Project Summary

Dave Hebert and Jack Barth

Cross-Frontal Fluxes and Mixing on Georges Bank

Collaborators: Dave Ullman, Dian Gifford, Karen Wishner, Igor Belkin, Jim Ledwell Robert Houghton

Our research effort in Phase III of the Georges Bank GLOBEC program consists of detailed hydrographic (CTD/ADCP), microstructure (MicroSoar) and optical (Fluorometer/Transmissometer) surveys of fronts on Georges Bank while following a subsurface isopycnal float (COOL float). With these instruments and surveys, we hope to determine the cross-frontal fluxes and the dynamics of the processes responsible for the cross-frontal exchange.

The details of our cruises depend on the plans and funding levels of other PIs as described below. Thus, we have proposed two scenarios. The first option is what we orginally proposed with refinements on timing. The second option would be done if Gifford/Wishner could not do the earlier cruise to the Northeast Peak front. For our work, the incremental cost of the field time compare to the preparation and analysis time allows us to still undertake two cruises even with the cuts we have taken.

Our basic field program for each frontal study is the same. I will describe it here and refer to it later as "frontal study". Upon arriving at frontal location based on SST maps and other anciliary information, we will conduct a radiator pattern CTD/ADCP/MicroSoar/optical survey to map the frontal region. Next, we will deploy the COOL float in the front at approximately mid-depth. While tracking the COOL float acoustically from the ship, we will make a butterfly pattern (10 km cross-front and 5 km along-front) CTD/ADCP/Microsoar/optical surveys centered about the COOL float's position for 3 days. The SeaSoar/MicroSoar would be recovered and MicroSoar data downloaded. Then, we would recover the COOL float, download its data and prepare it for the next deployment. This same procedure would be repeated for each frontal study.

Option 1:

This will a joint (two-ship) field program with Gifford et al. and Wishner. The ship requirements of Gifford/Wishner suggest that they would be best served by the Endeavor. We had also requested a mid-size ship. Given the Gifford/Wishner desires, the Oceanus should satisfy our requirements.

We had proposed two cruises, the first to study the Northeast Peak Front. The optimal time is the second half of March. Although we would prefer to have the cruise later when weather and sea state should be better allowing for greater success in tracking the COOL float and operation of the SeaSoar/MicroSoar. For the biological studies, the late March period would be better. Thus, we have compromised on this time period. Gifford/Wishner have proposed 18 days and we have proposed 16 days. This would allow us to complete at least 3 frontal studies over a fortnightly period. The location of this study would be near the moorings of Beardsley et al. Our survey data will be used to compliment the mooring data.

The second cruise is to study the fronts on the southern flank of Georges Bank. Again, the area of study will be located near the moorings proposed for that region. Again, we will coordinate our cruise with Gifford/Wishner. The optimal period, based on biological and physical processes, is the second half of June. We have both requested 18 days. We would request our shiptime to as earlier as possible since the SeaSoar is required on the west coast at the beginning of July for refurbishment before another major experiment out there. We have proposed to study both the shelf-break front and the tidal mixing front in order to determine the differences, if any, in the dynamics and cross-frontal exchange of these two fronts. We would do a frontal study at each site twice alternating between the two site. However, Gifford/Wishner would spend all of their time studying the tidal mixing front.

Option 2:

In this option, we have assumed that Gifford/Wishner are unable to do a late-March cruise to the Northeast Peak Front but plan to do the southern flank study in June. Thus, our second cruise plan as described above remains the same. In our mail reviews, it was suggested that we undertake a pilot study with the dye release experimentors (Ledwell/Churchill and Houghton) to compare the two methods on estimating cross-frontal exchange rates. We believe that this intercomparison would be great in terms of increasing our confidence in the interpretation of the other data we collect, for example, the June surveys. Ledwell/Churchill and Houghton are coordinating their cruises to have an overall for the later part of May and early June. They plan to use two ships and undertake two dye release experiments at the same time and location but at different depths. I believe that Houghton will be requesting the Endeavor and Ledwell/Churchill will ask for the Oceanus. Ledwell/Churchill are also proposing to undertake a dye release experiment on the southern flank before Houghton sails. Thus, we would propose to do our first cruise at the beginning of May and work with Ledwell/Churchill. We would request 16 days as we did for our Northeast Peak Front cruise (option 1). The tracking of the dye patch is for approximately 5 days. With the remainder of our scheduled time, we would proceed as outlined for our June cruise by studying both the tidal mixing front (which would be weaker than in June) and the shelf-break front. It is likely that we would use the Endeavor for this cruise. Given this assumption, it would be preferable for us to load/unload the Endeavor at the same port as the Oceanus which we would use for the June cruise. Otherwise, we would have to truck the SeaSoar and other equipment from one location to another and those items have not been budgetted for. As well, no cruise travel has been included in Hebert's budget.