Physical Setting

## The Physical Setting

           Regional Divisions
           Large Scale Features
           Mesoscale Features
           Seasonal Variability
           Interannual Variability
           Decadal Variability

### Regional Divisions

The California Current system spans more than 25 degrees of latitude along the west coast of North America (Figure 1). We divide the CCS into four regions--each is forced by somewhat different physical processes (see also Parrish et al. 1981). As a result, each region harbors a somewhat different ecosystem structure. The four regions are broadly defined, from north to south, as follows:

- Region I. Vancouver Island, Canada to Cape Blanco (Oregon, 50 deg N-43 deg N);
- Region II. Cape Blanco to Point Conception (California, 43 deg N-35 deg N);
- Region III. Point Conception to Punta Baja in northern Baja California (the Southern California Bight and offshore waters, 35 deg N-30 deg N);
- Region IV. Punta Baja to Cabo San Lucas (Baja California, 30 deg N-23 deg N).

Each of these regions is characterized by differences in wind stress, intensity of coastal upwelling, coastal morphology, freshwater inflow, large scale-advection and the level of mesoscale activity (Figure 1). Although we identify the boundaries by geographic features, above, the boundaries between these regions can be diffuse and some are known to change in response to interannual events, such as ENSO, and other long-term climate variation. In some cases, these same boundaries (or entire regions) also separate biological provinces.

### Large Scale Features

The CCS contains three major currents: the equatorward California Current, the poleward Davidson Current, and a poleward Undercurrent. In the long-term average of dynamic height, the California Current appears as a slow, equatorward flow that extends southward from the trans-Pacific flow of the West Wind Drift. The poleward Undercurrent is a sub-surface current, trapped along the continental slope (Huyer, 1983; Chelton, 1984; Hickey, 1979,1989). Poleward flow extends to the surface next to the coast from October through February and this surface poleward flow is referred to as the Davidson Current.

These currents and other more transient features of the CCS can be identified by unique water mass properties (Hickey 1979). California Current water is cool, fresh and high in dissolved oxygen and nutrients. The northward flowing poleward Undercurrent transports water of relatively high temperature, salinity and nutrients and low dissolved oxygen from equatorial regions. Subtropical water to the west and Equatorial Pacific water to the south are warmer and have lower dissolved oxygen and nutrients. Jets and meanders in Region II are often associated with upwelled water that is generally cool, salty, nutrient-rich and oxygen poor on the inshore edge of the jet (with opposite characteristics on the offshore edge) (Huyer et al. 1991).

### Mesoscale Features

Satellite images of sea surface temperature (SST) and ocean color, (Figures 2 and 3), and fields of sea surface height from altimeter data have revealed a rich structure of jets, filaments and eddies, especially in Region II in summer. These mesoscale features are superimposed on the slow, generally southward flow that is often shown as the typical eastern boundary current structure (Wyllie, 1966; Levitus, 1982).

### Seasonal Variability

Region I experiences the strongest winter storms with moderate summer upwelling. Region II experiences less intense winter storms than Region I. Following a 'spring transition', winds in Regions I and II become upwelling favorable, strongest is Region II (Huyer et al., 1979; Strub and James, 1988). Winds are weakest in the Bight (inshore part of Region III). Winds are moderately upwelling favorable all year in Region IV. A conceptual diagram of the seasonal surface currents is shown in Figure 4 (based on Hickey, 1979; 1989; Lynn and Simpson, 1987; and analysis of Geosat altimeter heights, Strub, unpublished). This shows the development of an equatorward jet off North America in spring next to the coast and the offshore movement of the jet in summer and fall, as the northward Davidson current develops next to the coast in winter. Inshore of the region occupied by the equatorward jet in spring and summer, colder and richer water is found. This is shown by satellite images of SST and pigment concentrations (Figures 2 and 3), as well as by field data (Brink and Cowles, 1991; other papers same volume).

### Interannual Variability

The major source of interannual variation in the Pacific Ocean is the ENSO cycle. Effects of this variability reach the CCS by two mechanisms: oceanic and atmospheric. During the warm phase of ENSO, the oceanic signal propogates poleward from the equator, and is manifested as an increase in northward transport, a deepening of the thermocline and a rise in surface temperature and sea level (Simpson 1983; Huyer and Smith 1985; Rienecker and Mooers 1986). SST variability associated with the ENSO cycle also cause changes in the position and strength of the atmospheric pressure (and wind) patterns which affect the California Current region (Philander, 1990). Since the ENSO cycle has periods of 3-7 years it can be expected to contribute to much of the variability seen over the 5-7 years of a U.S. GLOBEC study.

### Decadal Variability

Time series of physical and biological measurements in eastern boundary currents exhibit nonstationary properties-changes in temperature, accompanied by shifts in ecosystem structure, occur on a time scale of 30-50 years. Shifts in state have been documented for the mid-1940s when the system switched from warm phase to cool phase, and for winter 1976-77 (Miller et al. 1994a) when it switched from cool to warm phase (see Fig. 9). The 1977 shift is particularly well documented because atmospheric, sea surface and subsurface data were sufficient to demonstrate the basin scale nature of the shift. There is growing evidence that warm phase/cool phase shifts in the California Current are linked to the intensity of the Aleutian Low (Trenberth 1990; Graham, 1994; Miller et al., 1994b; Trenberth and Hurrel, 1994). A deepening of the Aleutian Low seems to result in more vigorous cyclonic circulation of the North Pacific subarctic gyre, and a deepening of the mixed layer in the North Pacific subtropical anticyclonic gyre.