Subtropical gyre variability observed by ocean-color satellites

The subtropical gyres of the world are extensive, coherent regions that occupy about 40% of the surface of the earth. Once thought to be homogeneous and static habitats, there is increasing evidence that mid-latitude gyres exhibit substantial physical and biological variability on a variety of time scales. While biological productivity within these oligotrophic regions may be relatively small, their immense size makes their total contribution significant. Global distributions of dynamic height derived from satellite altimeter data, and chlorophyll concentration derived from satellite ocean-color data, show that the dynamic center of the gyres, the region of maximum dynamic height where the pycnocline is deepest, does not coincide with the region of minimum chlorophyll concentration. The physical and biological processes by which this distribution of ocean properties is maintained, and the spatial and temporal scales of variability associated with these processes, are analyzed using global surface chlorophyll-a concentrations, sea-surface height, sea-surface temperature, surface winds from operational satellite and meteorological sources, and hydrographic data from climatologies and individual surveys. Seasonal and interannual variability in the areal extent of the subtropical gyres are examined using 8 months (November 1996–June 1997) of Ocean-color and Temperature Sensor (OCTS) and 6 years (September 1997–October 03) of Sea-viewing Wide Field-of-View Sensor (SeaWiFS) ocean-color data. The ocean-color data are interpreted in the context of climate variability and measured changes in other ocean properties (i.e. wind forcing, surface currents, Ekman pumping, and vertical mixing). The oligotrophic waters of the North Pacific and North Atlantic gyres are observed to be expanding over this period, while those of the South Pacific, South Atlantic, and South Indian Ocean gyres show much weaker and less consistent tendencies.