Project Summary

James R Ledwell, Carin J Ashjian, Scott M Gallager, James H Churchill, and Cabell S Davis

Water-Mass Exchange Across the Tidal Mixing Front over Southern Georges Bank and Its Impact on Larval Fish and Zooplankton Poplulations

The objectives of the work are:

• To examine the velocity and hydrographic structure of the tidal mixing front, and determine how this is influenced by factors such as changes in stratification, tidal current strength and wind forcing.

• To examine the 3-dimensional circulation over the upper tidal mixing front, and identify regions and rates of convergence onto the front and of cross-frontal exchange.

• To quantify diapycnal and isopycnal mixing rates in the frontal region.

• To investigate the impact of tidal mixing front circulation/exchange on the populations of cod an haddock larvae and their prey.

The field work would take place in May-June 1999, and involves 3 experiments. Each would include an ADCP/CTD/Drifter study of the front, lasting 4 or 5 days, followed by a dye release study, which also would last 4 or 5 days. The first two experiments would be on the southern flank, one in the surface layer, and the other in the pycnocline. The third experiment would be on the northern flank in the pycnocline.

We have submitted the following revised ship request for Oceanus, with coordination with the dye experiments of R. Houghton, who has requested Endeavor shown. This may serve as a strawman to start discussion at the PI meeting.

Dates	Ledwell/Churchill		Houghton

2  May	Depart W.H.
3	ADCP/CTD line
4	ADCP/CTD/drifter survey
5
6
7
8
9	Surface Dye Exp. S. Flank
10
11
12
13
14	
15	ADCP/CTD/drifter survey
16
17
18
19					Depart W.H.
20	Pycnocline Dye Exp.		Bottom Dye Exp.
21		S. Flank			S. Flank
22
23
24
25					Bottom dye exp.
26	Return to W.H.				N. Flank
27
28	Depart W.H.
29	ADCP/CTD Line			Bottom dye exp		
30	ADCP/CTD drifter survey			S. Flank
31		N. flank
1 June				
2			
3	Pycnocline Dye Exp		Bottom dye exp
4		N. Flank			N. Flank
5	
6			
7						
8	Return to W.H.			Return to W.H.				

Note that our ADCP/CTD drifter surveys will help set the stage for Houghton's 1st and last releases as well as our own, and then his first and last releases are simultaneous with the 2nd and third releases and are on the same side of the Bank. Although simultaneity is not essential, the synergy could be very great; we could get a much more complete picture of what is going on at the front. This is the justification for the coordination of the request.

It seems quite likely that surveys of biological populations on the Bank and across the fronts would best be done in the same time frame, bringing the program together. Hence, a third ship during the same period would be ideal. Also, of course, there is a window for the first 2.5 weeks during which we would be out and Houghton is not.