Gawarkiewicz Report

### U.S. GLOBEC: Modelling Studies of Coupled Biological/Physical Processes Affecting Recruitment on Georges Bank

    INVESTIGATORS:

    Glen Gawarkiewicz
    Woods Hole Oceanographic Institution

    Cabell Davis
    Woods Hole Oceanographic Institution

    Glenn Flierl
    Massachusetts Institute of Technology

    Changshen Chen
    University of Georgia

    GRANT PERIOD:

    July 1, 1993 - June 30, 1996

#### STATEMENT OF OBJECTIVES:

The objective of this project was to examine the effects of wind forcing on bank ecosystems using primitive equation numerical models with idealized topography. An important aspect of this effort was to examine simplification techniques (specifically empirical orthogonal function analysis) to study population dynamics.

#### STATEMENT OF WORK:

The work on wind forcing has been an extension of the previous work of Gawarkiewicz (1993), in which the interaction of wind forcing with a buoyancy-driven flow over a circular bank was examined. One of the primary limitations of that study relative to Georges Bank was that the bank was deep relative to the vertical scale set by the vertical mixing coefficients (the Ekman depth). The present work provides several important extensions; the vertical mixing scale is large relative to the minimum bank depth (20 m compared to 50 m from the previous work), the wind forcing varies in both magnitude and direction, to account for storm passage, and both steady and storm wind forcing has been run on both circular and elliptical isolated banks. The results show that, for the steady case without stratification, the flows in the direction of Ekman transport are strongest over the edge of the bank. The relative strength of the flows at the edge of the bank relative to the flow over the center of the bank increases as the vertical mixing scale increases. When fluctuating winds are applied (smoothed versions of a storm), the flows are far less spatially coherent, and the particle displacements off the bank are not as great as with strong uni-directional winds. A manuscript on this work is in preparation (Gawarkiewicz, 1995).

Work has also progressed on the understanding of the dynamics of tidal mixing fronts. Chen and Beardsley (1995) have explored the Lagrangian characteristics of tidal mixing fronts using a two-dimensional model. They find that particles over the north flank of Georges Bank are drawn from depth within the Gulf of Maine into the tidal mixing fronts at the northern edge of the bank, where they are pulled into strong vertical circulation cells. At the present time, Gawarkiewicz and Chen are both working on wind forcing over an isolated bank with tidal flows using a turbulence closure scheme, with these flow fields eventually being used with the ecosystem models.

Chen has coded an ecosystem model framework into the Mellor-Blumberg primitive equation model with turbulence closure. This ecosystem model includes sinking and swimming behavior as well as nutrient-phytoplankton- zooplankton dynamics. Franks and Chen (1995) have submitted a manuscript on this work, this work is described more fully in the Franks progress report.

In addition to the wind-driven work, Loder, Petrie, and Gawarkiewicz (1995) have submitted a review paper which describes the Labrador Shelf-Scotian Shelf -Gulf of Maine-Middle Atlantic Bight circulation. This work brings together previous transport estimates from various sub-regions to infer cross-shelf transports from the shelf to the slope. This synthesis is important in understanding the larger scale regional circulation to better understand implications of climate changes on this system for Georges Bank.

Gawarkiewicz et al. (1995) have also submitted a manuscript which describes aspects of shelfbreak frontal dynamics in the Middle Atlantic Bight. This work, which describes hydrographic observations during summer conditions, verifies that strong density gradients exist near the foot of the shelfbreak front. Previous modelling work has suggested that it is the density gradients at the foot of the front which allow the low salinity shelf flows to extend such large distances along-shelf (in the Northwest Atlantic- from Baffin Island down to Cape Hatteras). Sections near Cape Hatteras suggest that the shelfbreak density structure breaks down near Cape Hatteras because of the presence of extremely warm water over the upper slope. The climate implications of relevance to Georges Bank are that it is important for the density structure across the front to be maintained or else it is possible for shelf water to pass across the shelfbreak onto the upper slope relatively easily.

Gawarkiewicz has also co-advised a Master's student, W. Williams (with R. Beardsley as co-advisor) who has examined a numerical modelling problem of relevance to the passage of Scotian Shelf water across the Northeast Channel and onto Georges Bank. Williams found that barotropic flows concentrated at the outer shelf (similar to jets found near the shelfbreak over the Scotian Shelf) do not separate at sharp corners and do not form eddies, as coastal currents near corners do. The flows shoal as they turn the corner, adjusting their width and decreasing their speed at the corner. Williams will be continuing on for his Ph. D., and will be studying baroclinic flows turning corners. A manuscript on this work will be submitted to a journal in the fall.

Biological modeling by Davis, Flierl, Olson, and Lewis focused on three main areas: 1) development of a cohort copepod model with decoupled growth and development (Davis et al., 1994, in prep), 2) development individual-based and continuous models of copepod aggregation, and 3) development of a new technique for reduction of biological complexity in coupled physical - biological models (Flierl and Davis, in press).

