A Coupled Physical-Biological Model for the Gulf of Maine

 

 

1. The circulation model

Huijie Xue, Fei Chai, and Neal Pettigrew

School of Marine Sciences, University of Maine

 

This project carries out simultaneous prognostic integration of a three-dimensional circulation model coupled with a seven-component biological model. It considers the interaction of the important physical, chemical, and biological factors that control biological productivity in the Gulf of Maine. It also enables quantitative measures of the physical and biological coupling in the Gulf, reflected in the nutrient budget of the euphotic zone. We attempt to use the modeled annual nutrient budget, including estimates of atmospheric inputs, to reconcile the previously reported mismatch between nutrient flux to the euphotic zone and primary production in the Gulf of Maine (Schlitz and Cohen, 1984; Townsend, 1991). We expect this research to result in new insights into the rates of primary production in the Gulf as they vary seasonally and spatially in response to nutrient dynamics. Furthermore, the prognostic ecosystem model can be used to evaluate the sensitivity of the Gulf of Maine ecosystem to various natural and anthropogenic variations in the forcing functions.

As the first step, we have been successfully simulating the seasonal circulation of the Gulf of Maine using the Princeton Ocean Model (POM). The model has 103 x 151 horizontal grid points and 19 vertical levels to map the Gulf of Maine, Georges Bank, Scotian Shelf and the adjacent slope region with realistic topography to 4500 meter isobath. It is initialized and forced at the open boundaries with the results from the East Coast Forecast System (Aikman et al., 1996). At the surface, it is forced by the monthly winds, heat flux and fresh water flux from the Comprehensive Ocean and Atmosphere Data Set (COADS). The M2 tide is simulated using the model, Figure1. A seasonally-varying circulation pattern emerges in multi- year integrations. As an example, here are bi-monthly horizontal sections of salinity and velocity at 10m, Figure 2. The spinup of the cyclonic circulation between April and June is likely caused by the differential heating between the interior Gulf and the exterior shelf/slope region. From June to December, the cyclonic circulation continues to strengthen, but gradually shrinks in size. When winter cooling erodes the stratification, the cyclonic circulation penetrates deeper into the water column. The circulation quickly spins down from December to February as most of the energy is consumed by bottom friction (annual cycle). While inclusion of river discharge changes details of the circulation pattern, the annual evolution of the circulation is largely unaffected. On the other hand, inclusion of tide results in not only the anticyclonic circulation on Georges Bank, but also minor modifications to the seasonal circulation, Figure3.

Detailed discussions can be found in Xue, H., F. Chai, and N. R. Pettigrew, 1999: A model study of seasonal circulation in the Gulf of Maine. J. Phys. Oceanogr., 30, 1111-1135.

Abstract

The Princeton Ocean Model (POM) is used to study the circulation in the Gulf of Maine and its seasonal transition in response to wind, surface heat flux, river discharge, and M2 tide. The model has an orthogonal curvature linear grid in the horizontal with variable spacing from 3 km nearshore to 7 km offshore, and 19 levels in the vertical. It is initialized and forced at the open boundary with model results from the East Coast Forecast System (Aikman et al. 1996). The first experiment is forced by monthly climatological wind and heat flux from the Comprehensive Ocean Atmosphere Data Set (COADS); discharges from the Saint John, Penobscot, Kennebec, and Merrimack River are added in the second experiment; semidiurnal lunar tide (M2) is included as part of the open boundary forcing in the third experiment.

It is found that the surface heat flux plays an important role in regulating the annual cycle of the circulation in the Gulf of Maine. The spinup of the cyclonic circulation between April and June is likely caused by the differential heating between the interior Gulf and the exterior shelf/slope region. From June to December, the cyclonic circulation continues to strengthen, but gradually shrinks in size. When winter cooling erodes the stratification, the cyclonic circulation penetrates deeper into the water column. The circulation quickly spins down from December to February as most of the energy is consumed by bottom friction. While inclusion of river discharge changes details of the circulation pattern, the annual evolution of the circulation is largely unaffected. On the other hand, inclusion of tide results in not only the anticyclonic circulation on Georges Bank, but also modifications to the seasonal circulation.

Point of Contact: Dr. Huijie Xue.

 

2. Coupled physical-biological model

Fei Chai, Mingshun Jiang, and Huijie Xue

School of Marine Sciences, University of Maine

 

Beginning at the end of the third year, a 10-component biological model is integrated synchronously with the circulation model for 18 month. Results from the last 12 months are analyzed for seasonal variations in nutrient and plankton distributions.

 

Surface distributions

Cross sectional distributions

Point of Contact: Dr. Fei Chai.