From the Southern Ocean to the North Atlantic in the Ekman layer

Since the Southern Ocean encompasses the entire circumference of the globe, the zonal integral of the pressure gradient vanishes implying that the (meridional) geostrophic mass flux is zero. Conventional wisdom has it that, in view of this, the northward Ekman flux there must somehow find its way to the northern oceans, sink to the bottom (due to cooling) and return southward either below the topography or along the western boundary. Using recent (process oriented) numerical simulations and a simple analytical model, it is shown that most of the Ekman flux in the Southern Ocean does not cross the equator, nor does it sink in the northern oceans. Rather, the water that constitutes the link between the Southern Ocean and the deep water formation in the Northern Hemisphere originates in the eastern part of the southern Sverdrup interior. The associated path which takes the water from one hemisphere to the other resembles the letter "S", where the top of the letter corresponds to the sinking region in the Northern Hemisphere and the bottom to the origin in the Southern Ocean. Although it is true that the amount of water that is crossing the equator is equal to the integrated Ekman flux in the northernmost part of the Southern Ocean, it is merely the amount (and not the origin of the water) that is equal in these two cases. The width of the transhemispheric current in the south is betatauL/f(0)(partial derivativetau/partial derivativegamma), where tau is the wind stress, partial derivativetau/partial derivativegamma the curl of the wind, beta the familiar variation of the Coriolis with latitude, f(0) the mean Coriolis parameter, and L is the width of the basin.