Hydrographic conditions during two austral summer situations (2015 and 2017) in the Gerlache and Bismarck straits, northern Antarctic Peninsula

Abstract

Hydrography in the Gerlache and Bismarck straits is described based on data collected in Januaries of 2015 and 2017, during austral summer. These straits are in the northern Antarctic Peninsula (NAP) continental shelf. In the study area, temperature in the surface mixed layer during January 2017 was warmer than recorded in other years, contemporaneous with an extreme decrease in Antarctic sea ice extent at the end of 2016. This was probably caused by warm water advected from the north into Gerlache Strait, a consequence of northeasterly surface winds over the NAP, forced by a Southern Annular Mode (SAM) negative phase. Optimum multiparameter analysis indicated that relatively warm, salty and low-oxygenated modified Circumpolar Deep Water (mCDW) intrudes over the NAP continental shelf and dominates the Bismarck Strait, where it enters the Gerlache Strait, especially through its southern extremity. The mCDW mixes with relatively cold, fresh and recently ventilated modified High Salinity Shelf Water (mHSSW), producing temporal variations in water proportions. HSSW forms in the Weddell Sea and is advected southward along the deep basins of the Bransfield Strait, entering as bottom water into Gerlache Strait via its northern limit. There were two fronts in Gerlache Strait during both summers. First was a surface thermal front in the mixed layer represented by the 1 °C isotherm (~64.5°S), which separated colder waters toward Bismarck Strait. Second was a sub-pycnocline front, represented by the 0.4 °C and 220 μmol kg−1 dissolved oxygen contours below ~100 dbar, which separated mCDW toward Bismarck Strait from mHSSW on the northern side of the strait. This frontal position varies as a consequence of temporal variations in source water fractions. Moreover, changes in Ekman transport, forced by enhanced westerly winds associated with a SAM positive phase, appeared responsible for a major fraction of mCDW in Gerlache Strait during 2015 relative to 2017. The spatiotemporal variability of source water mixing in the water column structure of the Gerlache Strait might have implications for local biological and chemical cycles, and for regional climate feedbacks, because warm mCDW accelerates glacier retreat in the western Antarctic Peninsula.