You may not have noticed that I’ve stayed an arm’s length from discussing how oceanic circulation in the Pacific affects the movement and accumulation of marine debris. Believe me, it’s been intentional. We hear a lot of terms bandied about, and as someone whose sole graduate course in physical oceanography happened in roughly 1981, I’m no expert. However, I’ve been hanging around with some of these oceanographers for over a week, and I’m ready to tackle it.
First some basics. The sun warming the air, combined with the earth’s rotation, creates general wind patterns. Winds, again combined with the earth’s rotation, generally drive surface currents. So in the northern hemisphere, we have two major gyres, or circular circulation patterns. The first (if you’re from Hawaii, that is) is the clockwise North Pacific Subtropical Gyre; it’s made up of the Kuroshio Current, which flows north past Japan on the western side of the basin, the North Pacific Drift, which flows eastward along the northern part of the basin, the California Current, which flows southward past—you guessed it—California, and the Pacific Equatorial Current, which flows westward just north of the equator. To the north, there’s a smaller, counterclockwise pattern called the North Pacific Subarctic Gyre. OK, that’s the basic circulation pattern.
Now for a tiny bit of biology. You know how plants have chlorophyll—the green pigment? Well, in the ocean they do too. And chlorophyll is one of our big indicators of the boundary between the two gyres. Here’s why. The North Pacific Subtropical Gyre is closer to the equator, so the surface water is warm. Farther north, in the subarctic gyre, the water is colder at the surface (and on this cruise, we’ve experienced that cold water and the weather it creates!). Deeper water, no matter where you are, is cold and generally has lots of nutrients that allow water to be productive. We call the subtropical gyre “oligotrophic,” which means there’s relatively little productivity. One of the reasons? Stratification, by which I mean layering. The surface layer is warm, the deeper waters are cold, and not much mixing between the two happens between the warm and cold water, across the thermocline. By contrast, the waters to the north, in the subarctic gyre, are cold and rich in nutrients. That richness in nutrients is due to good mixing – when the surface water is cold, there’s little impediment to mixing of nutrient-rich (and cold) bottom water. As a result, the northern water mass sustains all kinds of life, particularly phytoplankton (tiny floating plants), which contain chlorophyll, the green stuff of plant life. Very productive waters have more chlorophyll and look a little greener. Those beautiful blue waters offshore of Hawaii don’t have much chlorophyll. So: subtropical gyre = warm, stratified water, low nutrients, low chlorophyll; subarctic gyre = cold, well-mixed water, higher nutrients, higher chlorophyll. Got it? OK, now take a break. I intend to—maybe some of that German chocolate cake from dinner is still down in the mess…