On the relationship between satellite‐estimated bio‐optical and thermal properties in the Gulf of Mexico

Three years of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color data were combined with three‐dimensional thermal fields generated by the U.S. Navy's Modular Ocean Data Assimilation System (MODAS) in order to examine the interdependencies between bio‐optical fields and their relationship to seasonal and mesoscale changes in upper ocean thermal structure. The combined data set suggests that the oceanic boundary layer within the Gulf of Mexico may be broadly defined by two seasonally occurring bio‐thermal periods. A winter mixing period, characterized by net heat losses to the atmosphere, deepening of the isothermal layer depth, and annual maxima of satellite‐estimated colored detrital matter (CDM) absorption coefficients and surface pigment concentration, was followed by a thermally stratified period characterized by net surface ocean heating, reduced isothermal layer depths, and annual minima in surface bio‐optical fields. Variability in the interdependencies of ocean color products was used to diagnose an attendant shift in the size‐structure of surface phytoplankton communities as well as identify CDM as the constituent responsible for the majority of blue‐light absorption in Gulf of Mexico surface waters. The mesoscale circulation, as resolved by MODAS thermal fields into cold and warm‐core eddies, appears to significantly modulate the seasonal bio‐optical cycle of CDM absorption and surface pigment concentration. An empirical model was developed to describe CDM absorption as a function of upper ocean thermal energy. The model accounted for nearly half the variance in the satellite‐estimate of this bio‐optical variable. Large mismatches between the model and satellite data implied episodes of shelf water export to the deep Gulf of Mexico.