The Change Of The Extratropical Zonal Flow In The Southern Hemisphere And Its Influence On The Southern Ocean

The atmosphere circulation in the extratropical Southern Hemisphere (SH) isdominated by the eddy-driven zonal flow. Due to the missing of the land in the SH,the extratropical zonal flow in the SH is characterized by substantial zonal symmetry.Changes in the extratropical zonal flow in the SH have significant implications forocean circulation and the global carbon cycle. In this thesis, we focus on the long termchange of the extratropical zonal flow in the SH. Based on the observation,atmosphere reanalysis data, ocean reanalysis data and MOM5ocean model, weinvestigate the change of the SH extratropical zonal flow in recent three decades andits influence on the Southern Ocean.The atmospheric reanalysis and climate models both suggest that theextratropical zonal flow in the SH has strengthened in recent three decades. Previousstudies attribute the strengthening of the SH extratropical zonal flow to the shift of theSouthern Annular Mode (SAM) index towards its high polarity due to dramatic ozonedepletion in the Antarctic. However, the development of the ozone depletion showssignificant seasonality, which only occurs in austral spring. The possible forcing fromthe ozone depletion should be confined to austral spring and summer. Here we analysethree factors that may be responsible for the trends of the extratropical zonal flow inthe SH: the SAM, the Antarctic ozone depletion, and SST. First, we use the regressionanalysis method to check the contributions of the SAM, ozone depletion, and SSTEOF1mode to the trends of the extratropical zonal flow in the SH. We find that, allthe three factors (the SAM, ozone depletion and SST variability) can drive the trendsof the extratropical zonal flow in the SH through eddy momentum flux convergenceand divergence. The contributions of the SAM and ozone depletion to theextratropical zonal flow in the SH are confined to the austral spring and summer, which is primarily due to the seasonality of the Antarctic ozone depletion. SSTvariability shows significant contribution to the trends of the extratropical zonal flowin the SH in almost all12months. We interprete the process that the SST influcencesthe extratropical zonal flow as below: SST EOF1mode shows relatively strong zonalsymmetry and significant meridional temperature gradient; the change of the SSTmode first induces the baroclinic response of the extratropical zonal flow in the SH;the baroclinicity associated eddy activity then reinforces the response in the uppertroposphere and reverses the response in the lower troposphere, which results in thetrends of the SH extratropical zonal flow with equivalent barotropic structure. Wefurther analyse the baroclinicity of the extratropical zonal flow at850hPa in the SH,and the result is consistent with the above interpretation.Then we focus on the change of the ENSO-SAM relationship and its possibleinfluence on the extratropical atmosphere circulation in the SH. The ENSO and SAMare the main climate modes that significantly influence the extratropical atmospherecirculation in the SH. Previous studies suggest that the relationship between theENSO and SAM may be statistically significant, and the ENSO-SAM relationshipmay influence the ENSO teleconnections in the extratropical atmosphere circulationin the SH. We calculate the sliding correlation of the ENSO and SAM, and find that:in austral spring the ENSO-SAM relationship shows significant long term change; inaustral summer the ENSO and SAM show long term stable significant relationship.Further analysis indicates that the long term change of the ENSO-SAM relationship inaustral spring is primarily due to the change of the alternated zonal flow associatedwith the ENSO, which regulates the development of the SAM events through eddymomentum flux convergence/divergence and results in the strengthening of theENSO-SAM relationship in austral spring. The significant ENSO-SAM relationshipin austral summer may be due to the alternated baroclinicity of the SH extratropicalzonal flow associated with the ENSO, which influences the eddy activity andregulates the development of the SAM events to be out of phase with the ENSOevents. We further investigate how the ENSO teleconnections change in the SH whenthe ENSO-SAM relationship shows long term change. Our analysis shows that, associated with the strengthening of the ENSO-SAM relationship in austral spring, theENSO teleconnections in sea level pressure (SLP) in the extratropical SH alsoexperience obvious long term change. This change in SLP further induces thestrengthening of the ENSO teleconnections in surface air temperature and sea surfacetemperature through temperature advection.Then we focus on the trends of the SST in the Southern Ocean. We investigatethe related mechanism of the SST trends and discuss the possible link between theSST trends and the change of the extratropical atmosphere circulation in the SH. Wefind that in recent three decades the SST over large parts of the Southern Ocean showsa cooling trend, although a warming trend is observed in the Indian sector of theSouthern Ocean. Mixed layer heat balance buget analysis indicates that oceanhorizontal advection and entrainment are the dominant contributors to the warmingtrend in the Indian sector, while ocean surface heat flux tends to cool the ocean in theIndian sector. Ocean surface heat flux is primarily responsible for the cooling trend inthe Pacific and Atlantic sectors. Further analysis indicates that the deepening of themixed layer depth in the whole Southern Ocean is primarily responsible for thedecreasing trend of the ocean surface heat flux term. The deepening of the mixedlayer depth may be due to the strengthening of the extratropical atmospherecirculation in the SH.Finally we do the sensitivity experiment to investigate the response of theSouthern Ocean to the strengthening of the westerlies based on the global coupledocean-ice model. We check how the ocean variables (surface temperature, salinity,and velocity) and sea ice variables (sea ice concentration, sea ice thickness and sea icetransport velocity) change when the surface wind strengthens. The model resultindicates that the most significant change of the surface temperature is in the Pacificsector, which shows substantial cooling change; the surface salinity shows increasingchange in the whole Southern Ocean; the most significant change of the surfacevelocity is in the Pacific sector, which shows anomalous flow towards the equator; thesea ice concentration and sea ice thickness decrease dramatically in the Weddell Sea;the change of the sea ice transport velocity indicates that the sea ice diverges in the Weddell Sea and converges to the east of the Weddell Sea. The decrease of the sea icein the Weddell Sea is primarily due to:(1) the surface wind transporting more warmair from the Antarctic Peninsula to the Weddell Sea;(2) the divergence of the sea icein the Weddell Sea and the convergence of the sea ice to the east of the Weddell Sea.