The contributions of atmosphere and ocean to North Atlantic Subtropical Mode Water volume anomalies

Subtropical Mode Water (STMW) in the western North Atlantic Ocean, or Eighteen Degree Water (EDW), as it is commonly known, is formed near the Gulf Stream in the wintertime, is dissipated by mixing and is removed by subduction. The ability of EDW to store and discharge large quantities of heat over periods of several years contributes to the memory of the climate system. To complement the 2-year field and modeling program of the CLIvar MOde Water Dynamics Experiment (CLIMODE) with a perspective of interannual-to-decadal variability (1985–2007), we used a simple box model to hindcast observed EDW volume anomalies in two regions: one in which EDW is formed and an adjacent region of subducted EDW. Estimates of the relative contributions of heat flux anomalies, vertical mixing from Ekman advection, mixing, and circulation are examined using proxy variables derived from winds, sea surface temperature, hydrographic data and altimetric sea level. The importance of each process is evaluated by its contribution to observed EDW volume anomalies in two regions. The study produced some robust conclusions: (1) anomalies of formation by surface heat fluxes are clearly reflected in EDW volume anomalies with some contributions by Ekman advection; (2) of the newly formed EDW about 65% is lost by mixing and about 35% is transferred to the subducted region; (3) mixing losses are well parameterized by the meandering of the nearby GS and (4) transfer and losses from the subducted region can be parameterized by the geostrophic surface flow.