How important are diapycnal mixing and geothermal heating for the deep circulation of the Western Mediterranean?

The dissipation rate of turbulent kinetic energy epsilon and the associated diapycnal turbulent mixing is inferred from a set of microstructure observations collected over several cruises from year 2012 to 2014. The geographical distribution of epsilon highlights several regions of enhanced levels of turbulence ranging from 10(-9) to 10(-6) W kg(-1): the Sicily Channel, the Corsica Channel, and the Ligurian Sea. Elsewhere, epsilon was small, often below 10(-10) W kg(-1). Below 1300 m, geothermal heating provides three-fold more buoyancy than small-scale turbulence. Geothermal heating and turbulent diffusion provide enough buoyancy to balance 15% to 50% of a mean yearly deep water formation rate of 0.9 to 0.3 sverdrup (10(6) m(3)/s), respectively. The remaining part has to eventually overflow through the Strait of Gibraltar. Plain Language Summary During the winter season in the western Mediterranean, an invisible river transports dense waters formed at the surface by cold winds down to the ocean bottom at a rate sixfold larger than the Amazon River discharge. This winter flow increases the volume of dense waters present at depth. However, two mechanisms supply buoyancy and erode the volume of dense waters. One is heat coming from the seafloor and is called geothermal heating. The other is heat coming from the surface and is powered by natural fluid turbulence, in the same way as producing turbulence with a teaspoon mixes milk and tea vertically. Here we use historical data of geothermal heating and observations collected in 2012-2014 that measure for the first time the intensity of the oceanic turbulence to diagnose whether the winter volume increase of dense water at depth can be balanced by heat coming from the surface and the seafloor. Observations suggest that geothermal heating is threefold more efficient than oceanic turbulence in bringing heat to the dense waters at depth. But the addition of heat by both oceanic turbulence and geothermal heating is not strong enough to erode the import of winter dense water at depth.