Seasonal and nonseasonal variability of satellite‐derived surface pigment concentration in the California Current

Abstract

Satellite‐derived pigment concentrations from the west coast time series (WCTS) are averaged into monthly mean fields over the California Current system (CCS) for the period July 1979 to June 1986. Errors caused by the scattering algorithm used in the WCTS are reduced by an empirical correction function, although winter values (November–February) remain unreliable. For the March–October period we look at both the mean seasonal development and the nonseasonal monthly anomalies of pigment concentration. These are compared with fields of alongshore wind stress, mixing power of the wind (u*3) and wind stress curl. Outside of the Southern California Bight there is a strong seasonal cycle with a spring‐summer maximum, a northward progression of high pigment concentrations from California to Oregon and a double maximum off Washington (spring and summer, with a lull in between). Within the Southern California Bight, seasonality is low, with a relative minimum in late summer. Off Baja California the pattern is similar to that off northern California. In regions where previous work has been done, there is general agreement with the seasonal cycles found here. Nonseasonal variability in pigment concentration over the large‐scale CCS (400 km wide) is most closely related statistically to synoptic fields of wind stress curl. Within 100 km of the coast, the strongest relations are between pigment concentration and both u*3 and alongshore wind stress. Correlations with these wind variables account for only 25% of the monthly variance in anomalous satellite‐derived pigment concentrations. This is partly due to the noise in both wind and pigment data sets but also demonstrates the fact that much of the anomalous pigment variability is not a response to anomalous wind forcing on these time scales. Correlations are also low between anomalous pigment concentrations and anomalous sea level heights, which serve as a crude proxy for the strength of the alongshore current over the shelf. The largest nonseasonal anomaly in the data occurred during the 1982–1983 El Niño, which caused a large‐scale decrease in pigment concentration, stronger and longer lasting in the south than in the north.