West Florida Shelf Interaction

Joint Measurements

Satellite AVHRR infrared images have been used to study SST patterns across the West Florida Shelf. This is an effective qualitative tool, but alone it offers little insight into ocean circulation patterns. Coupled with in situ wind or current measurements, these images may be used to infer circulation information beyond the immediate measurement site.

A current measurement program on the west-central Florida shelf has been underway since October 5, 1993. The first 16 months have been analyzed and when analyzed together with the AVHRR data, it offers some insight into the forces that drive the circulation over the West-Central Florida shelf. Some examples have been offered below.

Loop current influence upon the shelf via both eddy formation and strong weather system induced Ekman transport

The Loop Current is constrained from impinging directly onto the West Florida Shelf by depth related vorticity considerations. However, eddy formation and trapping of Loop Current waters on the shelf break combined with onshore Ekman transports provides a mechanism by which Loop Current waters can be transported onto the shelf. An example of this is found in March 1994. Toward the end of February, a Loop Current eddy formed near the shelf break. A storm with strong southerly winds (March 1994 in situ data SST & Water Level and Wind & Current Velocities) facilitated the trapping of this eddy onto the shelf break (March 4, 1994) and an onshore wind induced transport of Loop current waters onto the shelf. Subsequent satellite imagery then showed fairly steady surface conditions for the remainder of the month (March 18, 1994) and indeed despite relatively weak southerly winds, the near surface currents persisted toward the south consistent with the baroclinic effect of the relatively warm Loop Current water that had made its way onto the shelf.

A narrow band of coastal upwelling owing to the coalescence of Ekman induced upwelling off of Key West/Dry Tortugas and off of Pinellas / Manatee / Sarasota Counties

Very pronounced banded SST distributions owing to coastal jets are also observed nearshore as evident in May 1994. This is a particularly interesting example because it shows an upwelling feature that is continuous from the Dry Tortugas to the Big Bend. May 1994 in situ data SST & Water Level and Wind & Current Velocities show that the mid-shelf currents were southward (for baroclinic reasons) over most of the month. Strong northeasterly winds developed around May 19, which caused an offshore Ekman transport and an associated drop in sea level and SST. Prior to this a l ocalized region of upwelling was observed on the northern side of Key West / Dry Tortugas, but otherwise SST along the coast was warm. By May 22, a very pronounced narrow band of upwelling was observed along the entire length of the coast (May 22, 1994) linking the southernmost region with a maximum just off Tampa Bay.

Banded SST and baroclinic structures on the shelf owing to wind-forced coastal jets

For the same reasons that Loop Current intrusions are constrained by topography, circulation along the mid-portion West Florida Shelf also tends to be along-isobath (which is essentially along-shore). The shelf therefore responds to large scale wind forcing by forming along-shore jets. An example of this, discernable in satellite SST imagery, occurred in early October 1994 in response to a major storm (October 1994 in situ data SST & Water Level and Wind & Current Velocities). Prior to the storm SST was uniformly warm over the southern half of the shelf. After the storm a pattern of warm SST is observed to extend along the entire mid-shelf region past Cape San Blas and into the northwest portion of the shelf. This pattern retreated somewhat, but remained a distinctive feature sandwiched between relatively cold coastal and shelf break waters (October 21, 1994) before dissipating by the end of the month. Note that the cold shelf break waters mark the outer edge of the Loop Current. These waters are cold for two reasons: first, the thermocline is shallowest on the outer edge for geostrophic reasons and second, any flows occurring onshore will cause upwelling for kinematical reasons. A narrow band of cold water at the shelf break is a common occurrence on the West Florida Shelf.

Satellite SST imagery is an effective tool in recognizing surface circulation patterns, but when coupled with in situ measurements, its ability for describing the horizontal flow structure is improved.

Theme by Danetsoft and Danang Probo Sayekti inspired by Maksimer