GLOBEC 2000: Variability in Shelf Transports in the Gulf of Alaska
P. Stabeno [NOAA/PMEL], N. Bond [NOAA/PMEL], C. Mordy [NOAA/PMEL], J. Overland [NOAA/PMEL], J. Napp [NMFS/AFSC], D. V. Holliday [BAE Systems]

The proposal study tests the hypothesis that interannual variations in atmospheric forcing result in variations of on-shelf flow of nutrient- and zooplankton-rich slope water and of along shelf transport. The along shore transport is dominated by the Alaska Coastal Current (ACC). This westward flow is interrupted by cross shelf events that result from instabilities in the slope flow, eddies, interaction of currents with bathymetric features, and wind-driven downwelling. Changes in the large scale atmospheric forcing can modify on-shelf fluxes of nutrients and zooplankton through eddy formation and Ekman drift. Our results will support salmon studies, since it is hypothesized that salmon survival is dependent upon prey availability during the critical period when they first enter the ocean. Prey availability is likely related to bottom-up processes associated with the introduction of nutrient- and zooplankton-rich slope waters.

These hypotheses will be addressed through a combination of atmospheric modeling, observations and retrospective studies (Overland and Bond); biophysical moorings (Stabeno, Holliday, Mordy and Napp); and satellite-tracked drifters (Stabeno). The atmospheric modeling and observational studies will document and elucidate the downscale effects of coastal terrain on the local atmospheric forcing (along-shore winds and runoff) of the ACC. The retrospective study will describe how the local and basin wide forcing have varied over the last half century. Ten biophysical moorings will be deployed in years I and III, and five moorings in years II and IV. These moorings will measure temperature, salinity, fluorescence, and currents, and, at selected sites, nitrate. A surface mooring will be deployed each year and in addition to standard oceanographic variables will measure wind, barometric pressure, air temperature, and radiation, which will provide local observations for downscaling studies. A bioacoustics instrument to measure zooplankton will be used in years I and III to examine zooplankton and community size structure. Twenty-four (32 in years I and III), satellite-tracked drifters together with current measurements at the moorings will be used to study the currents. We will examine timescales from events to interdecadal. This proposal will address the effect of onshelf fluxes of slope water and its associated nutrients and zooplankton as factors affecting the ecosystem, particularly the foraging success of young salmon.

DATA COLLECTION DETAILS

During years I and III we plan to occupy 9 mooring sites. In years II and IV we will occupy 4 sites. The primary site will have a surface mooring with an adjacent ADCP near GAK 3. Moorings will be turned around every ~6 months. We plan to have continuous measurements at 4 sites over the 4 year period. Each site will be instrumented with : temperature sensors (throughout the water column), seacats (temp. and salinity), fluorometers (upper mixed layer), and current meters or ADCP. In addition, at the surface mooring site there will be a full weather system (winds, pressure, humidity, radiation, AT, etc), and a multi-frequency acoustic sensor to obtain time series of total biomass and size distributuon of zooplankton. Two nitrate meters will be deployed, one on primary mooring and the other near bottom nearer the shelf break. The acoustic sensor will only be deployed in years I and III. At present, moorings are on the Seward Line and off Gore Point. This may change.

We plan to deploy 24-32 satellite-tracked drifters each year. They will be deployed in February, May and September at three lines (Kayak Island, east of Prince William S. and the Seward Line). In addition drifters will be used to examine eddies, patches of zooplankton, etc.

The satellite-tracked drifter trajectories and some of the measurements from the surface mooring will be available in real time on a web page.

This page was last updated on September 22, 2000.

Maintained by:
Hal Batchelder [hbatchelder@coas.oregonstate.edu
College of Oceanic & Atmospheric Sciences
Oregon State University
Corvallis, OR 97331-5503
phone: 541-737-4500; FAX 541-737-2064