Coupled Bio-Physical Models for the Coastal Gulf of Alaska

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PROJECT SUMMARY A core hypothesis of the US GLOBEC Northeast Pacific Implementation Plan is that interannual to interdecadal variability in the circulation and hydrography of the Gulf of Alaska drives changes in productivity of zooplankton in the coastal zone, with consequent effects on the feeding success of salmonids and other species in the Gulf. Production regimes in the Coastal Gulf of Alaska (CGOA) and the California Current may covary on interannual to decadal scales, due to spatially correlated changes in physical forcing. To address these issues for the CGOA, a set of linked circulation models, coupled with a lower trophic level Nutrients Phytoplankton Zooplankton (NPZ) biological model, and an individual based model (IBM) of salmon are used. The eddy and tide resolving CGOA circulation model embedded in a basin/global scale circulation model. Specific questions to be addressed by this set of models include: 1) How do the relative strengths of the offshore Alaska Current Alaskan Stream (AC AS) and the Alaska Coastal Current (ACC) affect exchange between the shelf and slope, and hence the supply of nutrients and deep sea zooplankton to the shelf area? 2) Can wind driven Ekman flux account for most of the transport of oceanic zooplankton onto the shelf, or are more intricate cross frontal process involved? 3) How does interannual/decadal modulation of the seasonal pattern of the ACC affect secondary production and transfer to higher trophic levels on the shelf? Does a stronger ACC act to enhance or reduce productivity on the shelf? 4) Does tidal mixing significantly affect coastal production in the Gulf of Alaska? If so, can very low frequency modulation of tides account for some of the decadal change in the coastal dynamics, and hence production? The proposed set of coupled (global/basin and regional/coastal) circulation models are forced by realistic wind and river runoff time series. Output from the coastal circulation model will be used to drive the NPZ model of the near coastal area encompassing the shelf and shelf break. Circulation and prey fields produced in this manner will then be used as input to the spatially explicit IBM of juvenile salmon. This set of models will comprise a significant advance over past and existing models of the Gulf of Alaska, which have with few exceptions tended to use either coarse spatial grids or simplified physics, which exclude biologically relevant processes such as baroclinic instability in the coastal zone. Results of this proposed research will help identify past and future trends and spatial patterns of circulation, zooplankton and salmonid production within the Gulf of Alaska, and help identify mechanisms for shelf slope exchange. Output from the models will be made available to other PI's in Northeast Pacific GLOBEC, and should be especially useful as physical and biological boundary conditions for finer scale regional models (e.g. of Prince William Sound).