The Changes In The Southern Ocean Wind Stress And Their Effects On The Subpolar Ocean

The Southern Ocean(SO)surface westerly wind stress plays a fundamental role in driving the Antarctic Circumpolar Current and the global meridional overturning circulation.The SO westerly wind stress has strengthened significantly over the last few decades and is projected to continue to do so in the future.Studies on the changes of SO wind stress and their impact on the ocean need to take into account changes of not only the low-frequency variability of the westerly jet,but also wind fluctuations at much shorter time scales.However,so far,there have been few systematic and quantified researches about the effects of wind fluctuations on the wind stress changes and the impacts of wind stress changes on the subpolar ocean.In this dissertation,the changes of the Southern Ocean wind stress and its effects on the subpolar ocean are investigated.The reanalysis data and CMIP5 model results are used to quantify the contributions of wind fluctuations to the mean,seasonal variability,and trend of SO wind stress for the first time.Then,a high-resolution ocean-sea ice general circulation model(MITgcm-ECC02)is used to explore the effects of Southern Hemisphere(SH)surface westerlies on Antarctic Continental Shelf Bottom Water(ASBW)temperature by conducting the sensitivity experiments.The contributions of atmospheric wind fluctuations to the mean,variability,and trend of SO wind stress over the last four decades using NCEP and ERA-Interim reanalysis products are investigated at first.The magnitude of peak zonal-mean wind stresses is doubled when wind fluctuations are included in the stress calculation.Over 70%of this doubling effect is owing to fluctuations that last less than 8 days,that is,associated primarily with weather systems/baroclinic storms.The intensity of wind fluctuations exhibits a pronounced seasonal cycle,being highest in austral winter and lowest in austral summer.As a result,the peak zonal-mean wind stress is greater in austral winter than in austral summer,despite the mean westerly wind being stronger in austral summer than austral winter.Large discrepancies are found among the reanalysis products analyzed in this study regarding the contribution of wind fluctuations to the strengthening trend of SO wind stress from 1979 to 2016.The intensities of wind fluctuations in NCEP R1 and NCEP R2 have increased significantly over the last four decades and are found to contribute to about one-third and one-half of the increase in the strength of SO wind stress,respectively.In contrast,the intensity of wind fluctuations only experiences a very modest increase in ERA-Interim,and as such,the wind stress trend in ERA-Interim is explained almost entirely by the strengthening of the mean westerly wind.Then the results of historical simulations(1960-2005)from Coupled Model Intercomparison Project phase 5(CMIP5)models are used to investigate the influence of atmospheric wind fluctuations on the mean,seasonal variability and trend of SO wind stress.For all 12 models,wind fluctuations make important contributions to the mean and seasonal variability of SO wind stress.When annual wind fluctuations are included in wind stress calculations,the Multi-Model Mean(MMM)peak zonal wind stress increased by 126%with 75%of the increase being induced by wind fluctuations at synoptic frequencies(2-8 days)and higher.Meanwhile,larger multi-year-mean and zonal-mean SO wind fluctuations in austral winter than summer increase the corresponding wind stress in austral winter.The strengthening of mean wind stress from 1960 to 2005 is only significant in seven models and explained mostly by the strengthening of the mean westerly winds.The non-significant trends of wind fluctuations and austral-autumn wind stress imply the limitations of CMIP5 models in capturing the trend of SH westerly wind and the needs to improve the simulations of wind fluctuations changes in the future.The SH westerlies has strengthened and shifted poleward since the 1970s,which is connected to an increasing positive southern annual mode(SAM)index and this trend is about to continue under present anthropogenic forcing.The SAM is the dominant mode of atmospheric variability in the SH and we found that the SAM index is a good indicator for both the mean westerly wind and wind stress.The ASBW temperature is a key factor for ice mass balance of Antarctica.To better understand the impacts of SH westerlies on Antarctic coastal waters,we use atmospheric forcing of 1992 with weak westerlies and 1998 with stronger westerlies to drive the high-resolution ocean-sea ice general circulation model MITgcm-ECC02.Our model results show that under atmospheric forcing of 1998,the ASBW is warmer in most regions around the Antarctica except the coastal region between 60°-150°W than in the case under atmospheric forcing of 1992,which is consistent with the observational results.The main reasons for the warming of ASBW in the most regions around the Antarctica are owing to intense shoaling and warming of CDW associated with strengthened subpolar gyres induced by enhanced Ekman pumping.The strengthened subpolar gyres transport more warm water to the coast of the Antarctica.The cooling of ASBW in the coast of western Antarctic Peninsula is caused by stronger coastal currents,which bring much more colder water downstream from the northwest part of Weddell Sea.