Responses of the Southern Ocean in a Changing Climate
Based on Argo float observations, ocean heat content increase has mostly occurred in the Southern Ocean to the north of the Antarctic Circumpolar Current (ACC) over the past 15 years. In contrast, the upwelling of the pristine water dampens the warming within the ACC, creating an uneven warming in the Southern Ocean. In this dissertation, we utilize modern observational data sets and state-of-the-art climate model simulations to investigate the responses of the Southern Ocean in a changing climate.We show that the equatorward displacement of maximum subsurface warming relative to maximum heat uptake is associated with mean overturning circulation from the zonal mean perspective. Using idealized model experiments, we decompose the Southern Ocean climate change driven by buoyancy forcing and wind stress change, respectively. We find that the ocean subsurface temperature and salinity change primarily stems from buoyancy forcing change instead of wind change. The subsurface warming pattern due to overturning circulation increases the meridional density gradient and enhances zonal geostrophic velocity in the upper layer. Therefore, we find a significant acceleration of the Southern Ocean zonal flow driven by the buoyancy forcing change. Based on observations, we also document a robust acceleration of Southern Ocean zonal flow at latitudes 48˚S-58˚S since 1993 (satellite altimetry) and since 2005 (Argo floats), supporting the attribution of the ACC acceleration to the greenhouse warming. Moreover, the spatial pattern of Southern Ocean subsurface warming is found to be anchored by the mean ocean jets based on model simulations. For regional responses, jet-scale overturning circulation straddling the strong ocean jets, such as Subantarctic Front and Agulhas Return Current, facilitates the warming on the equatorward, downwelling flank and suppresses the warming on the poleward, upwelling flank. The research presented in this dissertation improves understanding of the physical processes controlling the Southern Ocean climate change and contributes to reliable future projections.