conversion term between eddy available potential and eddy kinetic energy?EKE?.As a result,the intensity of TIWs itself is weakened.The weakened TIWs inevitably reduces the meridional heat transport onto the equator.Consequently,less warming water is available on the cold tongue in the eastern equatorial Pacific.Thus,La Ni?a conditions tend to be intensified and ENSO amplitude is increased through atmosphere-ocean interactions.These results indicate that chlorophyll-induced feedback onto ENSO is very sensitive to the spatiotemporal scales for chlorophyll represented in the coupled model.That is,interannually varying chlorophyll induces a negative feedback on ENSO;TIWs-induced chlorophyll perturbation exerts a positive feedback on ENSO.These results provide some insight for biophysical interactions in the tropical Pacific,which may be a key factor to explain the large intermodel differences associated with the biological feedback on ENSO.???After studying the multi-scale feedbacks associated with chlorophyll onto ENSO,we also found that freshwater flux?FWF?and chlorophyll could produce nonlinear feedback onto ENSO.In the context of coupled ocean-atmosphere,FWF can increase ENSO amplitude:during El Ni?o,FWF increases in the western-central equatorial Pacific,which leads to a decrease in sea surface salinity and an increase in buoyancy flux,and consequently stabilizes stratification and shoals the mixed layer depth;therefore,the warm anomaly of SST is enhanced.However,interannual variability of chlorophyll induces a negative feedback onto ENSO.Due to the compensatory effect between FWF and chlorophyll,the combined net effect can give rise to an unexpected situation:an increase in the FWF forcing intensity can act to reduce the ENSO amplitude when the interannual variability of chlorophyll reaches to a certain intensity.The amplifying effect induced by FWF forcing can be compensated for by damping effect induced by interannual variability of chlorophyll,and consequently produces a nonlinear modulation onto ENSO.These interactions among different feedbacks may contribute to explain the difference and sensitivity for modulation of ENSO in the natural world and model simulations.In summary,chlorophyll-induced heating effect on ENSO depends on the different temporal and spatial scales;it also interacts with FWF and produces nonlinear feedback onto ENSO.These processes can further lead to diversity and complexity of ENSO.