Modeling the Coastal Ocean Processes within the U.S. Continental Margins

John R. Moisan

The human population of our planet lives, works, and plays primarily along the earth’s coastlines. Our coastal regions supply natural resources, serve as recreation areas, process human and industrial wastes, act as natural barriers for national security, and provide transportation avenues for commercial and agricultural products. While these regions provide us with a multitude of good and services, we are still largely unaware of their sensitivity to climate change and variability, to sea level rise, to human influence, and to other potential influences. The rise of anoxic conditions along the coasts from eutrophication due to agricultural and wastewater runoff, the loss of coastal habitats from coastal erosion and human encroachment, the overexploitation of large fisheries, the rise in extent and frequency of harmful algal blooms, the loss of recreational areas from water quality degradation — these and similar crises all point to the need for a detailed understanding of our coastal regions to improve our management of these areas and to reduce the risk of future degradation.

A Regional Earth Modeling System (REMS)
Understanding complex coastal regions requires both observations and models. Present global Earth system models have resolutions of about 1 degree, or approximately 111 km, and are not capable of resolving the spatial scales of less than 1 km required to simulate coastal processes and features. While achieving a higher spatial resolution of approximately 10 km is a long-term goal of global Earth system modeling, another decade will pass before improvements in computer capabilities resolve the circulation processes alone. Even more time will be required to resolve biogeochemical processes at the required finer resolution. Therefore, NASA has invested in the development of a Regional Earth Modeling System (REMS) to address coastal zone issues. When completed, the REMS will contain regional ocean circulation, ocean biogeochemical, coastal watershed, sediment transport, and atmospheric transport models. NASA’s goal is to develop a regional model of all U.S. coastal regions at very high resolution (less than 1 km) and to use remotely sensed data for model forcing and validation (such as wind stress and ocean color, respectively). This capability will allow scientists to investigate the sensitivity of coastal regions to potential human influences, to test the impacts of coastal eutrophication or erosion reduction efforts, or to predict the effects of storms. REMS with 500 m spatial resolution or less will produce these predictive results decades sooner than would global resolution models.
The REMS program is presently developing a modeling capability to assess the carbon and nitrogen budgets for the U.S. west, east, and Gulf of Mexico coastal regions. This collaborative effort involves researchers from NASA; the University of California, Los Angeles; the Scripps Institute of Oceanography; and Rutgers University. This project is using regional, eddy-resolving, coastal, ocean circulation, and biogeochemistry models. The biogeochemical model is capable of simulating the processes that control carbon and nitrogen cycles. The model is forced using wind stress and heat flux products derived from the European Remote Sensing Satellite and QuikSCAT satellite data. Validation is performed by statistical comparison with analyses from in situ measurements, from AVHRR sea surface temperature data, and from SeaWiFS integrated chlorophyll measurements (Figure 1).

U.S. West Coast Simulations
Winds blowing along the U.S. West Coast are the primary physical forcing mechanism that brings deep, nutrient-rich water to the surface-lighted regions. In these regions, single-celled algae use the nutrients to support the growth of coastal upwelling blooms. Coupled to these upwelling centers are offshore meandering filaments and subsurface clockwise eddies that move large volumes of carbon biomass offshore (Figure 2).
Modeling efforts on the U.S. West Coast have primarily focused on developing a closed carbon and nitrogen budget. In addition to this effort, we’re developing grid nesting techniques to create higher resolution subgrids to resolve smaller scale features and processes near regions of interest. These techniques are especially relevant for coastal regions in close proximity to sewage or pollution outfalls that are typically point sources of material flux. The West Coast modeling effort is being performed in collaboration with the city of Los Angeles and other local environmental agencies to develop a model that will simulate sediment transport and particle coagulation and transport along the sea floor. This modeling effort is also tracking the destination of pollution entering the coasts from runoff and sewage outfalls and of large DDT-contaminated regions located off Los Angeles, an application of interest to the Environmental Protection Agency (EPA) (Figure 3).

Northeastern North American Coast Simulations
The ocean circulation processes along the U.S. East Coast are directly linked to circulation and forcing processes that occur over the larger basin-scale North Atlantic. To accommodate this process, the East Coast model is actually an embedded domain nested within a larger, 10 km resolution grid encompassing the entire North Atlantic Basin domain. The boundary conditions for the coastal domain are obtained from results generated within the basin-scale model. As a result, the coastal circulation model simulates both local wind and buoyancy forced flow, and also the remotely forced deep-ocean shelf exchange processes that are the dominant source of new nutrients for the coastal marine ecosystem (Figure 4).
The model simulations are focusing on developing a carbon and nitrogen budget for the coastal ocean region. However, unlike the West Coast domain, the coastal ocean along the northeastern North American seaboard is heavily influenced by coastal river and stream runoff as well as by wet and dry deposition of nitrogen sources from polluted continental air. To address this difference, NASA has developed a dataset of selected water quality constituents and associated stream flow values for streams and rivers that flow into the Atlantic Ocean and the Gulf of Mexico using U.S. Geological Survey river gauge data. The data are presently being used as input into the coastal ocean model for sensitivity studies into how the coastal ecosystem and biogeochemical cycles respond to changes in climate, to large-scale eutrophication from industrial pollution, and to other anthropogenically induced changes.

Summary
Understanding the present and predicting the future state of our coastal ocean regions requires developing and using high spatial resolution (less than 1 km) coupled numerical models. These models are trying to simulate circulation, biological, chemical, watershed, and atmospheric processes. Their successful development is based in part on the availability of adequate datasets for which to force the models and compare results against. NASA is presently supporting such modeling efforts to understand and quantify the carbon and nutrient cycle of the U.S. coastal regions. The ultimate goal of this project is to develop a Regional Earth Modeling System that is able to provide regional to local scale forecasts to state and local coastal planning agencies through the use of remotely sensed data for model forcing and validation. This modeling capability will then provide the required links for delivering satellite-derived, value-added products in an end-to-end fashion to operational agencies such as the EPA.

Acknowledgments
This research has been supported by the National Aeronautics and Space Agency (NASA) under the Office of Earth Science Interdisciplinary Science Program.

About the Author
John R. Moisan is an oceanographer in the Laboratory for Hydrospheric Processes at the Wallops Flight Facility of NASA’s Goddard Space Flight Center. He participates in NASA-sponsored research to develop coastal biogeochemical models to study Earth’s climate. He obtained a Ph.D. in physical oceanography from the Center for Coastal Physical Oceanography at Old Dominion University in 1993. He can be reached at [email protected].
Collaborators on this project and on this article include Dale B. Haidvogel, Julia Levin, and John Wilkin of Rutgers University; Emenuele DiLorenzo, Arthur J. Miller, and Bruce Cornuelle of the Scripps Institute of Oceanography; and Patrick Marchesiello, James C. McWilliams, and Keith D. Stolzenbach of UCLA.

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