Open Ocean Polynyas and Antarctic Slope Current in High- Resolution Earth System Model Simulations

Abstract

The first part of my dissertation focusses on the intermittent occurrence of Open Ocean Polynyas (OOPs) in a 500-year-long High-Resolution Pre-Industrial (HR-PI) Community Earth System Model 1.3 simulation (Chapter 3). During the winter season, the near-surface salinity stratification is found to be a key condition for the intermittent occurrence of the OOPs. Increased/decreased stratification, resulting from strong/weak freshwater fluxes at the surface will hamper/favor the formation of polynyas. The surface freshwater flux varies with a regional Southern Annular Mode (SAM)-like index (measured over a longitudinal section instead of circumpolar) and the associated meridional shift of the precipitation-rich westerly winds. Based on this HR-PI simulation, I detected a new possible regional ocean-atmosphere coupled mechanism that explains both the intermittent occurrence of OOPs and the simultaneous change of the regional SAM index. When large Weddell Sea Polynya (WSP) emerge, they affect the regional atmospheric sea-level pressure, thereby feeding back onto the regional SAM index. The initiation of polynya events is controlled by changes in surface properties while the location of initiation is determined by bathymetric features.

The second part of my dissertation deals with the anthropogenic impact on the formation of WSPs and open ocean deep convection in an accompanying 250-year HR historical and future Transient (HR-TN) simulation (Chapter 4). In HR-PI, the (regional) SAM index does not have a clear trend, and only oscillates around its mean value. This provides a suitable environment for studying the intermittent occurrence of OOPs. In HR-TN, the anthropogenic impact forces the (regional) SAM index to become more positive. The associated poleward movement of the precipitation-rich Southern Hemisphere westerlies brings more freshwater and heat to the Weddell Sea region. At the same time, less sea ice forms due to the higher air temperature. These changes increase the stratification in the Weddell Sea, which in turn suppresses open ocean deep convection and the return of WSPs. However, shallow convection continues to occur intermittently over and around the Maud Rise seamount, even though this region turns ice-free in winter. Another noticeable feature is that the zonal asymmetry of the poleward movement of the westerlies leads to a drastic reduction of the winter ice cover in the Weddell Sea, but not in the Ross Sea. Corresponding Low-Resolution (LR) simulations do not produce any OOPs, and the anthropogenic forcing leads to an overall reduction of ice extent, but not to quasi-ice-free winters in the Weddell Sea as in HRTN.

For the third part of my dissertation, I investigated the anthropogenic impact on the Antarctic Slope Current (ASC) and the Dense Shelf Water (DSW) overflow in both HR and LR simulations (Chapter 5). In HR-PI and LR-PI, the generation of AABW from DSW overflow is partly captured on both the continental shelf and slope of the Ross Sea and the Weddell Sea. However, due to excessive convective entrainment, the bottom water potential density in the Ross Sea is lower than in observations in both HRPI and LR-PI. Due to HR-PI regularly simulating open-ocean deep convection in the Weddell Sea, the bottom water potential density there matches better with the observations than in LR-PI. In HR-TN and LR-TN, the generation of AABW from DSW overflow ceases as the shelf water becomes more buoyant. The latter is mostly attributed to a reduction of sea ice formation along the coast. ASC confines most freshwater input tightly along the Antarctic coast and increases the sea surface height (SSH) there. This, in turn, leads to an increased meridional SSH gradient, which then feeds back to further strengthen the ASC. This positive feedback isolates the Antarctic shelf waters, and eventually turns the three present-day characteristic shelf water regimes all into “Fresh Shelf” regimes.