Southern Ocean GasEx Blog

Dispatches from the Southern Ocean Gas Exchange Experiment

Satellites

Posted by sogasex on March 2, 2008

By Pete Strutton, Oregon State University

These days, oceanographers often take for granted the power of satellites to provide us with a picture of our environment. We can sit at our desks back home and view data from any corner of the globe. Thanks to email on ships, we can download images of the waters surrounding us. My first long cruises were done in the western Pacific and Antarctica not long before the launch of NASA’s SeaWiFS satellite in 1997. For biological oceanographers, this satellite has revolutionized the way we view the oceans. It measures surface chlorophyll concentrations, an indication of ocean productivity (see the 29-Feb blog) by observing ocean color or ‘green-ness’. This is particularly impressive given that more than 90% of what the satellite sensor sees is variability due to the atmosphere (dust, water vapor, absorbing gases etc). SeaWiFS recently celebrated it’s 10th birthday, far exceeding the projected life span.

In addition to ocean chlorophyll, NASA and other space agencies have launched satellites that measure sea surface temperature, winds and sea level, all important indicators of ocean physics. Since oceanic CO2 concentrations are largely determined by the interaction of biology and physics, maps of these parameters (see below) help us understand the measurements we’re making on the Brown. Both ship- and satellite-based measurements have their pros and cons. Ships can visit only a few stations in a day, but provide us with the opportunity to obtain water samples and make detailed measurements from surface to bottom. Satellites on the other hand see only the upper few meters at most, but can view the whole planet in one day. Combining the two types of observations is often very powerful.

Over the last month or so and during our transit to the SO GasEx location, we’ve been keeping tabs on the available satellite data and receiving daily updates from colleagues at the Plymouth Marine Laboratory in the UK. One of the features of a favorable experiment site, apart from high winds, is moderate-to-high productivity, indicative of a region where biology has drawn down the ocean CO2 concentrations to about 10% below atmospheric values. Looking at the data thus far, it seems the region NW of South Georgia has the qualities we’re looking for.

We don’t have satellites that can measure CO2 directly from space, but Burke Hales (also on the cruise) and myself, with colleagues at Oregon State University and elsewhere, have been developing techniques to determine sea surface CO2 from satellite data. As described above and elsewhere on this blog, biological productivity consumes ocean CO2, and temperature indicates processes such as upwelling and mixing that also govern oceanic CO2 concentrations. So using satellite measurements of chlorophyll and temperature we can generate predicted maps of what we think CO2 will look like. Again it looks like the proposed study area has low CO2, but we’ll see how accurate our predictions were once we get there and make the in situ measurements.

PS: It’s my brother’s birthday today: Happy birthday Dave.

first-sgsx-map-scaled.jpg

Three satellite-based maps from the SW Atlantic. South America and the Falkland Islands (Malvinas) are on the left, South Georgia on the right. The top panel is predicted CO2 concentrations from a model developed by Burke Hales. The middle and bottom panels are chlorophyll and sea surface temperature (SST) measured by the MODIS and AMSR-E satellites, respectively.

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