## Modeling

U.S. GLOBEC is committed to a vigorous program of modeling coupled physical and biological processes in the sea. We emphasize model development well in advance of field investigations. Relatively mature efforts from two modeling groups were ready for use on Georges Bank in advance of major field processes studies, and have provided directions for the field research that recently have begun.

Modeling studies of the relevant physics and biology in marine ecosystems will be used to investigate how changes in global climate affect the forcing and physical characteristics of the systems and how the distribution and abundances of animal populations respond to changes in that forcing. Models can integrate information from a variety of sources into a common context and link biological and physical information from various temporal and spatial scales. While the ultimate goal of the modeling studies is to predict and assess likely consequences of global climate change on marine animal populations, they can also assist in satisfying important nearer term objectives. These include hypothesis testing, sensitivity experiments, and planning and evaluating field research.

A long-term goal of U.S. GLOBEC is to bring predictive models for a limited set of ecosystem properties to an operational stage in the next decade. Efforts to date have been directed toward incorporating basic ecological process formulations into regional scale transport models. Several model activities must be expanded to achieve the operational model goal. First, ways must be developed to link existing regional scale bio-physical ecosystem models with ocean basin scale models which provide both the global physical forcing and the boundary conditions for the regional models. Only in this way can the large scale effects associated with climate change be accommodated, and their effects translated into specific phenomena at the regional scale. Thus, multiscale nested models must be developed. Second, many aspects of the basic ecological process formulations in these models need refinement. For example, animal behavior, such as vertical migration, may be critical; it can result in directed motion that is substantially different from that in the flow fields that transport passive organisms. The population effects of such small-scale individual organism behavior needs to be parameterized so that it can be incorporated into larger, e.g., regional, scale models. The effects of different ecological structures in coupled models must be investigated. When is a more complex food chain, perhaps including the microbial loop, needed to generate accurate predictions? Or, how will the episodic introduction of unusually large numbers of predators to a local environment alter mortality patterns, species composition and trophic structure? In fact, such introductions occur during strong El Niño's along the U.S. west coast. When is age- or stage-structure necessary in biological models of zooplankton populations, and when can it be neglected?

Many, if not most, models are not easily accessible to the wider oceanographic and fisheries community. This is particularly true of coupled biological and physical models. U.S. GLOBEC is committed to bringing such models into wider use within the appropriate scientific community.

Finally, much improved assimilation of biological and chemical data in interdisciplinary coupled models is necessary for efficient and rapid progress in research.

> Data assimilation provides a mechanism for adjusting model parameters relative to a known distribution, updating the model at various intervals and improving the accuracy of simulated distributions. Data assimilation has been used with meteorology and ocean circulation modeling studies and is now starting to be used with biological models....Data assimilation can compensate for model deficiencies, maintain synoptic phase information in the presence of loss of predictability, provide dynamical space-time interpolation of sparse data, and estimate poorly known parameters...[GLOBEC International Report No. 6 (p. 19, 1994)]

Improved data assimilation is crucial to the development of a prototype operational monitoring and prediction system (see section below; also GLOBEC International Report No. 6 and GLOBEC International Special Contribution No. 2). A lengthy bibliography of data assimilation issues can be found in GLOBEC International Spec. Contribution No. 2 (in press).