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Frontal Subduction |
| Daniel L. Rudnick |
The similarity of upper thermocline water to late winter surface water is a fascinating feature of the ocean. This observational fact suggests a circulation pattern where water moves from the surface layer downward and equatorward. The downward movement of water has been termed subduction, in analogy with the subduction of plates of the earth's crust.
Subduction is an important element in global ocean-atmosphere interaction. The surface layer of the ocean may exchange heat, fresh water, and gases with the atmosphere. This exchange stops once the surface water is subducted into the interior of the ocean, and the properties set by the atmosphere are carried away with the water. In this way heat and greenhouse gases, for example, are sequestered in the ocean.
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| Figure 1. Potential density and geostrophic velocity in the surface layer. The contour interval for potential density is 0.025 kg/m³, and the velocity scale is shown in the lower left corner. Note the strong jet along the density front. |
The Subduction Experiment, sponsored by the Office of Naval Research, was a multi-institution program designed to observe and quantify subduction. My contribution to the project was a study of the role of oceanic fronts in subduction. The density and horizontal velocity fields of the Azores Front in the North Atlantic were surveyed in May 1991 and March 1992 using a SeaSoar and a shipboard Acoustic Doppler Current Profiler (Figure 1). Fronts in density, such as the Azores Front, are usually accompanied by strong geostrophic currents directed along the front. Maximum velocities in the observed frontal jet approach 0.5 m/s. The flow is generally parallel to isopycnals, inferring that the sum of the rate of change and vertical advection of density must be small.
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| Figure 2. Across-front sections of the total potential vorticity including planetary and relative vorticity. The white lines are contours of potential density with an interval of 0.025 kg/m³. Note the tongue of constant density and low potential vorticity extending downward and southward from the north side of the front. |
The hypothesis of my study is that fronts are sites of active subduction. An essential characteristic of fronts leads to this hypothesis: fronts are often sites of surface convergence, which may lead to localized downwelling. Because we have concurrent density and absolute velocity measurements we have two tracers at our disposal for diagnosing the flow at the front: potential density and potential vorticity. A cross-front section indicates that water can flow downward from the north side of the front while conserving both tracers (Figure 2). The calculated potential vorticity includes contributions from planetary and relative vorticity in both horizontal and vertical directions. An important dyanmical feature of fronts is that the relative vorticity, particularly in the horizontal, is significant.
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| Figure 3. Vertical velocity in the surface layer. The contours of potential density (white lines, interval 0.025 kg/m³) help to locate the vertical velocity relative to the density front. Note the band of downwelling bordering the front to the north. |
A measure of the vertical velocity at the front is necessary to determine the subduction rate. Vertical velocity is notoriously difficult to measure in the ocean because it is small relative to the typical horizontal velocity. However, vertical velocity can be inferred using measurements of density and horizontal velocity combined with statements of conservation of mass, density, and momentum. Assuming the ocean to be quasi-geostrophic, an equation for vertical velocity, known as the omega equation, may be derived The solutions for vertical velocity reveal a band of downwelling on the dense side of the front. This downwelling, consistent with the tracers of Figure 2, peaks at about 8 m/day (about two orders of magnitude larger than Ekman pumping). A second region of intense downwelling occurs in the southwest corner of the survey, associated with southward flow. Upwelling is apparent on the south side of the front, especially in the center of the survey. The general pattern is one of dense northern water downwelling and light southern water upwelling. This circulation cell, which tends to stratify the water, is potentially an important mechanism of subduction.
Acknowledgment. SeaSoar operations during the Subduction Experiment were the responsibility of the WHOI SeaSoar Lab.
Rudnick, D. L., and J. R. Luyten, 1996: Intensive surveys of the Azores Front, 1, Tracers and dynamics. J. Geophys. Res., 101, 923-939.
Rudnick, D. L., 1996: Intensive surveys of the Azores Front, 2, Inferring the geostrophic and vertical velocity fields. J. Geophys. Res., 101, 16,291-16,303.
Fronts are a continuing topic of interest to me. Here are some other references involving fronts.
Rudnick, D. L., and R. E. Davis, 1988: Frontogenesis in mixed layers. J. Phys. Oceanogr., 18, 434-457.
Rudnick, D. L., and R. E. Davis, 1988: Mass and heat budgets on the northern California continental shelf. J. Geophys. Res., 93, 14013- 14024.
Eriksen, C. C., R. A. Weller, D. L. Rudnick, R. T. Pollard, and L. A. Regier, 1991: Ocean frontal variability in the Frontal Air-Sea Interaction Experiment. J. Geophys. Res., 96, 8569-8591.
Rudnick, D. L., and R. A. Weller, 1993: The heat budget in the North Atlantic subtropical frontal zone. J. Geophys. Res., 98, 6883-6893.
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