Vertical mixed layer convection in the Weddell Sea

A pressure‐dependent numerical model of vertical mixing in the surface mixed layer is developed, and applied to the problem of Antarctic Bottom Water (AABW) formation in the Weddell Sea. The results are used to formulate a new model of AABW formation. The surface salinity of water circulating around the southern edge of the Weddell Gyre gradually increases, possibly owing to horizontal exchange with water on the continental shelf and/or freezing along the edges of ice shelves. This permits surface mixed layer convection to penetrate deeper and deeper as the water drifts westward. By the time the water reaches the southwest corner of the Weddell Sea, the combined driving of cooling and salt release during freezing, both of which are important, can drive the convection to the bottom in deep water. This forms a water mass called Deep Mixed Water (DMW: θ = ‐0.6 to ‐0.8°C, S = 34.64‰). Upward mixing of Warm Deep Water causes intermittent melting (usually followed by refreezing) at all stations; at the westernmost deep station the ice cover disappears about two thirds of the way through the season. Meanwhile, over the continental shelf, where the bottom topography prevents Warm Deep Water from intruding, 2 m of ice forms in the same time, producing what is called Saline Shelf Water(SSW: θ = ‐1.9°C, S = 34.78‰). It is postulated that SSW flows down the continental slope and mixes with DMW in a ratio of about 1:8 to produce a previously (almost) unobserved water type called Original Bottom Water (OBW: θ = ‐0.6 to ‐0.8°C, S = 34.656‰), which is the true precursor of much of the bottom water in the world ocean. This mechanism is more direct than any other yet proposed as producing a significant amount of AABW, and therefore more plausible and potentially more efficient. These results suggest a need for more winter observations in the extreme southwestern Weddell Sea.