WHOI PO

Current meters at 36 W measured the flow into the western North Atlantic at the equator from 1992 to 2003. The flux had a steady mean value of Q=2.0 Sv averaged over more than one year, and potential temperature cooled at the rate of 0.0020 C/y. The steadiness differs from the transients such as warming of Antarctic Bottom Water (AABW) in the western Atlantic in the past decades reported elsewhere. The difference prompted a review of all temperature records from 30 S to 19 N deeper than approximately 4000 m. Results are consistent with eddy diffusivity values north of the region in the range 1.7-6.9x10-4 m2s-1. In addition, the deepest (“bottom”) CTD potential temperature records on the equator at 35 W show a warming rate of 0.00530 C/y.  Finally, improved bathymetric maps indicate that the water would pass through four previously unknown passages below 40 km wide between 30’ and 1 30’ N, and 37 to 40 W. The flatness of the bottom suggests that the current into the North Atlantic would accelerate to subcritical flow in each passage. This leads to two models of subcritical flow of nonrotating and rotating fluid. Archived bottom potential temperatures and bathymetry give volume flux predictions 0.98-2.64 Sv for nonrotating flow and 0.97-2.31 Sv for rotating flow (although near the equator). A byproduct is an estimate of downstream energy dissipation. Using this, estimates of eddy diffusivity can be made that depend on guesses of the area and volume of the mixing region and one is presented that produces values of order 10-4 m2/s. A useful extension would be to use worldwide data sets of bathymetry to locate passage saddle points along with potential temperature, salinity, and internal tides to estimate the rates of subcritical flow and dissipation through passages.