Ocean Circulation: A "Current" Topic

Terrible puns aside…

The Ricklefs reading mentions ocean circulation briefly, but I thought I'd expand on it a bit. So! Let's begin by noting that the ocean would have a significant impact on climate, even if it didn't circulate at all. Because water has the highest specific heat (that is, the heat per unit mass required to raise the temperature by one degree Celsius) of any common substance, it exerts a moderating effect on temperatures worldwide. Its "thermal inertia" means water temperature lags behind the seasonal atmospheric temperature, so water releases heat it acquired during the summers in winter, warming them, and gains heat lost during the winter in the summers, cooling them.

Oceans do circulate, of course. There are two majors kinds of ocean circulation: wind-driven, and buoyancy driven (thermohaline). We'll talk about wind-driven first. We know that atmospheric wind patterns are caused by the differential heating of the earth's surface. An easy example is the at the equator, where, because of the higher incidence angle of sunlight, the surface is hotter than the poles. This results in air rising at the equator, the air losing energy as it expands and cools, and rainfall as the water undergoes a phase change from gas to liquid (it DOES NOT have anything to do with warm air "holding" more water, like it was sponge or something. That explanation is completely wrong despite appearing in many textbooks, including ours - see http://www.ems.psu.edu/~fraser/Bad/BadClouds.html). The air, now depleted of moisture and cooler (hence, denser) circulates back down to the surface, forming a Hadley cell. The air flows toward the equator along the surface and is "deflected" westward by the Coriolis effect, resulting in the trade winds. Note that there is no such thing as the Coriolis force, so to speak. It is simply an effect of a rotating reference frame. See http://en.wikipedia.org/wiki/Coriolis_force for more.

This surface wind exerts a stress on the ocean surface, introducing momentum to the surface layer of the ocean. The ocean surface layer moves - but not in the same direction as the wind. Rather, because of, again, the Coriolis effect, the layer moves perpendicular to the wind direction (at 90 degrees). The net result of this movement on large scales is a gyre - a broad surface circulation pattern with a name like the North Atlantic Subpolar Gyre. Along the sides of the gyre you get currents. When they run along and are in fact shaped by continents, they are known as boundary currents, and in particular western boundary currents (i.e. the Gulf Stream). They're stronger in the west because the "westerlies" circulation pattern and their associated Hadley cell are stronger than the trade wind circulation pattern. This is because the Coriolis effect increases with latitude.

So far we've been talking about surface currents in a more or less 2-dimensional sense. Here's the cool part, though. We've got these surface wind patterns being deflected at 90 degrees resulting in gyres. But because of friction, the surface layer exerts a force on the layer below it. Again, due to the Coriolis effect, each conceptual "layer" of water moves in a direction perpendicular to the layer above it. The net effect is a "spiral" of water that, at the equator, transports water from the surface to the bottom. This is known as Ekman transport. See http://oceanmotion.org/html/background/ocean-in-motion.htm Because surface winds change in strength and direction from place to place (both at the Hadley cell scale and smaller scales), in some regions the Ekman transport piles surface water up (convergence) and in other regions it moves water down (divergence), resulting in "hills and valleys" on the sea surface with an amplitude of about 1.5 meters. This is also known as upwelling and downwelling. Upwelling brings cold nutrient-rich waters to the surface, resulting in increased phytoplankton growth and hence lots of food for fish. About half the world's total fish catch comes from upwelling zones.

These "hills" of water are centered in the middle of a gyre. Water does in fact flow along the pressure gradient from the top of a hill (high pressure) to the bottom of a valley (low pressure). However, this downward flow is again subject to the Coriolis effect, resulting in the water flowing around the center of a gyre and parallel to sea level elevation contours. This is known as geostrophic flow. See http://oceanmotion.org/html/background/geostrophic-flow.htm

So far we've only talked about wind-driven circulation. But there is also buoyancy-driven circulation. Two things make water denser - cooling, which results in the same mass of water taking up less volume, and evaporation, which results in a greater proportion of the water being made up of salt, which has more mass than freshwater per unit volume. Conversely, heat and the addition of freshwater make water less dense. The "oceanic conveyor belt," or thermohaline (thermo = heat, haline = salt) circulation, works based on the above principles. The "belt" begins in the North Atlantic, where dry, cold winds blowing from northern Canada cool surface waters and drive evaporation. Sea ice formation also locks up freshwater through a process called brine exclusion. Basically, the salt melts its way out of the ice lattice as it freezes. The saltier, colder water (known as North Atlantic Deep Water) now sinks and flows south along submarine valleys toward Antarctica and then branches off in two paths toward the Pacific and Indian Oceans. As it flows northward again toward India or the north Pacific, the deep water gradually warms and mixes with the overlying water. As it warms it rises, and loops back around toward the North Atlantic again, only this time at the surface. The cycle then repeats. There is some concern that melting polar ice caps will cause an influx of freshwater in the North Atlantic, reducing the salinity gradient and possibly disrupting or halting thermohaline circulation, with serious climate implications. The chance of this occurring and its effects are unclear at the moment.

The reality of ocean circulation is much more complicated, of course, than the above simplifications, and oceanographers are still trying to figure out the how all the above processes interact to create the observed currents. But I hope this at least serves as a good overview and definition of the terms.

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