Time-variable bottom water outflow in the Northwestern Weddell Sea

The Antarctic Bottom Water (AABW) has warmed in recent decades, with implications for the heat balance of planet Earth and thermosteric sea level rise. More than 60% of the AABW are supplied by Weddell Sea Deep Water (WSDW), which gains its high density from the Weddell Sea Bottom Water (WSBW). The WSBW descends down the continental slope along the western margin of the Weddell Sea as a northward flowing plume, thereby entraining warmer ambient waters.

Mooring array-based observations of the WSBW plume near the tip of the Antarctic Peninsula between 1989 and 1998 and between 2005 and 2012 reveal sizable intraseasonal and seasonal fluctuations of temperature and salinity. This poses a challenge to the use of nine cross-slope hydrographic sections acquired along the mooring array in different months in the different years in order to infer interannual changes in WSBW properties. WSDW in the interior Weddell Sea has been undergoing a decadal warming (Fahrbach et. al, 2011). We show that this is in striking contrast to the WSBW plume, which shows a cooling rather than a warming over the observational period. The cooling might be a consequence of changes in the source areas of the WSBW plume, namely the disintegrations of the ice shelves Larsen A and Larsen B in 1995 and 2002, respectively. Simulations based on the finite element ocean model FESOM are used to link the time-variable WSBW outflow near the tip of the Antarctic Peninsula to possible changes in the WSBW source areas upstream.

The velocities observed inside the plume were considerably lower before 1993 than afterwards. Based on records until 1998 the plume volume transport amounted to 1.3 ± 0.4 Sv (Fahrbach et.al, 2001). Continued records after 1993 indicate a stable a volume transport of 2.5 Sv. Our results suggest that the former estimate may have been biased low as a result of topographic blocking of the flow before 1993.