Investigating the Timescales and Pathways of Southern Ocean Water Masses Using Transient Tracers and ECCOv4

Abstract

It is widely accepted that we live in a changing climate, with rising global temperatures and an increasing concentration of atmospheric CO2 .The Southern Ocean, despite only accounting for 30% of global ocean surface area, is estimated to have been responsible for 43% of anthropogenic CO2 and 75% of heat uptake over the period 1861-2005 (Frölicher et al., 2015). This excess heat and carbon is transported from the surface ocean to the interior ocean predominately by two water masses, Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW). However, little is known about the timescales of this transport and how these timescales may be changing in response to climate change. This research uses transient tracers (CFCs and SF6) to identify these timescales. A Maximum Entropy Method (MEM) is applied to tracer measurements taken at 24°S in the South Atlantic in 2009 and 2018, in order to diagnose the location and point in recent history that these waters were last in contact with the atmosphere. Additionally, passive tracer experiments in ECCOv4 are used to validate the results obtained by the MEM. Multiple temporal origins are identified in AAIW at 24°S. They suggest a contribution from young waters of 5 - 25 years from north of the Subantarctic Front, and older waters of 35 - 55 years, from south of the Polar Front. When these ages are compared between the 2009 and 2018 data, the younger less dense waters appear to age by 3 years and the more dense older waters appear to age by 9 years. Multiple theories surrounding the ageing of this water are explored, including the possibility of an influence from water formed by the Weddell Polynya in the early 1970s. Analysis of the AABW at 24°S confirms a strong influence of water from the Weddell Sea, and suggests a timescale of 30 years for this surface water to reach 24°S.These results provide the information needed to estimate the uptake of anthropogenic carbon and added heat by Southern Ocean water masses. They also act as a baseline for assessing future changes in circulation, a task particularly pertinent in our warming world.