Hood¹, Raleigh R., Marjorie A. M. Friedrichs² and Jerry D. Wiggert³

¹University of Maryland Center for Environmental Science, Cambridge, MD 21613, ²Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, VA 23529, Tel: (757) 683-5560, Fax: (757) 683-5550, E-mail: marjy@ccpo.odu.edu and ³ESSIC, University of Maryland, College Park, MD 20742

Ecosystem model comparison in the Arabian Sea: A prototype regional modeling testbed

As part of the Joint Global Ocean Flux Study, many models have been developed to simulate biogeochemical cycling in various oceanographic regions; however, few quantitative comparisons of these models have been made. In order to critically assess which ecosystem structures and model formulations are best able to simulate observed biogeochemical cycling in the Arabian Sea, we apply three fundamentally different ecosystem models within a consistent one-dimensional framework (i.e. regional testbed) at the site of the WHOI mooring (15.5N, 61.5E). The testbed contains one-dimensional physical forcing fields from two different 3D physical models, and biogeochemical data that are used for assimilation and evaluation. The data include chlorophyll-a, zooplankton, nutrient and sediment trap observations and are assimilated using the variational adjoint method. The three ecosystem models that we examine consist of: a four-component model with diatom-like phytoplankton growth, a five component model emphasizing the microbial loop, and an eight-component model containing multiple plankton size classes. After objectively optimizing each model, we quantitatively compare the performance of the different models to assess which model structure best represents the fundamental underlying biogeochemical processes. Results suggest that after optimization, all three models behave very similarly, implying that the additional complexity of the multiple size-class model may not be justified. Furthermore, a change in physical model (mixed-layer depth and vertical velocity fields) typically produces a far greater change in plankton distributions than does a change in ecosystem model complexity, highlighting the fact that biological distributions are largely a result of the physical environment.