An Optimized Vertical Mixing Scheme And Its Performance In Ocean And Climate Models

Numerical simulation is one of the most powerful tools for studying the ocean and climate.However,model biases are still substantial in ocean and coupled ocean-atmosphere simulations in the tropical Pacific Ocean,including the too-cold tongue and too diffuse thermocline.These biases can be partly attributed to vertical mixing parameterizations in which the empirical parameters have great uncertainties.In this study,the bias problem is investigated using observations data and models,and a new scheme is proposed aiming to alleviate the temperature biases in the tropical Pacific Ocean.In the first part of this dissertation,we have studied the impact of two different vertical mixing schemes on the solution of a tropical Pacific OGCM.In the conventional KPP scheme,the vertical eddy viscosity and diffusivity are determined based on a diagnosed boundary layer depth and a turbulent velocity scale.In contrast,the Chen scheme is mainly based on the KTN mixed layer model and Peters'shear instability model.Overall,the Chen scheme produces more realistic SST simulations in the eastern equatorial Pacific than KPP scheme.However,the subsurface warm bias is enlarged,mainly due to the overestimated wind stirring effects off the equator.The improved SST simulation can be attributed to the Peters'shear instability model,which generally produces a lower diffusivity than its counterpart in KPP scheme.Introducing the Peters'shear instability model into the KPP scheme can reduce the cold bias by 30%,and does not cause the deterioration in subsurface temperature simulation.In the second part of this dissertation,we discuss the improvements in the Chen mixing scheme.As recognized,the mixed layer depth?MLD?plays an important role in the climate system through its influences on SST.The KTN bulk mixed layer model is designed for describing the MLD and has been adopted widely by many ocean modeling.However,large biases exist in the MLD simulation using the original KTN model in the tropical Pacific.This is partly due to the uncertainties in representing wind stirring effect in the model,which is scaled by a parameter?m₀?.Traditionally,m₀ is taken as a constant uniformly in space.In this study,the m₀ is estimated as spatially and seasonally varying through its inverse calculation from a balance equation describing the turbulent kinetic energy budget of the mixed layer.It is illustrated that the m₀ is spatially and seasonally varying over the tropical Pacific.The derived m₀ fields are then embedded into an OGCM.Compared with the observations and the Global Ocean Data Assimilation System analyses,the MLD simulations in the OGCM with varying m₀ are substantially improved in the tropical Pacific Ocean on seasonal and interannual time scales.Additionally,the Pacific subtropical cells become intensified,accompanied with the strengthening of upwelling in the eastern equatorial Pacific;thus,more realistic simulations are obtained when using spatially and seasonally varying m₀ case compared with the constant m₀ case.As the related cooling effect from the upwelling is enhanced,the simulated SST is slightly cooled down in the eastern equatorial Pacific.Further applications and implications are also discussed.In the third part of this dissertation,we optimize the depiction of background diffusivity and investigate its influences on tropical SST simulations.The background diffusivity,representing the integrated effects of diapycnal mixing processes in the ocean interior,is typically assigned with a constant value of 10^-5 m²/s in the current OGCMs.However,recent evidence shows that the diffusivity is reduced by about one order of magnitude in the tropics,implying that the tropical diapycnal mixing is overestimated in many ocean and climate modeling.The overestimated mixing can degrade the simulations in currents and water mass properties,but its relationships with the possible biases are not well demonstrated,mainly due to the sparsity of microstructure measurements for describing the spatial pattern of background diffusivity.In order to fill the gap,finescale parameterizations are proposed in recent studies,which provide an opportunity to estimate the background diffusivity with a global coverage by using the Argo profiles.In this study,the spatial structure for the background diffusivity in the tropical Pacific Ocean is derived based on the strain-based finescale parameterization,and is then employed into the MOM5-based ocean-only and coupled ocean-atmosphere simulations.Simulations of SST and upper-ocean temperatures are substantially improved by employing the Argo-derived background diffusivity compared with the original scheme,including a reduction in subsurface warm bias and thus a more realistic equatorial thermocline.The improved simulations in temperature can be attributed to the regulation in the currents system.By inhibiting the heat transfer into the ocean interior,the heat is accumulated below the mixed layer,resulting in the decrease in upper ocean stratification and the increase in Ekman layer depth.Meanwhile,the shallow meridional overturning circulation slows down and the related upwelling weakens,leading to the upper layer warming.The cooling effect beneath the thermocline is induced both by the reduced heat transfer from the upper layer and the convergence of the colder water from off the equator.By combining on the KPP mixing scheme with other schemes,a new optimized approach is proposed with Argo-derived background diffusivity and Peters'shear instability model embedded.This scheme can reduce the“cold tongue”errors by about 70%in MOM5,and can also be easily applied to other ocean and climate models.Therefore,this study has important scientific significance and practical applications in model improvements and climate modeling studies.