On The Responses Of Large-scale Ocean Circulation To Several Surface Forcing Changes

In this dissertation,the impact of synoptic atmospheric forcing on the mean ocean circulation,long-tern impact of including ocean surface currents in bulk formulas on surface air-sea fluxes and ocean general circulation,and the response of the Lorenz energy cycle for the Southern Ocean to intensified westerlies,are investigated.To carry out these studies,an eddy-permitting ocean-sea ice model,MITgcm-ECCO2,has been employed,and a series of sensitivity experiments has been conducted.To investigate the impact of synoptic atmospheric forcing on the mean ocean circulation,we compared simulations of MITgcm-ECCO2 forced with and without synoptic atmospheric phenomena.Consistent with previous studies,transient atmospheric motions such as weather systems are found to contribute significantly to the time-mean wind stress and surface heat loss at mid and high latitudes owing to the nonlinear nature of air-sea turbulent fluxes.Including synoptic atmospheric forcing in the model has led to a number of sigjnificant changes.For example,wind power input to the ocean increases by about 50%,which subsequently leads to a similar percentage increase in global eddy kinetic energy(EKE).The wind-driven subtropical gyre circulations are strengthened by about 10-15%,whereas even greater increases in gyre strength are found in the subpolar oceans.Deep convection in the northern North Atlantic becomes significantly more vigorous,which in turn leads to an increase in the Atlantic Meridional Overturning Circulation(AMOC)by as much as 55%.As a result of the strengthened horizontal gyre circulations and the AMOC,the maximum global northward heat transport increases by almost 50%.Results from this study show that synoptic atmospheric phenomena such as weather systems play a vital role in driving the global ocean circulation and heat transport,and therefore should be properly accounted for in paleo and future climate studies.The wind work and surface EKE can be reduced significantly when considering the relative motion between the ocean surface current and the surface wind(The Relative Wind Effect).The long-term impact of including ocean surface currents in the bulk formulas on surface air-sea fluxes and the ocean general circulation is investigated for the first time using MITgcm-ECCO2.It is found that including ocean surface currents in air-sea flux calculations significantly weakens the strength of the global ocean circulation in a number of ways:(1)reducing globally integrated EKE and mean kinetic energy(MKE)by about 27%and 12.5%respectively;(2)reducing the strength of the horizontal gyre circulations by 10-15%;(3)reducing the intensity of deep convection in the Labrador Sea by roughly 20%;(4)reducing the strength of the AMOC by about 12.6%.As a result,the maximum global northward heat transport drops by about 0.2 PW,which leads to a lower sea surface temperature and reduced surface heat loss in the northern North Atlantic.Additional sensitivity model experiments further demonstrate that it is including ocean surface currents in the wind stress calculation that dominates this long-term impact,with including ocean surface currents in the turbulent heat flux calculations making only a minor contribution.In addition,the potential impact of intensified westerlies on the Lorenz Energy Cycle for the Southern Ocean is examined by employing MITgcm-ECCO2.Two idealized sensitivity experiments are designed for this purpose:one is driven by 1992 forcing with weaker westerlies and the other driven by 1998 forcing with stronger westerlies.The intensified westerlies lead to the most significant increase of about 30%in the EKE reservoir,followed by the MKE reservoir increase(17.9%),eddy available potential energy(EAPE)reservoir increase(8.6%),and mean available potential energy(MAPE)reservoir increase(6.5%).In contrast,the increases in the generations of kinetic energy and available potential energy are quite similar,ranging from 21%for EAPE generation to 26%for MKE generation.There are considerablly increased energy transfers from MKE to MAPE(about 75%)and from MAPE to EAPE(about 78%),reflecting greatly enhanced baroclinic instability pathway.The conversion rates are strongly influenced by large topography;in particular,a relatively large energy conversion from EKE to MKE exists in the regions associated with large topography,in contrast to the energy flow from MKE to EKE over the broad Southern Ocean.Under stronger wind forcing,all energy conversions are enhanced,and the increases in the conversion rates from EAPE to EKE and from EKE to MKE are more prominent than the increases from MKE to MAPE and from MAPE to EAPE near large topography.