Similar downwelling-favorable, coastally convergent conditions occur off Oregon, Washington and northern California in fall and winter (with a much weaker fresh water input), causing the poleward Davidson Current. In this region, however, equatorward winds in spring and summer create moderate to intense upwelling, with divergence of the surface currents away from the coast (increasing in strength from north to south). Thus the flow works against retention of zooplankton and juvenile salmon near the coast, while it enhances nutrient enrichment. Retention near the coast may be accomplished by the alongshore jet that develops off Oregon and Washington (serving as a barrier), by the intense mesoscale eddy field that develops off California (populations remaining in eddies until fall and winter winds transport them back onshore), and by animals taking advantage of subsurface onshore upwelling flow.
Interannual and interdecadal changes in the strength and position of the major North Pacific atmospheric pressure systems (the Aleutian Low and the North Pacific High) apear to force these two boundary currents to covary out of phase, possibly changing the amount of transport into each system from the central North Pacific (in the West Wind Drift). The strength of the boundary current transports, coastal convergence/divergence and mesoscale activity within each system also change in response to atmospheric forcing, as do the surface temperature and mixed-layer depth. Changes in these physical variables appear to cause changes in the zooplankton (Fig. 4 & 5) and fish (Fig. 3 & 9) populations, and may move the boundaries between biogeographical provinces.
This phasing of the ocean environment and marine populations to common atmospheric forcing argues for concurrent studies of these regions (CGOA and CCS). Given these strong signals at interannual-to-interdecadal time scales, U.S. GLOBEC plans to use a 5-7 year program of observations (monitoring and process studies) to 1) document changes in the physics and biology of these regions, and 2) examine the mechanisms by which the changes in the physical conditions alter zooplankton and salmon populations in these two boundary currents. Retrospective analysis of longer historical data sets will address changes that might have occurred in a broader suite of species. Ecosystem modeling, linking the basin-scale to regional-scale physical processes with population and food web dynamics, will attempt to reproduce the observed variability at all time scales.