Water mass transformation in the North Atlantic over 1985-2002 simulated in an eddy-permitting model

[摘要] Water mass transformation in the North Atlantic is examined in aneddy-permitting simulation with the OCCAM ocean general circulation model,forced by realistic surface fluxes over the period 1985-2002. Three Atlantic regionsare considered - the subtropics, mid-latitudes, the northeast Atlantic - along withthe Labrador Sea. The oceanic boundaries of each region coincide with hydrographicsections occupied in recent years. These regions broadly represent theformation sites of Eighteen Degree Water (EDW), Subtropical Mode Water (STMW),Subpolar Mode Water (SPMW) and Labrador Sea Water (LSW).Water mass budgets are obtained for each region and year. Terms inthe budget comprise surface-forced transformation rates, boundary exchangesand unsteadiness. Transformation rates due to "total mixing" are obtained as the differencebetween net and surface transformation rates. Transports at the boundaries are evaluated alongsiderecent hydrographic section datasets, while surface-driven and mixing-driven transformation rates arecompared with estimates based on air-sea flux datasets and inverse analysis of hydrographic data.In general OCCAM compares well with the observations, although two particular discrepancies are identified:deep overflows at high latitudes too light by around 0.2 kg m^-3 and spurious heat gain ofup to 100 Wm^-2 east of the Grand Banks. Over 1985-2002, there is considerablevariability on a range of timescales, in the annual surface-driven and mixing-driven formation ratesof all four water masses.In the case of EDW and STMW, surface-driven and mixing-driven formation rates largely cancel.This is not so for SPMW and LSW, leading to regional net formation rates of up to 17 Sv and 15 Sv, respectively.In particular, OCCAM successfully simulates the strong LSW formation event of 1989-1994.

[学科分类] 海洋学与技术