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Seafloor Topography and Ocean Circulation

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Special Issue—Bathymetry from Spaceconstant f/H, where f is the Coriolis parameter and Hrepresents ocean depth. Barotropic theory is often sup-ported by observations. For example, floats in theAtlantic and Pacific Oceans preferentially spread alongf/H contours rather than across them, indicating thatflow responds to topography (LaCasce, 2000).In reality, because the ocean is stratified, and veloc-ities tend to be faster near the ocean surface than at mid-depth, flow does not literally follow contours of f/H.Gille (2003) used float data to examine whetherSouthern Ocean velocities could be assumed to beequivalent barotropic, meaning that velocities attenuatewith depth, with a fixed e-folding scale, Ho(Killworth,1992). Thus velocity v(z) = v(0) exp(-z/Ho). Under thisassumption, flow is predicted to follow contours of f/Fo,where Fo= Ho(1 - exp(-H/Ho)) (e.g. Marshall, 1995;Krupitsky et al., 1996). In the limit where the e-foldingscale, Ho, is infinite, this is equivalent to assuming thatflow follows f/H contours. Since only large-scale topo-graphic features of the sea floor are expected to steerlarge-scale circulation, topography was smoothed toeliminate variations with length scales less than 100 to200 km. As illustrated in Figure 2, Gille (2003) foundthat in the Southern Ocean the equivalent barotropicmodel explains the largest fraction of variance in theflow data when an e-folding depth of about 700 m isassumed. This is consistent with other analyses of theSouthern Ocean that have suggested that velocitiesdecrease with e-folding scales between 500 and 1000 m,depending on position within the ACC and computa-tion method (e.g., Karsten and Marshall, 2002).Research using the Naval Research Laboratory(NRL) Layered Ocean Model (NLOM) has shown theinfluence of abyssal layer flow on the upper ocean innumerical simulations. Hurlburt and Metzger (1998)demonstrated how topographically steered meanabyssal currents can steer upper ocean currents as theKuroshio Extension bifurcates in the vicinity of theShatsky Rise (158°E, 33°N). Surface currents can bendIntroductionSeafloor topography influences ocean circulationin two basic ways. First, it steers ocean flows. Second,it provides barriers that prevent deep waters from mix-ing, except within deep passageways that connectocean basins or in hydraulically controlled overflowregions. This paper explores the impact of both of theseprocesses on ocean circulation. The examples high-lighted here were among the broad range of topicsexplored at a workshop on “Ocean Circulation,Bathymetry, and Climate,” held at Scripps Institutionof Oceanography in October 2002.Topographic SteeringOcean currents cannot pass through ridges orseamounts. At ocean depths that are intersected bytopography, currents steer around major topographicfeatures. In addition, particularly at high latitudes,where the ocean is weakly stratified, geophysical flowstend to be vertically coherent (or barotropic) due to theEarth’s rotation. As a result currents near the oceansurface align in roughly the same direction as deepocean currents, and consequently often follow con-tours of constant depth, detouring around the bumpsand troughs in the seafloor (e.g., Schulman, 1975). Mostmajor currents respond to sea floor topography. TheAntarctic Circumpolar Current (ACC), the GulfStream, and the Kuroshio Extension all steer aroundridges and seamounts. Figure 1 shows estimates of thepaths of the Subantarctic and Polar Fronts, the twomajor jets that comprise the ACC, superimposed overthe seafloor topography of the Southern Ocean. Thefronts flow to the south of the Campbell Plateau nearNew Zealand, and through the Eltanin and UdintsevFracture Zones in the central Pacific Ocean. Just down-stream of Drake Passage, around 60°W, they veernorthward around the ridges of the Scotia Arc (Gordonet al., 1978; Gille, 1994).To the extent that oceanographic flows are strictlybarotropic, they should be steered along contours ofSarah T. GilleScripps Institution of Oceanography and Department of Mechanical and Aerospace EngineeringUniversity of California at San Diego•La Jolla, California USAE. Joseph MetzgerNaval Research Laboratory, Stennis Space Center•Mississippi USARobin TokmakianNaval Postgraduate School•Monterey, California USA47Oceanography • Vol. 17 • No. 1/2004Seafloor Topography and Ocean Circulation90253_OCEAN 3/18/04 1:45 PM Page 47This article has been published in Oceanography, Volume 17, Number 1, a quarterly journal of The Oceanography Society. Copyright 2003 by The Oceanography Society. All rights reserved.Reproductionof any portion of this article by photocopy machine, reposting, or other means without prior authorization of The Oceanography Society is strictly prohibited. Send all correspondence to: [email protected], or5912 LeMay Road, Rockville, MD 20851-2326, USA.48Oceanography • Vol. 17 • No. 1/2004contribute to the separation of the East Korean WarmCurrent (EKWC) near 37° – 38°N as a result of theupper ocean-topographic coupling described above.These results demonstrate that the bottom topographyin this region is critical for the EKWC to separate fromthe coast at these latitudes. An experiment thatremoved the ridge near 39°N, 130°E eliminated theoffshore abyssal steering and consequently changedthe separation latitude of the EKWC as shown inFigure 3.The mean pathways of major current systems canalso be significantly affected by accurate topographicinformation. Metzger and Hurlburt (2001) studied thisin the vicinity of Luzon Strait, which connects thePacific Ocean and the South China Sea (SCS). As theNorth Equatorial Current bifurcates along the east coastof the Philippines, the northward branch forms thebeginning of the Kuroshio. Upon entering Luzon Strait,the Kuroshio intrudes westward into the SCS beforeretroflecting and continuing its poleward journey alongthe east coast of Taiwan. Using a 1/16° Pacific Oceanversion of NLOM, the authors found that the westwardextent of Kuroshio intrusion is highly dependent uponthe accuracy of the coastline geometry of the islandchain within Luzon Strait. Two small-scale shoals werefound to have a significant blocking effect on theKuroshio. Inclusion of the shoals had two effects onNLOM. First, they narrowed Luzon Strait and thusreduced the westward bending (a result consistent withLi et al. (1996)). Second, more importantly, they deflect-ed the inflow angle making it more northward, thusreducing the westward intrusion


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