11.4 Observed Circulation in the Atlantic
The theories by Sverdrup, Munk, and Stommel describe a very simple flow. But the ocean is much more complicated. To see just how complicated the flow is at the surface, let's look at a whole ocean basin, the North Atlantic. I have chosen this region because it is the best observed, and because mid-latitude processes in the Atlantic are similar to mid-latitude processes in the other oceans. Thus, for example, we use the Gulf Stream as an example of a western boundary current.
Let's begin with the Gulf Stream to see how our understanding of ocean currents has evolved. Of course, we can't look at all aspects of the flow. You can find out much more by reading Tomczak and Godfrey (1994) book on Regional Oceanography: An Introduction.
North Atlantic Circulation
Let's begin with the traditional view of the time-averaged flow in the North Atlantic based mostly on hydrographic observations of the density field (Figure 2.8). It is a contemporary view of the mean circulation of the entire ocean based on a century of more of observations. Because the Figure includes all the oceans, perhaps it is overly simplified. So, let's look then at a similar view of the mean circulation of just the North Atlantic (Figure 11.7).
The figure shows a broad, basin-wide, mid latitude gyre as we expect from Sverdrup's theory described in §11.1. In the west, a western boundary current, the Gulf Stream, completes the gyre. In the north a subpolar gyre includes the Labrador current. An equatorial current system and countercurrent are found at low latitudes with flow similar to that in the Pacific. Note, however, the strong cross equatorial flow in the west which flows along the northeast coast of Brazil toward the Caribbean.
If we look closer at the flow in the far north Atlantic (Figure 11.8) we see that the flow is still more complex. This Figure includes much more detail of a region important for fisheries and commerce. Because it is based on an extensive base of hydrographic observations, is this reality? For example, if we were to drop a Lagrangean float into the Atlantic would it follow the streamline shown in the figure?
To answer the question, let's look at the tracks of a 110 buoys drifting on the sea surface compiled by Phil Richardson (Figure 11.9 left). The tracks give a very different view of the currents in the North Atlantic. It is hard to distinguish the flow from the jumble of lines, sometimes called spaghetti tracks. Clearly, the flow is very turbulent, especially in the Gulf Stream, a fast, western-boundary current. Furthermore, the turbulent eddies seem to have a diameter of a few degrees. This is much different that turbulence in the atmosphere. In the air, the large eddies are called storms, and storms have diameters of 10° - 20°. Thus oceanic "storms" are much smaller than atmospheric storms.
Perhaps we can see the mean flow if we average the drifter tracks. What happens when Richardson averages the tracks through 2° × 2° boxes? The averages (Figure 11.9 right) begin to show some trends, but note that in some regions, such as east of the Gulf Stream, adjacent boxes have very different means, some having currents going in different directions. This indicates the flow is so variable, that the average is not stable. Forty or more observations do not yields a stable mean value. Overall, Richardson finds that the kinetic energy of the eddies is 8 to 37 times larger than the kinetic energy of the mean flow. Thus the oceanic turbulence is very different than laboratory turbulence. In the lab, the mean flow is typically much faster than the eddies.
Further work by Richardson (1993) based on subsurface buoys freely drifting at depths between 500 m and 3,500 m, shows that the current extends deep below the surface, and that typical eddy diameter is 80 km.
Gulf Stream Recirculation Region
The observed increase, and the large transport off Hatteras, disagree with transports calculated from Sverdrup's theory. Theory predicts a much smaller maximum transport of 30 Sv, and that the maximum ought to be near 28°N. Now we have a problem: What causes the high transports near 40°N?
Niiler (1987) summarizes the theory and observations. First, there is no hydrographic evidence for a large in flux of water from the Antilles Current that flows north of the Bahamas and into the Gulf Stream. This rules out the possibility that the Sverdrup flow is larger than the calculated value, and that the flow bypasses the Gulf of Mexico. The flow seems to come primarily from the Gulf Stream itself. The flow between 60°W and 55°W is to the south. The water then flows south and west, and rejoins the Stream between 65°W and 75°W. Thus, there are two subtropical gyres: a small gyre directly south of the Stream centered on 65°W, called the Gulf Stream recirculation region, and the broad, wind-driven gyre near the surface seen in Figure 11.8 that extends all the way to Europe.
The Gulf Stream recirculation carries two to three times the mass of the broader gyre. Current meters deployed in the recirculation region show that the flow extends to the bottom. This explains why the recirculation is weak in the maps calculated from hydrographic data. Currents calculated from the density distribution give only the baroclinic component of the flow, and they miss the component that is independent of depth, the barotropic component.
The Gulf Stream recirculation is driven by the potential energy of the steeply sloping thermocline at the Gulf Stream. The depth of the 27.00 sigma-theta (σθ ) surface drops from 250 m near 41°N in Figure 10.8 to 800 m near 38°N south of the Stream. Eddies in the Stream convert the potential energy to kinetic energy through baroclinic instability. The instability leads to an interesting phenomena: negative viscosity. The Gulf Stream accelerates not decelerates. It acts as though it were under the influence of a negative viscosity. The same process drives the jet stream in the atmosphere. The steeply sloping density surface separating the polar air mass from mid-latitude air masses at the atmosphere's polar front also leads to baroclinic instability. For more on this topic see Starr's (1968) book on Physics of Negative Viscosity Phenomena.
Let's look at this process in the Gulf Stream (Figure 11.10). The strong current shear in the Stream causes the flow to begin to meander. The meander intensifies, and eventually the Stream throws off a ring. Those on the south side drift southwest, and eventually merge with the stream several months later (Figure 11.11). The process occurs all along the recirculation region, and satellite images show nearly a dozen or so rings occur north and south of the stream (Figure 11.11). In the south Atlantic, there is another western boundary current, the Brazil Current that completes the Sverdrup circulation in that basin. Between the flow in the north and south Atlantic lies the equatorial circulation similar to the circulation in the Pacific. Before we can complete our description of the Atlantic, we need to look at the Antarctic Circumpolar Current.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, firstname.lastname@example.org
All contents copyright © 2005 Robert H. Stewart,
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Updated on October 25, 2006