G. Maze, G. Forget, M. Buckley, J. Marshall and I. Cerovecki
JOURNAL OF PHYSICAL OCEANOGRAPHY
Walin’s (1982) water mass framework quantifies the rate at which water is transformed from one temperature class to another by air-sea heat fluxes (transformation). The divergence of the transformation rate yields the rate at which a given temperature range is created or destroyed by air-sea heat fluxes (formation). Walin’s framework provides a precise integral statement at the expense of losing spatial information. In this study we plot out the integrand of Walin’s expression to yield transformation and formation maps and use them to study the role of air-sea heat fluxes in the cycle of formation/destruction of the 18 ± 1 o C layer in the North-Atlantic. Using remotely-sensed sea surface temperatures and air-sea heat flux estimates based on both analyzed meteorological fields and ocean data-model syntheses for the three-year period 2004– 2006, we find that EDW is formed by air-sea heat fluxes in the western part of the subtropical gyre, just south of the Gulf Stream. The formation rate peaks in February when the EDW layer is thickened by convection due to buoyancy loss. EDW is destroyed by air-sea heat fluxes from spring to summer over the entire subtropical gyre. In the annual mean there is net EDW formation in the west to the south of the Gulf Stream, and net destruction over the eastern part of the gyre. Our results suggest that annual-mean formation rates of EDW associated with air-sea fluxes are in the range 3 to 5 Sv. Finally, error estimates are computed from sea- surface temperature and heat flux data using an ensemble perturbation method. It is found that transformation/formation patterns are robust and that errors mostly affect integral quantities.