The influence of natural climate modes on the interannual variability of the deep-water plume in the northwestern Weddell Sea

Abstract

A fraction of the deep-water plume that flows along the slope in the NW Weddell Sea eventually leaks from the Weddell Gyre through deep passages on its northern margin. This provides source waters for the Antarctic Bottom Water that ultimately fills the ocean abyss as the lower branch of the Meridional Overturning Circulation (MOC). Despite the importance of this supply, uncertainties still remained associated to its interannual variability. Here we investigate the role played by the combined effect of two natural climate modes in the interannual variability of the densest  water mass found within this plume, the Weddell Sea Bottom Water (WSBW). Previous studies found that both the Southern Annular Mode (SAM) and the El Niño-Southern Oscillation (ENSO) influence the winds around Antarctica, and suggested that their overlapping effects on the along-shore winds are reinforced when they occur at opposite phases (i.e. a positive SAM with a La Niña or a negative SAM with an El Niño). We prepared a combined SAM-ENSO climate index (SEI) that takes into account their overlapping effects on the winds and performed a lagged cross-correlation analysis with a 2005-2022 timeseries of WSBW thermohaline properties measured at the bottom instrument of a mooring redeployed in the NW Weddell Sea (AWI207). The significant correlations found suggest that a positive SAM occurring in summer, reinforced by a La Niña event, can influence the WSBW properties measured in the NW Weddell Sea at two different time scales. First, it would produce a warming of the WSBW reaching our mooring in the NW Weddell Sea between 4 and 5 months later. We propose that this warming is caused by the entrainment of a less modified WDW during the formation of WSBW. This is enabled by the weaker along-shore winds induced by a positive SAM and a La Niña event. Second, it would induce a freshening  of the WSBW that can be measured in the NW Weddell Sea between 13 and 14 months later. This freshening is probably related to the first mechanism proposed by McKee et al. (2011), i.e. negative anomalies in the meridional winds in the eastern side of the Antarctic Peninsula in summer would induce a reduced HSSW formation during the next winter and a decrease in the export of dense shelf waters during the next summer. However, the freshening mechanism proposed by Gordon et al., (2020), i.e. the wind-driven deepening of the V-shaped double front located at the shelf break in the western Weddell Sea, might also contribute to this freshening by enabling the injection of fresh shelf waters into the WSBW plume.