The Ventilation of the Deep Gulf of Mexico

Recent measurements over the sill in the Yucatan Channel indicate that the deepest flows between the Caribbean Sea and the Gulf of Mexico, those that take place below the sill level at the Florida Straits, have zero mean net mass transport but carry significant amounts of heat and oxygen. The heat flux associated with the mean exchange exports approximately 150 GW from the deep Gulf toward the Caribbean and may be related to the formation of the Yucatan Undercurrent. The eddy heat transfer is also significantly different from zero and exports on average an additional 60 GW. This eddy transfer is attributable mostly to events that last from a few days to about 1.5 months, during which colder water from deeper levels in the Caribbean (beneath 2000 m) flows over the sill within a bottom boundary layer close to 200 m thick. The colder water is also very rich in oxygen, and the deep exchange sustains the near-bottom oxygen maximum in the Gulf of Mexico, whence that cold water must slide down the northern slope of the Yucatan Sill. Estimates of oxygen transport by diffusion from the deep water into the overlying intermediate water (∼50 m3 s−1) and the oxygen consumption reported in the literature (∼100 m3 s−1) are balanced by the rates of mean and eddy transfers over the sill (∼150 m3 s−1). The near-bottom mass transport [∼0.32 Sv (1 Sv ≡ 106 m3 s−1)] measured across the deepest portion of the central Yucatan Channel suggests a residence time for the deep waters of the Gulf of about 250 yr.