I thought that I would take a few minutes at the beginning of the cruise to explain why we are going to what might be described as “the middle of nowhere” to look for plastic debris in the ocean. Plastic comes from people, so why don’t we go somewhere near a big city like Los Angeles to look for it? Interestingly enough, the answer has to do with the weather and the rotation of the earth.
There is a high pressure weather system in the North Pacific Ocean. Air wants to move from high pressure towards low pressure, but the rotation of the earth forces the moving air to curl to the right. This force is called the Coriolis Force, and it causes the wind to blow in a clockwise circle around high pressure weather systems. Figure 1 below shows the wind patterns in the Pacific Ocean near California. The wind pushes water at the surface of the ocean. The Coriolis Force acts on the moving water too, which curls to the right of the wind. This causes water to converge towards the middle of the rotating winds. Logically enough, we call this a “convergence zone.” Sea level is actually a little bit higher in the middle of convergence zones like this. The force of gravity acting on the “extra” water is in balance with the Coriolis Force.
As the winds move new water into the middle of the convergence zone, the water that was there before sinks. The sinking water leaves floating material (like plastic) behind. The “new” water in the convergence zone brings new plastic with it, and this is why plastic accumulates there. New plastic comes in and the old plastic stays behind. The whole process works in reverse around a low pressure weather system. The winds blow counter-clockwise around it, water is pushed away from the rotating winds, sea level is lower in the middle, and water diverges away from the middle of the rotating winds.
So we would expect to find a plastic patch in the convergence zones in the ocean, but not divergence zones. Our destination is roughly 35 degrees north and 140 degrees west, which can be seen to be very close to the middle of the rotating winds in Figure 1.
Figure 1: North Pacific winds (from Tomczak, M. and Godfrey, J.S., 2002. “Regional Oceanography: an Introduction,” online version.). The red square (A) is a convergence zone, and the purple square (E) is a coastal upwelling divergence zone. Black arrows are winds, green arrows illustrate water movement.