Using Transformation and Formation Maps to Study the Role of Air–Sea Heat Fluxes in North Atlantic Eighteen Degree Water Formation

The Walin 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 the integrand of Walin's expression to yield transformation and formation maps is plotted and used to study the role of air–sea heat fluxes in the cycle of formation–destruction of the 18° ± 1°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 3-yr period from 2004 to 2006, the authors find that Eighteen Degree Water (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 owing 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. Results suggest that annual mean formation rates of EDW associated with air–sea fluxes are in the range from 3 to 5 Sv (Sv ≡ 106 m3 s−1). Finally, error estimates are computed from sea surface temperature and heat flux data using an ensemble perturbation method. The transformation/formation patterns are found to be robust and errors mostly affect integral quantities.