GAGE (Guianna Abbysal Gyre Experiment)

Principal Investigator

Michael McCartney

Project Description

The Hypothesis: There exists a western intensified cyclonic deep-recirculation gyre beneath the tropical and subtropical thermocline of the North Atlantic, the "Guiana Abyssal Gyre." This gyre causes the transport of the Deep Western Boundary Current to be more than double that of the net cold-water export in the meridional overturning circulation.

In summary, the foundation of the hypothesis of the Gyre rests on two strong cornerstones: the robust estimates of the primary MOC amplitude of order 15 Sv, and the DWBC amplitude larger than that by a factor of 2 or 3. Together these demand recirculation: that there be a large northward flow of cold water across the rest of the basin offshore of the DWBC at all latitudes where that large DWBC is observed, to partially compensate that large transport and reconcile the observed large DWBC and the MOC amplitude. The Gyre hypothesis is of a strong western intensification of that recirculation, and is built from the cornerstones by indirect evidence: geostrophic shear and water mass distributions. The proposed GAGE intends to provide a third cornerstone to the foundation of the Gyre hypothesis, the direct measurement of the recirculating flow: its existence and its intensity.

We believe that the proposed measurement will change the Gyre from a hypothesis to an observed phenomenon. As such it will move significantly forward the quantitative description of the North Atlantic general circulation including the kinematics of the flow underlying the order 15 Sv MOC.

A merger has been effected with a German program, Meridional Overturning Variability Experiment (MOVE). This enlarges the array to 10 moorings, and includes augmented sampling at two additional levels besides those of GAGE: nominally 1600 m aned 1300 m. Three of the German moorings are additionally 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 baroclinic transport variability estimate for the cold limb of the overturning circulation.

Abstract

The PIs hypothesize that an abyssal gyre fills the western basin of the North Atlantic from the Equator to near 300°N, with the strongest recirculation located in the tropical Guiana Basin. To date, measurements of the deep western boundary current portion of that flow show it to be so large as to upset the overall balance of the meridional overturning circulation. The hypothesized gyre is one mechanism, which, if shown to be of sufficient magnitude, would account for this discrepancy. It has never been observed directly, and the PIs propose a field experiment to to so. The study would focus on the narrow Guiana Basin between 80°N and 150°N. Current meters and temperature sensors, supported by shipboard hydrography and ADCP measurements would be arrayed so as to directly determine the northward flow in this gyre.

The Guiana Abyssal Gyre Experiment (GAGE) placed a moored array of current meters across the Guiana Basin, to measure the deep boundary current and to attempt to detect and quantify the hypothesized northward flow to its east. As implemented the array was placed at 16°N for 27 months, and GAGE was enhanced through collaboration with a German .Meridional Overturning Variability Experiment. (MOVE, based at U. Kiel). Together the GAGE/MOVE program fielded 10 moorings with 72 current meters (measuring temperatures and currents) and 48 moored temperature/salinity instruments (CTDs). Considerable shipboard CTD measurements were made in the Guiana Basin in concert with mooring deployments and recoveries. GAGE completed its fieldwork in May 2002, with the exception of one GAGE mooring that failed to release and will be recovered in March of 2003. A first look at the results from the moored instruments, confirms the Gambia Abyssal Gyre hypothesis. The deep western boundary current at 16 N is strong, estimated as 38 Sverdrups. It has an unexpected two-regime structure in hydrography and in measured currents: an onshore regime with strong southward flow near 1600 m, declining with depth to weak flow near 4000 m, and an offshore regime with strong flow near 3600-4600 m, declining in strength upwards and reversing to northward flow at 1600 m. The hypothesized northward recirculation is found in three regimes. Immediately offshore and adjacent to the deep western boundary current the entire water column between 1600 m and 5400 m flows northward. On the opposite side of the Guiana Basin adjacent to the Mid-Atlantic Ridge northward flow extends from the seafloor at 5400 m to 3600 m. All array instruments across the Basin at or below 3600 m, except one in the deep boundary current, exhibit northward flow. This yields a surprisingly strong flow of what is called Antarctic Bottom Water, the coldest abyssal water. Together these northward flow elements are preliminarily estimated as 19-23 Sverdrups, yielding a net transport of 15-19 Sverdrups, the expected amplitude for the cold limb of the overturning circulation. The large GAGE/MOVE datasets will be worked up in collaboration with the MOVE scientists. The measured currents and the measured hydrography will be combined to produce a complete quantitative characterization of deep flow at 16 N, including its variabilities in amplitude and structure. With that as a powerful constraint, the full hydrographic dataset within the Guiana Basin will be reexamined to fully characterize the Guiana Abyssal Gyre and establish its role in the cold limb of the meridional overturning circulation.