Deep western boundary current east of Abaco: Mean structure and transport

Abstract

To assess whether the southward deep water flow of the Atlantic meridional overturning circulation can be monitored by measuring the deep western boundary current, the structure and transport of the time-averaged deep western boundary current at 26.5N are estimated from long current meter records deployed in a series of 7 mooring deployments over 11 years from March 1986 to June 1997 east of Abaco Island in the Bahamas. Time-averaged meridional velocities for each current meter record are combined into estimates of mean velocity at 3 to 10 depths at 10 mooring locations extending eastward from the continental slope out to an offshore distance of 580 km. The mean deep western boundary current below 1000 m depth exhibits a core of strong southward velocities above 15 cm s-1 at an offshore distance of about 55 km and extends out to an offshore distance of about 160 km with an estimated total southward transport of 34.6 Sv. We estimate the error in this time-averaged transport to be 3.7 Sv due to uncertainties in the mean velocities due to temporal variability and up to 4 Sv due to the choice of methods to integrate spatially the time-averaged velocities across the section. Offshore from 160 km out to at least 580 km, there is broad, slow northward flow that recirculates deep water northward. While the mooring array is not sufficient to accurately measure the northward recirculation, a simple spatial average of the time-averaged currents indicates a northward recirculation of about 13 Sv in this offshore region. Daily estimates of the deep western boundary current from two arrays with reasonably complete coverage of the deep western boundary current suggest that the instantaneous boundary current has a width of about 100 km, naturally narrower than the mean boundary current. Daily southward transports vary between 5 and 75 Sv but the array does not fully resolve the boundary current during offshore meander events that occur about 20% of the time. We conclude that it is problematic to monitor the net southward flow of the Atlantic meridional overturning circulation with boundary current measurements at 26.5N because the narrow deep western boundary current transport has substantially larger transport than the net southward flow and the offshore northward recirculation is broad and diffuse so that accurate estimates of the recirculation would require an extensive array of moored instruments.