How do you study a moving target? -by Liz

The final two days of the cruise found us frantically chasing an eddy in the lee of the islands. Eddies are temporary features caused by winds and currents that form regularly near Hawaii, and may be up to 100 km across. Physical changes they cause in the ocean make them visible in satellite data collected by NOAA, NASA, and even the US Navy. Seems like it should be pretty simple to look at the data, locate a few sampling stations in the eddy, and take some samples, right? Not exactly, because eddies move. Scientists can predict where they think the eddy will be up to a month in advance, but that doesn’t mean the eddy will be where they expect. It’s a little like predicting the weather. The eddy we planned to study shifted eastward after the cruise began, and all of the carefully planned locations of our sampling stations went out the window. Lucky for us, the ship’s instruments continuously collect sea surface temperature and ADCP data that we could use to figure out what direction the eddy was traveling, and tell us when we were in the right place to collect samples for the biological and chemical oceanographers in the group. The ADCP projects acoustic waves of around 70 kHz into the ocean, and the way the waves reflect back to the ship’s sensors tell us current direction, and even the best depth for finding fish and crustaceans. The current wizards in our group, Matt, Zach, and Charlie, translated this data for the rest of us, and spent all night tracking eddy currents until we found the new location of the eddy center at 4:30 in the morning. The biological samples we collected in the eddy were amazingly diverse, and included tiny cephalopods, pteropods (marine snails with wings), radiolarians, copepods, fish larvae, and lots of shrimp.