Collapses, maxima, multi-year modulation and trends of the Zapiola Anticyclonic Circulation: insights from Mercator Reanalysis.

The Argentine Basin hosts a unique oceanic feature: the Zapiola Anticyclonic Circulation (ZAC) located above a sedimentary deposit. Taking advantage of a high-resolution (1/12º) global ocean reanalysis (GLORYS12) we examine the ZAC over 27 years (1993-2019). The mean ZAC is bottom-intensified with bottom currents reaching 0.10 m s^-1. The ZAC volume transport ranges from -18.5 Sv to 268 Sv with a mean of 122 Sv. The strong negative peaks correspond to occasional ZAC collapses. During large transport events (>195.4 Sv) the ZAC shows a well defined  coherent gyre. Strong transport events are associated with high eddy kinetic energy (EKE) at the periphery of the ZAC (especially to the west and south). In contrast, during weak transport events (<49.8 Sv), EKE increases at the center of the ZAC and decreases at the ZAC periphery. 
A weak ZAC is more permeable to external mesoscale structures. Each weak event features a cyclonic eddy at the center of the ZAC carrying subantarctic cold and fresh waters. 
The ZAC exhibits a multi-year modulation, with periods of 4-5 years (1993-1997, 1998-2003 and 2004-2009) of low salinity corresponding to low transport, and high salinity to high transport.
Over the last 27 years, transport time series exhibit a significant negative trend of -15 Sv.decade^-1 associated with a  negative trend in EKE (-0.015 (m/s)².decade^-1) to the north west of the ZAC. Waters in the Zapiola region become warmer and saltier in the first 2000 m of the water column because of the southward migration of the subtropical front.