Meridional Overturning and Heat Transport From Argo Floats Displacements and the Planetary Geostrophic Method (PGM): Application to the subpolar North Atlantic

A direct method is presented to obtain the meridional overturning and heat transport in oceanic basins from observations under the sole assumptions of geostrophy and hydrostatics. The method is made possible because of the rising Argo float displacements database that can provide a reference level at 1000 dbar for the time mean circulation at 1 degrees x 1 degrees resolution. To achieve the overturning and heat transport objectives, the absolute geostrophic time mean circulation must have nondivergent barotropic transports and this requires the solutions of two Poisson equations with suitable boundary conditions, one for the geopotential at 1000 dbar and one for the barotropic stream function. Applied to the subpolar Atlantic for the period 2000-2009, an overturning of 16-18 Sv is found around 40-50 degrees N, and a meridional heat transport of 0.59 petawatt (PW, 1 PW = 10(15) W) is found at 40 degrees N (0.23 PW at 60 degrees N) so that on average similar to 50 W/m(2) is exported from ocean to atmosphere to feed the atmospheric storm track. The zonally averaged flow (the overturning) falls short of explaining the observed heat transport, and the barotropic component of the circulation accounts for up to 50% of the heat transport poleward of 55 degrees N. With the rising Argo float database, the method offers high potential to reconstruct the World Ocean time mean circulation and its heat transport away from the equator at higher resolution. The drawback is that it requires in some critical places additional current observations on the shallow shelves that are not sampled by the Argo floats.