Cushman-Roisin Report

### Numerical Investigation of Currents on Georges Bank

    INVESTIGATORS:

    Benoit Cushman-Roisin
    Thayer School of Engineering
    Dartmouth College
    Hanover, NH 03755
    (603)646-3844
    (603)646-3856 fax
    B.Cushman@Dartmouth.EDU

    Daniel R. Lynch
    Thayer School of Engineering
    Dartmouth College
    Hanover, NH 03755
    (603)646-2308
    (603)646-3856 fax
    Daniel.R.Lynch@Dartmouth.EDU

    GRANT PERIOD: July 1, 1993 - June 30, 1996

### STATEMENT OF OBJECTIVES:

The recent collapse and closure of the Georges Bank fishery highlights the impact environmental variability can have on fish populations, especially populations already under intense fishing pressure. On Georges Bank, recruitment of young fish is critically dependent on the highly variable currents acting to keep eggs and larvae on the bank. In years when the currents sweep many eggs and larvae off the bank, recruitment is very poor. A few consecutive years of low recruitment can cause a fish population to crash, at tremendous ecological and social cost.

We use a three-dimensional finite-element coastal ocean numerical model developed at Dartmouth College to model the critical current system on the southern flank of Georges Bank. The currents on Georges Bank are largely controlled by five external factors: density differences between continental shelf and continental slope waters, tides which resonate in the Gulf of Maine, summer heating, seasonal winds, and episodic events such as strong storms and Gulf Stream rings. Taken together these forcings create a variable and complex current system on the bank. Our approach is to break this complex system into its fundamental components and then rebuild the complex current system block by block, at each step analyzing the effects of the latest block on the system as a whole. In analyzing the current system at each stage we ask several important questions:

- What is the dynamical composition of the current as a function of both position and time?

- What is the structure of the flow in the vertical and cross-stream directions?

- What are the possible instabilities?

- And, what are the possibilities for water exchange across the current system?

### STATEMENT OF WORK:

We have created a finite-element mesh of more than 13,000 nodes and 25,000 triangular elements to cover the Gulf of Maine/Georges Bank area. Mesh resolution is less than 1km in critical areas, and no element has greater than a 30% change in depth among its nodes. Runs of the model for the first steps of the study show a large influence on the currents by the steep bottom topography along the sides of Georges Bank. The sharp change from the relatively flat bank top to the steep bank sides creates an area of vertical mixing known as the shelf break front. This front creates a partition in the water that tends to keep eggs and larvae from being swept off the bank. In addition, the runs for the first steps highlight the importance of an area where the currents turn into the Great South Channel. Here, the currents split, with one part turning to wrap around back to the North that keeps young fish on the bank, and the other part turning to the South that sweeps young fish into the deep water of the Mid-Atlantic Bight.

We are in the process of making more complex simulations with the model. These runs include the tides, seasonal heating and seasonal winds. Tidal resonance in the Gulf of Maine produces a strong clockwise residual flow around Georges Bank. Combined with tidal mixing, seasonal heating produces a tidal mixing front on the bank top. This front separates well mixed shallow regions on the bank top from the stratified deeper regions of the bank sides. Both the clockwise flow and the tidal mixing front effects serve to help keep young fish on the bank.

The coastal ocean model as well as the finite element mesh used for this study will be available on the World Wide Web via a home page dedicated to coastal and climate change research conducted at Dartmouth.

### SUMMARY OF KEY FINDINGS:

Simulations to date show that the currents on Georges Bank are largely controlled by the steep and complex topography. As currents climb up or down slopes they are vertically squeezed or stretched. Because of the earth's rotation, this stretching or squeezing causes the currents to turn to left or the right, respectively. This is the critical factor that causes the along shelf jet on the southern flank to veer into the Great South Channel and then turn South again to continue into the Mid-Atlantic Bight. The topography also affects the currents through friction with the sea floor. This bottom friction tends to slow down currents in shallow regions and reduces the turning effects of the earth's rotation. In addition, bottom friction creates boundary layers where the water behaves differently than it does above. Preliminary numerical as well as early analytical results show that these effects are very important to the fate of young fish on Georges Bank.

In addition to offering important insights into the dynamics of the currents on Georges Bank, this study provides a test for the powerful new three-dimensional finite-element coastal ocean model developed at Dartmouth. The success the model has had in simulating currents for a variety of cases in the difficult Gulf of Maine-Georges Bank system shows that it is a robust tool that will be useful in studies of other coastal areas, as well as future, more detailed studies of Georges Bank.

### REFERENCES:

Lynch, D.R., J.T.C. Ip, C.E. Naimie, F.E. Werner, 1995: Comprehensive Coastal Circulation Model with Application to the Gulf of Maine. Continental Shelf Research, in press