EXPERT OPINION: Ocean physics with Greg Gerbi
Physical oceanography... animal, vegetable, or mineral? It’s fundamentally interdisciplinary. Geol- ogy and topography affect ocean physics, ocean physics and circulation affect biol- ogy and chemistry, and biology and chemistry can also feed back on physics. My focus is on the interaction of the ocean with its surroundings—land and the atmosphere. Most of my work has been in the upper ocean, roughly the top 100 meters. This is where oxygen and carbon dioxide get mixed down from the atmosphere, nutrients get brought up- ward from depth, and sunlight fuels phy- toplankton growth. I’m interested in the turbulence that mixes these things and transports these things within the sur- face boundary layer. I am interested in coastal regions,
partly because they’re more convenient to study than the open ocean, but mostly because they’re the areas of ocean with which humans interact frequently. For example, pollution from the land comes out with river water through estuaries. Understanding the near-surface and coastal dynamics is essential to predict- ing where and how our runoff will affect water quality and ecosystem health. Every estuary and every shelf is differ- ent. Water from the Chesapeake comes out into a nice straight coast and tends to flow southward. Most of the water from the Hudson eventually flows southward as well, but because its mouth is in the corner of the Long Island and New Jer- sey coasts, the circulation is much more complicated.
How does the ocean influence hurricanes? The warm waters in the upper ocean are an important source of that energy for hurricanes. In the late spring, summer, and into fall, sunlight is absorbed by par- ticles of sediment and plankton in the upper ocean. When there are more parti- cles near the surface, most of the sun- light gets absorbed there; fewer near- surface particles will allow a thicker layer of warm water, but the layer will be less
warm. Some computer models have shown that changes in light absorption can affect the number and intensity of hurricanes. Seasonal growth and decay of phytoplankton communities is one fac- tor that can alter the particle density, and thereby the light absorption and tempera- ture, in the upper ocean. This is one case in which biology affects physics.
What technologies do oceanographers use? We work with electrical sensors for tem- perature and salinity, and we use both acoustic and optical instruments to study currents and particles in the ocean. Acoustic current-measuring equipment sends out sound pulses, and we can ex- amine the pulses bouncing back off small particles in the water to determine how fast the water is moving. Differ- ent types of these instruments can measure at a single point or tens or hundreds of meters away. I work with a colleague at Rutgers who uses particle image velocimetry, a high- frequency, high-resolution technique to map the flow field in laboratory experi- ments. We’re working on a project to study how the swimming behavior of snail larvae changes in different turbulence conditions. From only a few days of field
work, I can collect enough data to keep me busy with analysis for a long time. I also use computer models similar to the ones meteorologists use to predict the weather. My students and I can run simple models on a laptop, but right now I'm setting up a new server to handle more data and more complicated simula- tions that take a day or two to run. Currently, with collaborators at the
University of Maine, I’m measur- ing light intensity with what we call autonomous plat- forms—four-foot-long cylinders that drift in the ocean and then come to the surface, measuring
temperature, salinity, particle concen - trations, and sunlight as they rise; from the surface, they send us their data and receive instructions via satellites. Our project's goal is to help calibrate satel- lite-based imaging systems that use ocean color to estimate chlorophyll and particle concentrations in the near- surface waters. A much larger study using similar floats is the international Argo program; you can learn about that at
www.argo.net.
Greg Gerbi, who arrived at Skidmore in 2011, teaches courses in physics, geo- sciences, and geophysics. He earned his doctorate from a joint program of MIT and Woods Hole.
MARK BOLLES
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