How Does Southern Ocean Mode Water Ventilate the Subtropics?

Southern Hemispheric subtropical mode waters are formed and subducted via narrow windows of intense mixing and weak stratification just north of the Antarctic Circumpolar Current. These mode waters eventually ventilate the subtropical thermocline, potentially trapping anthropogenic carbon in the deep interior ocean for centuries to millennia. However, the manner in which these mode waters ultimately ventilate the subtropics is not well known. In this work, we aim to (i.) identify the primary mode water export pathways out of the Southern Ocean and to (ii.) better understand the factors that control the location and intensity of these pathways. Using passive tracers and Lagrangian floats in an eddy-permitting Southern Ocean model, we examine the three-dimensional structure of mode water export routes on annual to decadal timescales. Specifically, we quantify how mode waters mix with surrounding water masses along export pathways (e.g. mixing between Subantarcitc Mode Waters and Pacific Deep Water) and highlight the longitudes and density classes along which mode water leaves the Southern Ocean. We find that more than 80% of each mode water tracer crosses 27°S below 200 m. The Eastern Pacific pathway (see attached figure) is an especially efficient mode water export route, with over 90% crossing 27°S below 200 m. The time required for 50% of the mode water tracers to leave the Southern Ocean domain varies significantly between mode water pools, from 9 years for the Indian mode water pool to roughly 38 years for the Central Pacific mode water pool. The Indian and Pacific mode water pools are primarily driven by large-scale gyre circulation, whereas the Australian and Atlantic pools are heavily influenced by the Antarctic Circumpolar Current. We also discuss the relative influence of mean and eddy circulation in setting the structure of the export pathways and discuss the potential impacts on anthropogenic carbon sequestration.

Image caption

Cumulative column sum of a passive tracer over the course of a 50-year numerical integration. All values are scaled by the maximum cumulative column sum. Tracer was released in a mode water pool in the Pacific between 300-500 m depth (approximate position indicated by a white triangle). The Subantarctic Front of the Antarctic Circumpolar Current is shown (solid black line).