Observations of Entry and Exit of Potential Vorticity at the Sea Surface

Although potential vorticity (PV) is central to many theories of the oceanic circulation, the entry–exit of PV at the sea surface has not been thoroughly discussed from an observational perspective. After clarifying the notion of “PV entry and exit,” and the mechanisms responsible for it, a climatology of this quantity for the Northern Hemisphere is presented. It is found that surface PV loss over western boundary current regions and their interior extension is a robust feature over the North Pacific and Atlantic basins. At high latitudes, mechanical and diabatic effects act in concert in the North Atlantic to drive the net PV exit. In the Pacific, however, these effects oppose each other and the net entry–exit of PV is more uncertain. At low latitudes, surface winds are found to be particularly important in setting the surface PV exit in the Pacific, equatorward of the intertropical convergence zone.