Project Summary

J. J. Bisagni

Satellite-Derived Estimates of Mixing Across Sea Surface Temperature Fronts in the Georges Bank Region

Georges Bank is a large submarine bank located southeast of Cape Cod, Massachusetts, separating the Gulf of Maine from the western North Atlantic. Near-resonance of the Gulf of Maine with the North Atlantic M2 tide causes strong rotary tidal currents on Georges Bank due to vertical constriction of flow. Vertical heat flux into the ocean's surface from late-spring through early-fall causes vertical density stratification over deeper portions of Georges Bank where the thermally-induced buoyancy input is greater than the tidal mixing-induced buoyancy removal. A tidal mixing front (TMF) forms along the loci of points where the thermally-induced buoyancy input is equal to the tidal mixing-induced buoyancy removal, separating vertically well-mixed from stratified waters. Locations of the Georges Bank TMF and nearby shelf/slope front (SSF) can be determined using sea surface temperature (SST) fronts, which can now be mapped from satellite-derived SST images using automated methods. We hypothesize, based on evidence from continental shelf seas, that variation in tidal mixing due to the spring-neap cycle and so-called "bolus-transfer", i.e., growth, separation and flux of eddies across fronts due to baroclinic instability, may be mechanisms which control mixing across the Georges Bank TMF and SSF.

In this proposal we describe a three-year data analysis project which will:

  1. Test our hypothesis that variation in tidal mixing due to the spring-neap cycle together with "bolus-transfer" of water may be two mechanisms which largely control mixing across the Georges Bank TMF and SSF,
  2. Quantify the mean seasonal cycle of cross-frontal mixing by each of the two mechanisms described above using in situ data and SST from long time series (1985-2001) of cloud-cleared Advanced Very High Resolution Radiometer (AVHRR) satellite images,
  3. Quantify interannual variability in cross-frontal mixing as departures from the mean seasonal cycle, including data collected during the 1999 US-GLOBEC Phase III field year, for each of the two mechanisms described above,
  4. Provide SST frontal positions to research vessels and collaborators, in real-time and on the World Wide Web, using daily cloud-cleared AVHRR satellite images which cover Georges Bank, the Gulf of Maine, the western Scotian shelf and nearby slope regions.

Our proposed work includes the following elements:

  1. Assemble a multi-year (1985-2001) time series of SST frontal locations produced from cloud-cleared AVHRR satellite images using automated methods and covering the TMF and SSF regions on Georges Bank,
  2. Estimate the mean seasonal cycle and interannual variability for the spring-neap fluctuation in the position of the TMF and SSF, cross-frontal bolus transfer and mixing,
  3. Estimate the mean seasonal cycle and interannual variability in number, size and rotational sense of eddies separating from the TMF and SSF by inspection of SST images,
  4. Maintain a very large data archive, containing all raw AVHRR SST images, acquired and processed in near real-time, together with corresponding SST frontal location images and data, which is accessible to research vessels and collaborating investigators over high-speed dial-in telephone lines and the World Wide Web.