Cross-shelf phytoplankton pigment variability in the California Current

Five years (1979–83) of satellite ocean color data from the coastal zone color scanner (CZCS) data are used to calculate cross-shelf transects of phytoplankton pigment concentrations in the California Current and quantify their seasonal and interannual variability. Alongshore means over six regions, Washington–northern Oregon (48.4–42.8°N), Cape Blanco to Cape Mendocino (42.8–40.4°N), northern California (40.4–37.8°N), central California (37.8–34.5°N), northern Baja (32.5–29.4°N) and southern Baja (27.9–22.9°N) are used to contrast latitudinal differences. The general seasonal cycle shows minimum cross-shelf pigment extensions in March (2.0 mg m −3 isoline within 80 km of shore), expansion of pigment concentrations offshore in April and May often to the annual maximum (2.0 mg m −3 isoline up to 200 km offshore), a mid-summer (July–August) reduction in cross-shelf extension followed by a late summer (September) expansion and a fall collapse back towards the coast by late October. Amplitude of this seasonal pattern is minimum along the northern Baja coast and strongest off central California. Strong interannual variability is evident in both the timing and strength of these seasonal features. The most latitudinally consistent pattern of interannual variability is of negative anomalies and reduced concentrations during the 1983 El Nino. Time series of cross-shelf pigment distribution are compared quantitatively with concurrent alongshore wind stress and wind mixing ( U* 3) from each region. Maximum correlations occur when pigment lags wind forcing by 1 or 2 ten-day periods. Strongest correlations are in the region from Cape Blanco to Cape Mendocino. Correlations are weaker both in the Pacific Northwest and off Baja. Correlations which excluded the anomalous conditions of 1983 are stronger in the three regions from Cape Blanco south to Point Conception suggesting that connections to wind forcing are stronger during non-El Nino years in these regions. Lastly, preliminary comparisons of the CZCS seasonal cycles are made to those evident in the first 19 months of SeaWiFS chlorophyll data which include the strong 1997–98 El Nino.