The Spatiotemporal Structure of Diabatic Processes Governing the Evolution of Subantarctic Mode Water in the Southern Ocean

A coupled ice-ocean eddy-permitting Southern Ocean State Estimate (SOSE) for 2008-10 is used to describe and quantify the processes forming and destroying water in the Subantarctic Mode Water (SAMW) density range (sigma(theta) = 26.7-27.2 kg m(-3)). All the terms in the temperature and salinity equations have been diagnosed to obtain a three-dimensional and time-varying volume budget for individual isopycnal layers. This study finds that air-sea buoyancy fluxes, diapycnal mixing, advection, and storage are all important to the SAMW volume budget. The formation and destruction of water in the SAMW density range occurs over a large latitude range because of the seasonal migration of the outcrop window. The strongest formation is by wintertime surface ocean heat loss occurring equatorward of the Subantarctic Front. Spring and summertime formation occur in the polar gyres through the freshening of water with sigma(theta) > 27.2 kg m(-3), with an important contribution from sea ice melt. Further buoyancy gain by heating is accomplished only after these waters have already been transformed into the SAMW density range. The spatially integrated and time-averaged SAMW formation rate in the ocean surface layer is 7.9 Sverdrups (Sv; 1 Sv equivalent to 10(6) m(3) s(-1)) by air-sea buoyancy fluxes and 8.8 Sv by diapycnal mixing, and it is balanced by advective export into the interior ocean. Maps show that these average rates are the result of highly variable processes with strong cancellation in both space and time, revealing the complexity of water mass transformation in the three-dimensional Southern Ocean overturning circulation.