Interesting float data for the week of October 19, 2014

We'll provide updates on interesting oceanographic observations reported by the SOCCOM profiling floats in this space.  Our first report doesn't actually involve the Southern Ocean.  It's from a profiling float in the North Atlantic that was near Hurricane Gonzalo and it concerns mixing of the ocean and possible biological responses.  But we will compare the impact of a tropical cyclone to the amount of mixing seen in the Southern Ocean in a later post. So read on.


Hurricane Gonzalo pretty much went over float 7663, which was in the Atlantic Ocean southwest of Bermuda (missed by about 50 km – that was still was in the band of hurricane force wind). The following plot from the FloatViz web site (http://www.mbari.org/chemsensor/floatviz.htm) shows the last 2 profiles for temperature and nitrate before the hurricane and the profiles about 1 day after the hurricane passed. Max wind speeds were about 140 mph when Gonzalo was at this point. Sea surface temperature clearly drops about 1.5 C as the mixed layer deepens from 50 to 90 meters. Nitrate shows no change at all because the nitrate-cline at around 130 meters is already deeper than the hurricane mixing depth. So surface temperature and nitrate are decoupled.

Why is this important? There are numerous papers that use satellite remote sensing data to show cold water and chlorophyll increases in the wake of hurricanes. One explanation is that deep mixing from the hurricane is bringing nitrate (a limiting nutrient) into surface waters and fertilizing the ocean. But there are very few (maybe none) direct measurements of nitrate in hurricane wakes (who wants to do that!). A profiling float with chemical sensors allows us to test this idea without going into very stormy seas. At the location of float 7663, a biological response to Gonzalo would just be mixing a deep chlorophyll maximum to the surface and not increased growth due to nitrate entrainment. The profiling float data, combined with satellite remote sensing, provide a much clearer interpretation of oceanic processes.

Ken J.


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