The Arctic Ocean double estuary: Quantification and forcing mechanisms

The Arctic Ocean double estuary is a “three-legged” overturning system in which inflowing waters are converted into both lighter and denser waters before being exported equatorwards. As the northern terminus of the Atlantic Meridional Overturning Circulation (MOC), it thus both affects, and is affected by, the Atlantic MOC. Here we quantify the magnitudes of the two overturning cells in density space, and then decompose the water mass transformation rates into net pan-Arctic contributions from surface forcing and diapycnal mixing. We use a high-resolution, quasi-synoptic ice and ocean hydrographic data set spanning the four main Arctic Ocean gateways—Fram, Davis and Bering Straits, and the Barents Sea Opening. Two surface flux reanalyses and a hydrographic climatology are used to generate estimates of surface water mass transformation rates by density class. A box model then determines the profiles of turbulent mixing transformation rates, and associated turbulent diffusivities. We show that turbulent mixing and surface forcing drive transformations of similar magnitudes, while mixing dominates in the upper cell and surface fluxes in the lower cell. Consideration of uncertainties and timescales leads to the tentative suggestion that our results might be representative of recent decades. We discuss the possible significance of tides and sea ice brine rejection as energy sources driving turbulent mixing. Finally, we speculate as to whether water mass transformation rates may change in future as ocean heat transport into the Arctic increases. As sea ice declines and the efficiency of atmosphere-to-ocean momentum transfer increases, the Arctic Ocean is expected to “spin up,” causing more intense turbulent mixing, with uncertain consequences.