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Analysis and Interpretations

Phase 1 (August 2002)

Summary

Within the North Atlantic about 15-20 x 106m3s-1 of warm water flows northward in the upper ocean, transforms at high latitudes, and returns at depth in the cold limb of the meridional overturning circulation. The main vehicle for that cold limb flow is the deep western boundary current - but measurements of that current suggest that southward of 30°N it transports as much as 30-45 x 106m3s-1. The Guiana Abyssal Gyre hypothesis reconciles this disparity with an opposing northward interior recirculation east of the deep boundary current; with enough cancellation to yield the smaller net cold limb flow. The Guiana Abyssal Gyre Experiment (GAGE) was funded to place 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 2003. This proposal is for a two-year analysis phase of the GAGE/MOVE datasets.

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 x 106m3s-1. 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 x 106m3s-1, yielding a net transport of 15-19 x 106m3s-1, 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 above summarized first look results will be carefully refined, with the combining of the measured currents and the measured hydrography requiring methodical work. The product of the work will be 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.

The intellectual merit of the proposed work lies in it completing the process of evolution of the Guiana Abyssal Gyre from a hypothesized major oceanic circulation feature, through a defining first measurement, and on to become a fully documented phenomenon.

The broader impacts of the work are as follows. The quantitative characterization of this major North Atlantic circulation element will lead to improvements in modeling ocean circulation through its use in validating ocean model performance. Ocean models are embedded in larger climate models that are used to address issues of coupled climate variability including global warming scenarios and carbon dioxide sequestration. Refinement of the "image" of the oceanic meridional overturning circulation will yield a better graphical image of how the ocean "works" that will help convey important science to a larger public audience. The success of the GAGE/MOVE may foster future international collaborations on experiments, bringing more resources to bear on important oceanographic issues.

Also in this section

Figure 1. A schematic of deep and bottom water circulation pathways as synthesized for the GAGE proposal in 1998 and representing an update from Schmitz and McCartney [1993]. The abyssal western basin has plumes of Antarctic Bottom Water (AABW, blue) and Denmark Strait Overflow Waters (pink) that converge at mid-latitude, warmed by exchanges with warmer overlying waters to transform [Luyten et al., 1993] into North Atlantic Deep Water (NADW) . In the schematic the solid brown pathways indicate the DWBC elements of the circulation of the NADW, emphasizing the deeper LNADW elements. The nested set of dashed brown contours represent the estimated pathways for the northward recirculation. The southward recirculations of AABW are dashed blue curves, and the diapycnal flows from AABW into the LNADW are brown stars. At low latitudes the AABW is an entraining plume warming by volumetric dilution by LNADW, this aspect is omitted from this schematic.
Figure 1. A schematic of deep and bottom water circulation pathways as synthesized for the GAGE proposal in 1998 and representing an update from Schmitz and McCartney [1993]. The abyssal western basin has plumes of Antarctic Bottom Water (AABW, blue) and Denmark Strait Overflow Waters (pink) that converge at mid-latitude, warmed by exchanges with warmer overlying waters to transform [Luyten et al., 1993] into North Atlantic Deep Water (NADW) . In the schematic the solid brown pathways indicate the DWBC elements of the circulation of the NADW, emphasizing the deeper LNADW elements. The nested set of dashed brown contours represent the estimated pathways for the northward recirculation. The southward recirculations of AABW are dashed blue curves, and the diapycnal flows from AABW into the LNADW are brown stars. At low latitudes the AABW is an entraining plume warming by volumetric dilution by LNADW, this aspect is omitted from this schematic.

Preliminary Conclusions from the Moored Array as of August 2002

The measured hydrography and the measured currents at western three GAGE/MOVE moorings establish the two regimes of the DWBC, an onshore regime with maximum flow in the UNADW and an offshore regime with maximum flow in the LNADW and no southward flow of UNADW. No southward flow of AABW occurs in the DWBC, confirming the break in southward DWBC flow of AABW between the Guiana Nares Abyssal Gyres diagrammed in Fig. 1.

Immediately offshore of the DWBC in the Western Trough is one element of the hypothesized Guiana Abyssal Gyre recirculation of amplitude about 11-13 Sv. It has a significant AABW component, and a significant part of the recirculating NADW is a barotropic flow "invisible" in hydrographic shear.

The moored measurements show that there is poleward flowing AABW throughout the 16°N array with no significant equatorward flow of AABW. The AABW flow is large, about 11-14 Sv. The GAGE array measured weak flow at the top of the LNADW layer in the Eastern Trough. The array supports the amplitudes of geostrophic flow beneath this level of no motion in the Eastern Trough where there is little organized shear above that level.

The direct recirculation of LNADW across 16°N occurs in two branches. One is immediately offshore of the DWBC, the other is on the opposite side of the basin from the DWBC in a current adjacent to the MAR. In the MNADW and UNADW, recirculation is found only in the branch offshore of the DWBC. Because of a shear geostrophic concentration at mid-Basin at the site of the yet un-recovered mooring, it is possible that an additional branch will be found there.

The direct recirculation of NADW is estimated as 8-9 Sv and occurs in two branches. A branch of 6 Sv occurs immediately adjacent to the DWBC, and involves all levels of the NADW. A distant narrow branch of 2-3 Sv occurs adjacent to the MAR, and is restricted to the LNADW.