Cohort model - The cohort model includes temperature- and food-dependent growth and reproduction and temperature-dependent development. Development proceeds as a function of temperature and is independent of weight. For each life stage, mortality decreases with increasing body weight, so that smaller animals in a given stage die more rapidly. This model also includes a species-specific temperature optimum for growth and development. With this formulation, the model generates animals which have a size-at-stage that is inversely related to temperature, matching observation. The model will be used to compare spring and fall conditions on Georges Bank. This model will be used as Lagrangian particles embedded in the 3-D flow field of ECOMsi. The model is coupled with eulerian-based nutrient-phytoplankton dynamics.

Aggregation model - The aggregation modeling has focused on interacting particles in a 2-D turbulent flow field. We have explored the effect of individuals attracted to neighbors within a certain detection range but repulsed at very close range. We find that the organisms become clumped in large groups in the absence of flow and that these groups are broken up by the flow into smaller groups that have different spatial characteristics. We have also explored the effects of taxis to a gradient on patch formation. We are currently developing a more detailed model of the swimming behavior of copepods.

Simplification method - Perhaps the most significant work completed thus far is our development of simplification methods for reducing complexity in biological/physical models (Flierl and Davis, in press). Since biological models of the life--cycle of an organism or of many interacting species are complex with large numbers of variables, they cannot be incorporated into a full three--dimensional model of a oceanic region due to computational limitations. In our work, we developed a technique for reducing the complexity of such a model (using as an example, the age-within-stage model of Davis, 1984, for the life cycle of the copepod Pseudocalanus), while retaining the essential aspects. The full model was run in a system with no spatial variation, but with forcing representing the expected influence and time scales of the physical forcing. From this calculation, the Empirical Orthogonal Functions (EOF's), representing the principal temporal and age--stage variability, are extracted and a small set of the age--stage EOF's are used as basis functions. Simplified dynamics are derived by projecting the original dynamical system onto this set of basis functions.

We examine development of an initial pulse, flow over one-- and two--dimensional topography, and nonlinear interactions with a food source. In all these problems, the full 200 variable system can be adequately reproduced using between 5 and 15 modes. The reduced models are generally much more accurate that the ``grouped'' model (e.g., an Eggs--Nauplii--Copepodid--Adult representation). A second advantage is that the EOF reduction does not have adjustable parameters (other than the number of modes to include). The EOF approach allows the important aspects of detailed biological interactions (i.e., more realism) to be included in a large--scale physical model.

Biological-Physical coupling - As part of the third phase of our modeling project, Craig Lewis working with Gawarkiewicz and Chen, has developed code for including 16 biological compartments into ECOM-si. These compartments can be used for a variety of biological state variables including multiple life stages with nutrients, phytoplankton, and zooplankton dynamics, EOF modes, or complex food-web dynamics including the microbial loop.

Using the population and aggregation models we have developed together with the simplification method and the coupled biological-physical code in ECOM-si, we are now in a position to explore a more rich and realistic set of biological dynamics affecting population abundance in a 3-D flow field. Examining the effects of event-scale forcing on these dynamics is the goal of this year's study.

In addition to these modeling studies, we have spent a portion of our time working on related topics including modeling north equatorial current meandering on a pelagic ecosystem (Dadou et al., in press), modeling salp grazing in the Hauraki Gulf of New Zealand (Zeldis et al., in press), and visualization of plankton distributional data in 3-space (Davis et al., in press).

#### Publications Crediting this Grant:

Chen, C., and R. Beardsley, 1995: Behavior of Lagrangian Properties of Flow within a Tidal Mixing Front over Georges Bank. Submitted to the Journal of Physical Oceanography.

Dadou, I. V. Garcon, V. Anderson, G. Flierl, and C. Davis. Impact of the north equatorial current meandering on a pelagic ecosystem: A modeling approach. Deep Sea Res., Deep Sea Res., (in press).

Davis, C. S., S. M. Gallager, M. Marra, and W. K. Stewart. Rapid visualization of plankton abundance and taxonomic composition using the Video Plankton Recorder. Deep Sea Res., (in press).

Davis, C. S., D. B. Olson, M. Pascual, and J. Steele. An N-P-Copepod cohort model: temperature and food-dependent growth and development. (in prep)

Flierl, G. R. and C. S. Davis. Reduction of complexity in coupled biological-physical models. J. Mar. Res. (in press)

Franks, P.J.S. and C. Chen. Plankton production in tidal fronts: a model of Georges Bank in summer. Submitted ms.

Gawarkiewicz, G, 1995: Steady and Fluctuating Wind Forcing of a Shallow Bank. Manuscript in preparation.

Gawarkiewicz, G., T. Ferdelman, G. Luther, and T. M. Church, 1995: On the Interaction of Gulf Stream, Shelf, and Slope Water on the Continental Shelf North of Cape Hatteras. Submitted to Continental Shelf Research.

Loder, J., B. Petrie, and G. Gawarkiewicz, 1995: The Coastal Ocean of Northeastern North America (Cape Hatteras to Hudson Strait). Submitted to The Sea, Special Volume on Continental Shelves of the World.

Williams, W., 1995: The Adjustment of Barotropic Currents at the Shelf Break to a Sharp Bend in the Shelf Topography. M.S. Thesis, WHOI/MIT Joint Program.