Methods

The RAPID-MOCHA observing system consists of the following in-situ elements, which link together and integrate the U.S. and U.K. contributions to the total observing system (Fig. 2):

-A trans-basin array of moored current meters and hydrographic sensors/profilers (Fig. 3 and Fig. 4 ), and repeat hydrographic sections along 26.5°N (U.K. (NERC) + U.S. (NSF))

-Florida Current monitoring with submarine cable and quarterly hydrographic/ LADCP sections (NOAA), and semi-annual hydrographic/LADCP sections off Abaco, Bahamas (NOAA + NSF)

-Quarterly high-resolution VOS XBT sections across the Atlantic, from Gibraltar to Miami (the AX-7 line, NOAA).

The overall strategy for monitoring the MOC relies on deep water endpoint “dynamic height” moorings on either side of the basin to monitor the basin-wide geostrophic shear, combined with observations from clusters of moorings up the western (Bahamas) and eastern (African) continental margins, and direct measurements of the flow though the Straits of Florida. Moorings are also included on the flanks of the Mid-Atlantic Ridge to resolve flows in either sub-basin. Ekman transports derived from satellite winds are then combined with the geostrophic and direct current observations and an overall mass conservation constraint to continuously estimate the basin-wide MOC strength and vertical structure. Precision bottom pressure gauges are also employed to monitor absolute transports including barotropic circulation.

Figure 2. Joint UK/US Monitoring Array: Western Boundary Current Observations.

Figure 3. Schematic of the 26.5°N mooring array during its 2004-2005 deployment. The array comprises 3 sub arrays: the Western Boundary with 9 moorings, the Mid-Atlantic Ridge with 4 moorings (2 on either side of the Ridge), and the Eastern Boundary with 9 moorings. Moorings are not drawn to scale but illustrate the instrument and buoyancy distribution (courtesy of NOC).

Figure 4. Cross-sectional view of the Western Boundary array over topography, showing the instrument types and depths on the moorings. (Note the expanded distance scale near the western boundary, west of 76°W.) Little data were recovered from the moored profiler (MMP) on WB4 for the first year of deployment, and no data from WB4 is used in the analysis. Moorings WBA–WB4 compose a tightly-spaced array near the Bahamas escarpment, while mooring WB5 lies 500 km offshore and forms a “dynamic height” endpoint for the array.