A poleward flow of AABW of 11-14 Sv is estimated, with Western and Eastern Trough branches of 3-4 Sv and 8-10 Sv.

The combined flows of AABW and NADW in the Guiana Basin interior offshore of the DWBC are estimated as 19-23 Sv. The estimated DWBC transport is 38 Sv, and the net equatorward cold-water flow through the array is therefore estimated as 15-19 Sv.

The first look at the array measurements has yielded poleward cold-water flow elements adequate to confirm the Guiana Abyssal Gyre hypothesis, as the estimates in hand even without the results from the one mooring exceed the net cold-water export of the MOC. There is an interesting twist, however. The majority of that poleward flow is colder that 1.8°C, and can be characterized as "massive" relative to the delivery of AABW across the equator.

Our estimate of 12-13 Sv flow colder than 1.8°C through the GAGE array requires large diathermal flow across the 1.8°C isotherm between the equator and the array, as there are no other sources for AABW in the domain.

The GAGE data has revealed the expected large recirculation of NADW to be partially hidden by LNADW being volumetrically subsumed into the warming AABW plume.

The first look at the array measurements has yielded poleward cold-water flow elements adequate to confirm the Guiana Abyssal Gyre hypothesis, as the estimates in hand even without the results from the one mooring exceed the net cold-water export of the MOC. There is an interesting twist, however. The majority of that poleward flow is colder that 1.8°C, and can be characterized as "massive" relative to the delivery of AABW across the equator.

Our estimate of 12-13 Sv flow colder than 1.8°C through the GAGE array requires large diathermal flow across the 1.8°C isotherm between the equator and the array, as there are no other sources for AABW in the domain.

The GAGE data has revealed the expected large recirculation of NADW to be partially hidden by LNADW being volumetrically subsumed into the warming AABW plume.

Questions and Preliminary Answers for GAGE Analysis as of August 2002

The preliminary-processed GAGE current meter records have been in our hands only a week at this writing. They and the associated hydrography clearly are a rich dataset. Here are the issues we will address, some of which we already have remarked upon above, others that are more of the "big picture, with more certainly to emerge as we progress in the analysis.

Is there a "large" DWBC in the Guiana Basin?
Yes. A first estimate is 38 Sv. The analysis will combine the measured currents and "hydrography" and the shipboard hydrography, to fix the DWBC amplitude and variability, and further develop the image of two shear regimes within it. Comparisons will be made with the measurements at 26.5°N [Lee et al., 1996; Chave et al., 1997], 8°N [Johns et al., 1993], 2°N [Fischer and Schott, 1997], 1°S [Hall et al., 1997], and 18°S [Weatherly et al., 2000].

Is there a large recirculation east of the DWBC?
Yes. A first estimate is 19-23 Sv. The analysis will refine the estimates of the recirculation elements adjacent to the DWBC (with their barotropic component) and at the MAR, and the basin wide massive northward flowing AABW. This analysis will also delineate the areas with little recirculation revealed by the interior array instruments.

Are there basin scale fluctuations of the Guiana Abyssal Gyre flow elements?
Yes. The 60-day pressure fluctuations revealed by the MOVE. The analysis will look for fluctuations in currents and/or isotherm depths that should be associated with those large-scale pressure fluctuations (and look for lower frequency elements).

Are there "lessons" about the reference level issues for the various current elements that can be generalized for use in other hydrographic sections?
Yes. It appears that the two regimes of the DWBC the eastern area of northward flow (AABW and LNADW), and the central area with no significant flow above the AABW may yield an equivalence between the directly measured shear and flow and the hydrographic shear combined with a current meter-based level of no motion. On the other hand the recirculation element adjacent to the DWBC involves a barotropic reference velocity rather than a level of no motion.

Figure 6 was used in the GAGE proposal to illustrate the implied overturning circulation. The AABW plume flows north beneath the LNADW to converge with DSOW and warm and upwell into the LNADW at mid latitudes. Similar to the DSOW plume, as the ABBW descends into the mid-latitude abyss it entrains warmer waters, warming the plume and increasing its transport. As is often the case with upwelling and downwelling the entrainment of the LNADW into the AABW may be predominantly by horizontal flow through tilted isotherms, rather than vertical flow. At the equator the AABW enters the Guiana Basin at relatively shallow depths: the 1.8°C isotherm is at 4000 m, and the coldest waters, ~ 0.6°C enter above 4500m.
Figure 6 was used in the GAGE proposal to illustrate the implied overturning circulation. The AABW plume flows north beneath the LNADW to converge with DSOW and warm and upwell into the LNADW at mid latitudes. Similar to the DSOW plume, as the ABBW descends into the mid-latitude abyss it entrains warmer waters, warming the plume and increasing its transport. As is often the case with upwelling and downwelling the entrainment of the LNADW into the AABW may be predominantly by horizontal flow through tilted isotherms, rather than vertical flow. At the equator the AABW enters the Guiana Basin at relatively shallow depths: the 1.8°C isotherm is at 4000 m, and the coldest waters, ~ 0.6°C enter above 4500m.