Long-term changes on the Bransfield Strait deep water masses: Variability, drivers and connections with the northwestern Weddell Sea

Abstract

The Bransfield Strait receives substantial input from recently ventilated Dense Shelf Water (i.e., High Salinity Shelf Water – HSSW or Low Salinity Shelf Water – LSSW) formed in the Weddell Sea continental shelf. The bathymetric restricted connections to the surrounding oceans make the Bransfield Strait's deep basins proxy regions to study the temporal variability of these waters. HSSW is an important precursor to Antarctic Bottom Water, a water mass with a global impact on the deeper branches of the global overturning circulation. The deep waters in the central basin of the Bransfield Strait have unveiled significant long–term cooling, freshening, and lightening between 1963 and 2019. Conversely, in the eastern basin of the Bransfield Strait, signals of freshening, lightening, and decreasing of dissolved oxygen have been present since 1975, but somewhat less robust than those founded on the central basin. Hence, that dichotomy in neighboring regions influenced by same source water masses suggests different mixing proportions and variability, likely resulting from distinct source regions of the dense waters precursors. In addition, the deep water masses in the Bransfield Strait have shown a strong interannual variability of thermohaline properties, which is mainly explained by the El Niño–Southern Oscillation and the Southern Annular Mode. These climate modes are likely to influence the periods of increasing salinity, such as 2010 to 2016, favoring higher intrusions of HSSW rather than the less dense variety into the Bransfield Strait. Evidence to the increased contribution of HSSW during that period may be explained by an increase in the sea ice formation in the Weddell Sea. During the sea ice formation, higher rates of Dense Shelf Water was likely formed in the northwestern Weddell Sea due to brine rejection. The changes reported here are key to add new insights on a better understanding of how changes in the source water masses can affect the deep branches of the global overturning circulation cell, which is still a challenging question for the community. Finally, the hydrographic properties time series in the Bransfield Strait used here spanned through six decades, which is much longer than those previously reported in the Weddell Sea, providing further and stronger evidence on how climate variability affects bottom water in the Southern Ocean.