If you read the last oceanography post and are back for the stunning finale, I just have this to say to you: Are you kidding me? No, I mean, thanks. I hope I’m making sense and not telling a pack of lies. If you happen to be an oceanographer, please let me know how to fix this. I’ve been thinking so hard by brain’s about to pop, and I really do want to understand it and be able to communicate it. Thanks in advance for your help.
The Transition (and convergent) Zone: As you head north from the subtropical gyre (characterized by chlorophyll under 0.15 mg/m3) to the subarctic gyre (chlorophyll over 0.25 mg/m3), you go through a transition zone between them. The transition zone moves by roughly 1000 kilometers north and south throughout the year, from about 30-35 to about 40-45 degrees north. In the summer months, it moves to the north (as the sun heats up the water). Right now, at the end of winter, it should be near its farthest south position, or maybe heading north already. There’s a chlorophyll front at the boundary between the two gyres, which scientists have called about 0.2 mg/m3 of chlorophyll. This generally occurs near the 18 degrees C isotherm (line of temperature). (Exception to the “generally” rule: on this cruise, we’ve actually been seeing the chlorophyll front to the north of the 18 degree isotherm.) The chlorophyll front is generally a convergent zone as well. The easterly trade winds we experience in Hawaii result in surface water moving north (a rule of thumb is that the Coriolis effect makes the top ~100 meters of water move 90 degrees to the right of the wind direction, at least in the Northern Hemisphere). To the north, westerly winds result in surface water moving south. Where these waters come together, that’s convergence. A number of scientific papers have hypothesized and in some cases seen that marine debris is concentrated by this convergence.
We’re comparing the DELI (debris estimated likelihood index) maps, which have been prepared based on satellite-derived data on chlorophyll, sea surface temperature, and wind stress, with on-the-ground measurements of chlorophyll and sea surface temperature, as well as our debris sightings.
If I haven’t lost you yet, congratulations! You’ve pretty much reached the end of my understanding (or ignorance!) of oceanography. Two more thoughts. Variations from year to year can make a very big difference. In El Nino years, the chlorophyll front and the associated convergence move much further southward. A number of papers have been published showing a correlation between El Nino years and increased marine debris deposition or seal entanglements in the Northwestern Hawaiian Islands. Conversely, in La Nina years, the convergence is likely weaker and the convergence zone doesn’t move as far south. We seem to be in a La Nina right now, and we had to go quite a lot farther north than we’d anticipated to find the chlorophyll front. We weren’t quite as successful (so far!) in finding large concentrations of debris.
OK, that’s one thought. The other? It’s a biiiiggg ocean. Let’s say a DELI map shows one pixel covering about 6 square kilometers. Our ship is 68.3 m long by 13.1 m wide. That makes us 895 m2. That means 7,000 ships the size of the SETTE could fit inside just one of the pixels on a DELI map. The map I’m looking at shows moderately high likelihood of encountering debris across 2 degrees latitude by 13 degrees longitude, roughly 228,000 km2, or about 40,000 pixels. If the DELI map (which is just giving us an estimate anyway) is off by a few pixels because the data are a few days old, we can be driving almost blind. That’s our challenge. By improving our understanding of the oceanography of this transition zone between the two gyres, by improving our protocols to observe debris from ships, by testing and improving the technology to use other observing platforms, like the UAS: these are the ways this cruise has helped get us a bit closer to our goal of finding and removing the debris before it gets into sensitive areas like the Northwestern Hawaiian Islands.