Study On The Variations And Dynamic Mechanisms Of The Wind-driven Ocean Circulation In The Pacific Section Of The Southern Ocean

The Southern Ocean, where the sea takes lager proportion in the land-seadistribution and contacts the southernmost waters of the Southern Hemisphere, plays amuch significant and profound role on the responses and feedback of the exchanges ofthe mass and energy to global climate changes. As the most important driving factor,the Southern Hemisphere subpolar westerlies drives directly the ACC system a ndmaintains the Southern Ocean MOC, thus influencing indirectly the formation andproperties of water masses; the property of the wind stress curl in the midlatitudesassociated with the westerly winds permits an inter-oceanic connection of the threesubtropical gyres and the formation of the Southern Hemisphere subtropical supergyre.As a new found character of the ocean general circulation, the supergyre has closeconnection with the southern ACC system. In this paper, we provide somecomprehensive and systematic researches on their interaction, multidecadel changesand the climate impact under the influence of the wind changes in the Pacific sectionof the Southern Ocean, and reveal the key mechanisms and processes of the oceanicresponses to this wind changes through sensitivity experiments. Some main newreseareh results are listed as follows:1. Driving by the poleward-intensifying basin-scale westerly winds associated withthe SAM changes, the water masses in the Southern Hemisphere subtropicalsupergyre interiors become cooler/fresher, with the significant exceptions of theAgulhas Current system and Agulhas leakage. The results also exhibit a strongsouthward shift in the central-south Pacific, suggesting a pronounced strengthening ofthe inter-basin connection of the supergyre.2. The simulated Tasman Outflow (TO) off eastern Tasmania originates in theremainder of the East Australian Current (EAC) and joins the ACC recirculation toturn west. After flowing past southern Tasmania, the TO splits a weak northwardbranch flowing along the western slope off Tasmania constitutes partly the FlindersCurrent (FC) while others merge ultimately into a much stronger westward flowacross the South Australian Basin. The simulated transport variability of TO is largestin the decadal band, slightly smaller in the mesoscale and quasi-annual bands, andsmallest in the interannual band. In particular, the modeling results fairly support thatthe energetic offshore eddies associated with the EAC migrate poleward andwestward around the Tasmania and play a key role in modulating the current path and the mesoscale variability. And it also reveals that the quasi-annual changes intransport are highly correlated with changes in wind stress curl over the South Pacificand the EAC.3. The full-depth CTD observations reveal the net westward transport at the northend of WOCE I9S section, determined by the south Australian circulation system,corresponds to the above Tasman leakage. Relying on a consistent set of water masscriteria and baroclinic transport maxima, the ACC fronts are identified. Mesoscaleeddy features are identifiable in the high-resolution CTD sections and tracked inconcurrent maps of altimetric sea level anomalies (SLA). Because of the existenceand propagation of the remarkable mesoscale eddy features within the SAF, theeastward transport of the SAF presents at two latitude bands separating by1oand thecross-front transport occurs. Both the CTD surveys and the altimetric data suggestthat the mesoscale variability is concentrated around the APFZ and causes the ACCfronts to merge, diverge, and to fluctuate in intensity and position along their paths.4. The above researches suggest the dominated role of the wind-driving on thesupergyre and ACC system. The European Centre for Medium-Range WeatherForecasts (ECMWF) Re-Analysis (ERA-40) westerly winds represent a significantmultidecadal poleward-intensifying changes in the Pacific section, which is associatedwith the typical positive phase of the SAM. Using the MITgcm s imulations at thecoarse and eddy-resolving resolutions, we discuss the mechanisms of the oceanicresponses to this wind changes in detail. The results indicate that the first response isthe oceanic Ekman dynamical processes. The strengthening and the poleward shift ofthe northward Ekman velocity as well as the Ekman pumping rate led to acorresponding strengthening trend in the southern branch of the Deacon Cell. Thisstrengthening, in turn, intensified the meridional density gradient and the tilting of theisopycnal surfaces, so the southern ACC transport and the subpolar gyre strengthened.The intensified wind stress curl over the midlatitude ocean leads an enhanced andexpanded subtropical gyre, making the Tasman leakage be more crucial in thePacific/Indian connection. The changes of the gyres then lead a considerablenarrowing of the ACC that tends to reduce the ACC transport. However, there is nosignificant correlation between the ACC position change and the poleward-shiftingwesterly wind jet. The modeling reveals that the enhanced and poleward shift of the Ekman transport carries more lighter water from the subpolar region and results in theSAMW and AAIW presenting a robust cooling/freshening in the central-south Pacific,east of New Zealand. The combined influences of the poleward-intensifyingsubtropical gyre and strengthening southward transport of the subtropical deep MOCmake the ACC region significantly warm. The SAM-like wind changes alsocontribute to warming and reducing the formation rate of AABW. In the longer term,the eddy permitting simulations demonstrate that changes in poleward eddy fluxespartially compensate for the enhanced equatorward Ekman transport and makes thewind-driven changes is only moderate and not as strong as the coarse-resolutions. Theenhanced eddy state is more effcient at transporting poleward heat, leading to awarming of the Southern Ocean.Above research results lay the foundation for the further exploration of the keyrole of the Southern Ocean on the global climate change.