An abyssal gyre is hypothesized as filling the western basin of the North Atlantic from the equator to near 30°N, with strongest recirculation in the tropical Guiana Basin (north of South America and east of the Caribbean) and over the subtropical Nares Abyssal Plain. This "Guiana Abyssal Gyre" is comprised of a southward deep flow along the western boundary of the Basin, the deep western boundary current, and a northward interior flow in the eastern part of the western basin. Near 10°N, the eastern limb of the gyre is estimated at up to 27 Sverdrup (Sv) northward flow and is in opposition to the western limb's southward flow of up to 40 Sv in the DWBC. The latter transport estimate typifies estimates and direct measurements of the DWBC. The flow of the eastern limb has not been directly measured, but is inferred as a rationalization of several aspects of the regional circulation. The deep western boundary current as measured and estimated by several means at several locations is too big: If that amplitude of flow represented the net export of cold water to the South Atlantic in the transformation of warm water to cold water by the meridional overturning circu­ lation, then the meridional transport of heat by the Atlantic would be larger than consistent with heat transport estimates from independent oceanic, sea surface, and atmospheric budgets (all of which roughly agree). The disparity resolution requires a substantial northward flow in the interior to bring the net cold water export to order 15 ­ 20 Sv, and the hypothesized Guiana Abyssal Gyre is the means of achieving that compensation. The distributions of geostrophic shear and water mass tracers across the Guiana Basin allow recognition of the likely pathway and depth extent of the opposing northward flow. This proposal is to place an array of current meter moorings across the pathway of the northward flowing limb of the Guiana Abyssal Gyre to measure its amplitude of flow. The array will include concurrent sampling in the deep western boundary current. This measurement, combined with existing DWBC measurements, will fix the amplitude of what may be the second strongest coherent deep recirculation element of the North Atlantic (after the strong, compact counter­rotating recirculations to either side of the deep levels of the Gulf Stream). The net flow through the Guiana Abyssal Gyre to a large extent sets the amplitude of the cold limb of the meridional overturning circulation, while the temperature contrast between its opposing limbs determines the temperature transport of that cold limb, which combined with that of the warm limb sets the meridional heat flux of the meridional overturning circulation. More importantly, the demonstration that the cold limb transports are achieved as the difference between op­ posing strong flows is important for determining the abyssal circulation dynamics active within the cold limb. Quantifying this circulation feature will provide an important benchmark for both numerical simulations of oceanic circulation (in isolation and in the climate context), and theorizing on the interplay of source ­ driven abyssal circulation with bottom topography and mixing.

The proposed GAGE will focus on the narrow Guiana Basin near 16°N. The GAGE is principally a moored current meter and temperature sensor experiment (6 moorings instrumented at 5 levels at and below 3000 m), supported by shipboard hydrography and acoustic Doppler current profiler measurements during the array deployment cruise in early 2000 and the array recovery cruise in early 2002. Integration of the directly measured velocities will form one flow estimate. A second flow estimate will be the hydrographic sections with reference levels guided by the direct velocity measurements. These will be cross­validated by using the moored temperature records to construct a time series of baroclinic shear (taking advantage of the constancy of deep temperature­salinity here), and comparing that to the directly measured current meter shears, with the time series of baroclinic shear also providing information on the representativeness of the hydrographic sur­ veys at array deployment and recovery.

The above is a synopsis of the plan as proposed to NSF and approved for funding. A merger has been effected with a German program (U. Send, W. Zenk, and M. Rhein, funded, see abstract for IFM­Kiel Pro­ grams). This enlarges the array to 10 moorings (see figure below for array location), and includes augmented sampling at two additional levels besides those of GAGE: nominally 1600 m and 1300 m. Three of the German moorings (stars on the figure) will be heavily instrumented with fixed point ctds to enable full water column hydrographic data collection and thus dynamic height. Their data will be combined with bottom pressure measurements to make a baroclinic transport variability estimate for the cold limb of the overturning circulation. During the combined phase of the experiment the direct current measurements will provide ab­ solute transport and shear measurements. The German program will continue with a third year after the recovery of the GAGE equipment in early 2002, and it also is intended as a pilot program for a long term monitoring activity in the German contribution to CLIVAR.

Schedule. No formal ship schedule available until later in 1999. The scheduling goal is a combined deployment cruise for the merged experiment in January 2000, a German cruise (Sonne) to turn around their moorings in late 2000, and a recovery of the GAGE component in early 2000, probably jointly with the turn around of the German gear. The final recovery of the German gear would be late 2002 or early 2003.

In the current cycle of the U.S. NSF funding process there are additional supplementary programs under consideration that would add to this Joint U.S. ­ German experiment a deep RAFOS float element (D. Fratantoni and P. Richardson) for mapping out the Guiana Abyssal Gyre geography) and a higher horizontal resolution of the flow across the array line by the addition of HEF's and PIES's (A. Chave and D. Luther). Decisions of those pending elements will not be known until summer. Under discussion with W. Johns and Doug Wilson are possibilities for monitoring of the Caribbean Passages between Guadelope and South Amer­ ica, which combined with the German dynamic height time series would allow estimation of the upper warm limb of the meridional Overturning circulation simultaneous with the GAGE ­ German measurements of the cold limb west of 51°W. The GAGE intensive phase 2000 ­ 2002, the following year with the four German moorings, and the German plan for those moorings subsequently becoming a long term monitoring sites for the meridional overturning circulation provide an opportunity for "upgrading" one or more of the sites to included profiling CTD's and surface flux measurements as westward extensions to the initial Pirata array.