Deep-Water Exchange Between the Atlantic, Caribbean, and Gulf of Mexico

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The deep waters of the Caribbean Sea are renewed by sporadic flow over the two deep sills connecting with the open Atlantic. In turn, the deep waters of the Gulf of Mexico are renewed by flow from the northwest Caribbean over the sill in Yucatan Channel, between Mexico and Cuba. So we examine the obvious question: if all this renewal water is coming in, what is going out'? The view that emerges is a three-layer mean deep flow in the Caribbean and the Gulf. There is strong vertical shear in the water above 800 m associated with the primary upper-layer flow. Surprisingly, this shear continues to at least 1400 m. In the three-layer scenario described here, the upper-most deep layer flows into the Gulf at depths of ∼800-1100 m as an extension of the upper-layer circulation. The deepest inflow is sporadic, just above sill depth. The third layer, containing the required mean return flow to achieve mass balance, is found between these two mean flows at depths of ∼1100 to ∼1900 m. There is little information about the horizontal pattern of this deep return flow. The transport in these deep flows into the Gulf and in the return flow is slightly less than 1 Sv. These conclusions are inferred from the computed mean vertical shear, conservation of potential vorticity in the deep layers, the observed salinity gradients, and the assumption that there can be no net flow between 800 and 2000 m. The deep return flow of water from the Gulf of Mexico, having salinity ∼34.96 psu, appears to flow from Yucatan Channel all the way back to Windward and Jungfern Passages, leaving the Caribbean above the entering deep renewal water.