## Cross-Frontal Exchange (1998)

Georges Bank circulation can be thought of as a "leaky gyre", due to cross-frontal exchange processes as well as on-off bank mean flow and episodic forcing from storms and rings as discussed in the previous section. Cross-frontal flows potentially can cause chronic "leakage" of plankton and nutrients across the boundaries of the bank along the shelf-slope front to the south and along the Gulf of Maine/Georges Bank front to the north. Cross-frontal exchange also occurs at the seasonal tidal front positioned along the 60 m isobath and separating the well-mixed central area from the surrounding stratified region. Since nutrient level and plankton taxonomic composition in well-mixed and stratified areas are different, flows across this tidal front potentially can have significant effects on the target species populations in both areas. These flows must be better understood.

Questions: Cross-frontal exchange studies should address the following questions:

1. What are the exchange rates of target species from Georges Bank due to cross-frontal mixing?

2. What is the three-dimensional circulation associated with the shelf/Slope Water front, the northern front, and the tidal front?

3. How does seasonal stratification affect exchange rates at the shelf/Slope Water front?

4. How do biological and physical processes interact to control cross-frontal exchange of target organisms?

5. What are the exchange rates across the tidal front and how do these affect the nutrient and species distributions in stratified and well-mixed area?

Strategy: These questions can be addressed by process-oriented studies in each frontal system (i.e., shelf/Slope Water, northern edge, well-mixed/stratified) during stratified and unstratified periods. Quantifying cross-frontal exchange rates of target species requires rapid sampling of biological and physical properties of the frontal system in 3-dimensions over time. Abundance of target species should be quantified using depth-stratified sampling with nets, acoustics, and optical devices. Vertical migration behavior in relation to vertical shear and bottom-boundary layer flows should be determined by high frequency stratified sampling. Physical variables (salinity, temperature, advection) should be measured using "towyo" deployment of a CTD or SEASOAR and shipboard ADCP. Biological/physical moorings should be used to examine scales of variability in the frontal system. Instrumentation to be moored includes CTD, fluorometer, acoustics, and plankton pumps and video imaging systems for collection of taxonomic data.