Mediterranean outflow transports and entrainment estimates from observations and high-resolution modelling

We use cross-slope sections of direct current observations together with a high resolution numerical simulation to revisit estimates of transports and entrainment in the Gulf of Cadiz. We provide a three dimensional picture of the outflow from the Mediterranean into the intermediate layers of the Atlantic. In the model, the time-averaged Mediterranean Undercurrent is characterised by two cores of zonal velocity at 8°30′8°30′W: one at 500 m (in the σ1σ1 interval 31.6-31.831.6-31.8 kg m-3) and another around 1100 m (∼32.2∼32.2 kg m-3) with maximum westward velocity of 0.36 m s-1. A single well defined vein of saltier and warmer water (salinity maximum ∼36.9∼36.9 psu, 1 psu = 1 kg salt/1000 kg seawater) is found attached to the slope, centred at 1300 m (32.2-32.432.2-32.4 kg m-3). The observational sections corroborate this description but instant maximum velocity reaches 0.6 m s-1 whereas salinity peaks just above 36.5 psu. Unlike what was previously thought, the velocity veins and the thermohaline anomaly cores are not co-located. At the Strait of Gibraltar, we estimate that the transport of pure Mediterranean Water (S>38.4S>38.4 psu) is about 0.48 Sv (1 Sv=106=106 m3s-1). Near the Portimão Canyon, the results for westward transport (31.6-32.631.6-32.6 kg m-3) computed from observations are within the range 2.6-3.62.6-3.6 Sv and the time-mean estimate from the numerical simulation is of 3.5 Sv. The westward salinity and heat transports are of ≲1.3≲1.3 psu Sv (1 psu Sv =103=103 m3s-1) and ∼20×1012∼20×1012 W, from observational and numerical data alike. By tracking water masses within a closed domain in the Gulf of Cadiz, we find that most of the North Atlantic Water entrained into the undercurrent is supplied through the south and southwest borders. After analysing volume balances per layer, we conclude that the entrainment from shallower layers is around 1.1 Sv in total: 0.32, 0.34, 0.40 and 0.04 Sv distributed by four equally spaced density intervals (0.2 kg m-3) between isopycnals 31.8 and 32.6 kg m-3.