Long-term Changes Of Subduction And Obduction Over The Global Ocean And Their Mechanisms In The20^th Century

The process of subduction and obduction, which are known as the ventilationprocess, has been recognized as a primary channel for the communication between thesurface layer of the ocean and the subsurface. The water mass in the mixed layercould be transported to the main thermocline through the subduction process. On thecontrary, water mass from the main thermocline could be transported to the mixedlayer through the obduction process. Through the atmosphere-ocean interface, theventilation plays a key role to the link between the active atmosphere and the changeof the ocean subsurface layer. Meanwhile, the ventilation may impact both of theocean thermal structure and circulation substantially. Especially under the background of global warming, the interannual and decadal variability of the ventilationand its response to the climate system have been increasingly concerned.Based on the gradually plentiful ocean observation data and improved climatemode results, abundant studies have been carried out for the subduction and obductionprocess. The ventilation has been proved to have interannual and decadal variabilityand the evolution of the subduction and obduction has been certified to closelyassociate with the global warming. However, comprehensive studies on the variabilityof the trend of the subduction and obduction under global warming are still weak, andfurther investigations on the mechanism and inducement of the trend of the ventilationare urgently in demand.In this paper, we focus on the variability of the trend of the ventilation and itsmechanism as a key scientific issue. Based on a recently developed20thcentury oceanreanalysis product SODA and an atmospheric reanalysis dataset20CRv2, the annualmean subduction rate (1900-2007year) and obduction rate (1901-2008year) have been assessed through a calculation program of water parcel tracer under theLagrangian coordinate. First, through congregating the results of every basin, thetrend of the subduction and obduction and their mechanism have been studied overthe global ocean point of view. And then, the trend of ventilation and its mechanismunder global warming in every basin has been analyzed separately. As the mostimportant production of the subduction process, mode water, it has been proved tohave remarkable influence on both ocean thermal structure and circulation. So wefurther investigated the production of the mode waters separately in every basin,which is the subduction rate in the formation region of the mode waters.According to the calculation program of the subduction rate and obduction rate,the ventilation rate consists of the vertical pumping term and the lateral inductionterm. The Ekman pumping rate arisen by the wind stress curl plays a dominant role tothe vertical pumping term, and the lateral induction term is influenced by the wintermixed layer depths and the currents. In order to quantitatively assess the trend of thesubduction rate and obduction rate, through analyzing the potential vorticity on theisopycnals, the trend of the Ekman pumping rate, the distribution and the change ofthe winter mixed layer depth, the change of the currents in the upper layer of theocean and a MLD budget equation, we studied the mechanism of the trend ofventilation in every basin separately.Through the quantitative analysis of results, it suggests that the subductionvolume of the global ocean is intensified in the20thcentury. The contribution of thevertical pumping term and the lateral induction term all display an intensification, butthe vertical pumping term plays a dominant role in the intensification of the globalsubduction process. An intensification trend of subduction has been found in theSouth Pacific, the South Atlantic, the Indian Ocean and the equatorial region, whilethere is no significant change in the North Pacific and the North Atlantic. Under thecontribution of the vertical pumping term and the lateral induction term, thesubduction of the South Pacific displays a intensification trend, and contribution from the lateral induction surpass the vertical pumping term. The contribution of thevertical pumping term and the lateral induction term to the intensification ofsubduction in the South Atlantic and the Indian Ocean are similar with each other.Through analysis to the subduction volume of mode waters in each basin separately, itsuggests that different response to the global warming of mode waters has been foundand the dominant roles to their intensification or weakening are different. Especiallyin the North Pacific, the subtropical mode water displays a weakening trend, while thecentral mode water displays a significant intensification trend, although there is nosignificant change in the whole basin of the North Pacific.Similar to the trend of the global subduction, an intensification trend of obductionvolume has been found in the global ocean. The contribution of the vertical pumpingand the lateral induction all displays an intensification trend in the North Pacific, sodoes it in the South Pacific and the South Atlantic, but the contribution from thelateral induction surpasses that of the vertical pumping term. The contribution of thevertical pumping term and the lateral induction term to the intensification ofobduction in the Indian Ocean are similar with each other. In the North Atlantic, thecontribution of the lateral induction surpasses that of the vertical pumping term to theintensified obduction. Although a weakening trend of the lateral induction term hasbeen detected in the equatorial region, but under the dominant influence of the verticalpumping term an intensification trend has been presented.