Decadal Abyssal Ocean Stratification Change

Cold and dense Antarctica Bottom Water (AABW) forms near the coastline of Antarctica, sinks to the ocean bottom, moves northward in deep branches of the Meridional Overturning Circulation, becomes lighter through mixing, and eventually upwells to shallower depths. The AABW properties change under the changing climate. Repeat hydrography sections occupied approximately once per decade since the 1990s have revealed continuously warming and freshening of the AABW. In many regions of the ocean, the warming and freshening trends are strongest at the bottom. This causes a decrease in the vertical gradient of density, namely stratification. As the level of mixing is strongly related to stratification, it is important to quantify its trend and understand its spatial structure. I quantify the trend in stratification globally and find a reduction in stratification in the Southern Ocean and along the spreading pathways of the AABW at lower latitudes.