Antarctic Bottom Water Variability in a Coupled Climate Model

Abstract

The natural variability of the Weddell Sea variety of Antarctic Bottom Water (AABW) is examined in a long-term integration of a coupled climate model. Examination of passive tracer concentrations suggests that the model AABW is predominantly sourced in the Weddell Sea. The maximum rate of the Atlantic sector Antarctic overturning ( atl ) is shown to effectively represent the outflow of Weddell Sea deep and bottom waters and the compensating inflow of Warm Deep Water (WDW). The variability of atl is found to be driven by surface density variability, which is in turn controlled by sea surface salinity (SSS). This suggests that SSS is a better proxy than SST for post-Holocene paleoclimate reconstructions of the AABW overturning rate. Heat-salt budget and composite analyses reveal that during years of high Weddell Sea salinity, there is an increased removal of summertime sea ice by enhanced wind-driven ice drift, resulting in increased solar radiation absorbed into the ocean. The larger ice-free region in summer then leads to enhanced air-sea heat loss, more rapid ice growth, and therefore greater brine rejection during winter. Together with a negative feedback mechanism involving anomalous WDW inflow and sea ice melting, this results in positively correlated -S anomalies that in turn drive anomalous convection, impacting AABW variability. Analysis of the propagation of -S anomalies is conducted along an isopycnal surface marking the separation boundary between AABW and the overlying Circumpolar Deep Water. Empirical orthogonal function analyses reveal propagation of -S anomalies from the Weddell Sea into the Atlantic interior with the dominant modes characterized by fluctuations on interannual to centennial time scales. Although salinity variability is dominated by along-isopycnal propagation, variability is dominated by isopycnal heaving, which implies propagation of density anomalies with the speed of baroclinic waves.

Warming of Global Abyssal and Deep Southern Ocean Waters between the 1990s and 2000s: Contributions to Global Heat and Sea Level Rise Budgets*

“…These stronger winds may also spin up the Weddell gyre, increasing the temperature of abyssal waters escaping that gyre to flow northward (Jullion et al 2010), and perhaps similarly the Ross gyre. Finally, wind strength and position also affect ice coverage and warm water transport into formation regions, changing formation rates of AABW (Santoso and England 2008).…”

Impact of the Antarctic bottom water formation on the Weddell Gyre and its northward propagation characteristics in GFDL CM2.1 model

“…This is the first study using a fully coupled model to systematically examine the relationship between AABW cell and the Weddell Gyre on decadal time scales. The export of AABW from the Weddell Sea has been investigated by sparse observation and coarse ocean‐only and coupled models [e.g., Naveira Garabato et al ., ; Orsi et al ., ; Stössel and Kim , ; Santoso and England , ]. However, their results sometimes are contradictory.…”

“…Analyses of water mass and oceanic chlorofluorocarbon budget estimate that 60–70% of AABW is formed in the Weddell Sea [ Carmack , ; Orsi et al ., ]. Coupled climate models suggest that Southern Ocean deep convection mainly occurs in the Weddell Sea, which also implies a strong AABW formation in the Atlantic [e.g., Santoso and England , ; Martin et al ., ; Latif et al ., ].…”

“…Using a coarse global ocean‐sea ice model, Stössel and Kim [] suggested that the AABW anomaly can propagate to the subtropical Atlantic a few years later through baroclinic Kelvin waves with phase speed corrected for the model's grid resolution. Santoso and England [] argued that the temperature anomaly induced by AABW fluctuation is dominated by isopycnal heaving, while the salinity anomaly is dominated by along‐isopycnal propagation. This implies that the density anomalies associated with AABW fluctuation can propagate to the interior ocean with the speed of baroclinic waves in their coarse resolution fully coupled climate model.…”

Dense shelf water spreading from Antarctic coastal polynyas to the deep Southern Ocean: A regional circumpolar model study

“…There have been several modeling studies on the pathways of AABW across the Southern Ocean. Doney and Hecht [] and Santoso and England [] investigated modeled AABW pathways, using passive tracers in coarse resolution models. Their horizontal resolutions ( ) were suitable for long‐term integration, but too coarse to realistically resolve the bottom topography of the Southern Ocean and to reproduce Antarctic coastal processes for dense water formation.…”