Multiyear Eulerian Current Observations Near the Florida Big Bend Coast

The northern part of the West Florida Shelf (WFS) bends sharply to form Florida's Big Bend coast. Eulerian acoustic Doppler current profiler measurements on this narrowing shelf, the first multiyear in situ measurements in this region, have been obtained at K-Tower, an Air Force navigational tower in 19 m of water about 33 km south of the Big Bend coast, at 5 m and 10 m depth along a section perpendicular to the coast from K-Tower, and in 19 m of water off the Florida peninsula about 45 km south of K-Tower. Analysis of these observations and their dynamics on subinertial and seasonal time scales led to the following conclusions. (i) Apart from the frictional surface and bottom boundary layers the flow is approximately barotropic for all seasons of the year. Except for the 5 m depth site A, where turbulent friction dominates the flow, the depth-averaged flow approximately follows the isobaths provided that the local radius of curvature of the isobaths is 5 km or larger. This is consistent with the approximate conservation of potential vorticity. (ii) At the 19-m Big Bend site K-Tower, consistent with the coastally trapped wave (CTW) theory, the subinertial along-isobath flow is not proportional to the local along-isobath wind stress, but rather to the along-shore wind stress to the south along the WFS. At the southern 19-m site S, consistent with previous work, the subinertial along-isobath flow is driven by the along the Florida peninsula wind stress, but is weakened by an along-shore pressure gradient "brake". Via CTW dynamics this "brake" is due to the abrupt end of the WFS at the Florida Keys. (iii) In contrast to the along-isobath flow at the 19-m sites, the subinertial along-isobath flow at the 5-m site A is driven by the local wind in a constant stress turbulent frictional layer for most of the year. (iv) Typically, because of the fresh water flux near the coast, density increases seaward. This leads to a strong asymmetry in the cross-isobath flow. Specifically, the seaward cross-isobath bottom boundary layer flow associated with downwelling involves pushing less dense water under more dense water. This leads to vertical mixing and cross-isobath bottom boundary layer flow that is much weaker than that due to upwelling. Sometimes in winter temperature can reverse the mean seasonal density gradient and then, by the above mechanism, the asymmetry is reversed such that the bottom boundary flow associated with upwelling rather than downwelling is much reduced. (v) At 19-m depth K-Tower the depth-averaged seasonal flow is along the isobaths and, as expected, dynamically is remotely-driven by the wind blowing along the axis of the Florida peninsula south of the Big Bend. The depth-averaged along-isobath flow is eastward along the Big Bend coast during November - February and June - July and westward in April - May and August - September. The seasonal along-isobath depth-averaged flow varies from a maximum westward flow in May of 3 cm/sec to a maximum eastward flow in November of 4 cm/sec with an overall mean of about 1 cm/sec eastward. (vi) Fresh water input from the Big Bend coast and, to a lesser extent, seasonal changes in the water temperature, give rise to a seasonal along-isobath vertically sheared flow in accordance with the thermal wind relation. (vii) Seasonal flows at the shallower 5-m site A and 10-m site B are comparable to those at 19 m and, especially in winter at 5-m site A, are mainly driven by the local wind stress in a constant stress, vertically sheared turbulent layer. (viii) Surface gravity wave observations suggest that surface waves likely have a negligible influence on the seasonal and mean flows. (ix) The seasonal across-isobath flow is influenced by the rectification of the higher frequency subinertial flow by the mechanism discussed in (iv). Thus, typically, when density increases seaward, even if the mean monthly wind is zero, the subinertial upwelling and downwelling fluctuations will lead to a rectified bottom boundary layer toward the coast.