Impacts Of Changes In Topography And Freshwater Forcing On Thermohaline Circulation

The ocean is an important component of the climate system. Due to its vast areaand large heat capacity, the ocean plays a vital role on the heat redistribution. Due toits dominate role in controlling water properties in the ocean, changes ofThermohaline Circulation (THC), in particular the mode shifting, may induce drasticchanges in climate. THC can be affected by many factors, such as topography, wind,surface forcing of heat and freshwater, and sea ice. On the geologic time scale, majorchanges in global topography induced major changes in ocean circulation and climate.One of the most significant events is the opening of the Drake Passage (DP). After theopening of the DP, the perennial ice sheet on Antarctic gradually built up, thetemperature of deep ocean decreased sharply, and the temperature difference of upperand bottom water in equatorial regions more than doubled than before the opening ofthe DP. On the millennial scale, mode shifting of THC is often closely linked tofreshwater input. For example, changes of freshwater forcing may induce thecatastrophic phenomenon such as the Younger Dryas.Due to THC’s long time scale and large space scale, observations are sparse. As aresult, numerical models are widely used as the tool in research of THC. Comparedwith those complicated atmosphere-ocean-sea ice-land coupled models, box modelsare much simpler, and have some unique advantages because they require much lesscomputer power and can be used to explore the fundamental physical processes in theoceanic general circulation. Therefore, they can be implemented for long time scalesimulation. In this study, a simple box model is designed to explore the impacts of theopening of the DP and changes of freshwater forcing on THC.The opening of the DP has a major impact on the oceanic general circulation are examined. When the DP is open, wind stress at mid-and high latitudes in theSouthern Hemisphere produces a wind-driven gyre, which induces a meridional heatexchange between mid-and high latitude in the Southern Ocean. After the DP opens,the Antarctic Circumpolar Currents (ACC) forms and its associated strongtemperature front blocks the heat transport from mid-latitudes to high latitudes. Asimple box model is formulated, in order to explore the effects of the wind stress (forthe case of the DP closed) and the thermal front (for the case of the DP open) on thevariability of Antarctic Bottom Water (AABW) and North Atlantic Deep Water(NADW). Our sensitivity experiments demonstrate that:(1) When the DP is closed,the enhancement of the wind-driven gyre leads to the decline of AABW formation inthe Southern Ocean and the increase of NADW formation in the North Atlantic. As aresult, water at high latitudes in the Southern Ocean becomes warmer, so does thebottom water of global ocean.(2) When the DP is opened, there are two types ofmode. If the intensity of thermal front along ACC is below a threshold value (it is4.03℃in our model), there is no formation of AABW and ocean temperature in most of areasis relatively warm. When the thermal front is stronger than the critical value, AABWstarts to form, ocean temperature in most areas starts to decline. In particular, highlatitudes of the South Hemisphere and the bottom water in global ocean cools down.These results demonstrate that during the opening of the DP changes in wind stressand the formation of the thermal front in the Southern Ocean can substantially affectthe formation of AABW and NADW, thus changing the state of meridionaloverturning circulation in the global ocean.To explore the effect of changes in freshwater forcing on THC, threeexperiments are carried out: A) Changing the freshwater forcing distribution inthe Southern Hemispheres and Antarctic region; B) Changing the freshwaterforcing distribution in the Northern and Southern Hemispheres; C) Changing thefreshwater forcing distribution in the Northern Hemisphere and Antarctic region.In Experiment A, there is no significant change in THC, and the adjustment timeis much longer. In Experiments B and C, there are shifting in mode and formationof the deep water; in addition, the adjustment time is relatively short. The major feature s in Experiments B is the significant change in the formation andadjustment of AABW; on the other hand, in Experiment C there are significantchanges in the formation and adjustment of NADW. Salinities and temperaturesin the middle and bottom layers tend to be the same in Experiments B and C,which suggests that freshwater forcing can regulate the circulation and waterproperties in the deep ocean through affecting the formation of deep/bottomwater.