Seasonal Along-Isobath Geostrophic Flows on the West Florida Shelf with Application to Karenia Brevis Red Tide Blooms

TOPEX/Poseidon (T/P) sea surface height (SSH) measurements along tracks 91 and 15 crossing the West Florida Shelf (WFS) are used to estimate seasonal across-shelf SSH gradients. These gradients and the knowledge that geostrophic flow approximately follows the isobaths enables an estimation of the seasonal along-isobath geostrophic flows. The seasonal geostrophic along-isobath flows are found to be highly correlated with the seasonal along-shore wind stress. Southeastward directed along-isobath flows are predicted in December, January, February, and March. Northwestward directed along-isobath flows are predicted in June, August, and September. The along-isobath geostrophic component of the flow is small in the remaining months. Karenia brevis, the Florida red tide organism, usually blooms on the south/central WFS in the summer and fall months. It is likely that the northwestward along-isobath flow in June, August, and September transports the red tide blooms northward to the Big Bend shelf region during these months. K. brevis blooms in 2005 and 2006 are used as a case study to examine the northward transport mechanism. Above average northwestward along-shore wind stress caused by hurricane activity in the Gulf of Mexico most likely resulted in the northward transport of K. brevis in 2005 while below average along-shore wind stress in 2006 suggests why a K. brevis bloom on the central WFS was not observed farther north.