Decorrelation scales of chlorophyll as observed from bio-optical drifters in the California Current

The California Current System is characterized by intense mesoscale variability, with meandering jets that can create small regions of strong vertical motion. To study physical and biological scales of variability, 24 near-surface drifters were released in this system, each equipped with a spectroradiometer to measure upwelling radiance. Sensors also measured downwelling irradiance as well as sea surface temperature (SST). Data were relayed to shore via satellite and processed into biological quantities, such as chlorophyll. Several drifters were trapped by mesoscale eddies, and the cross-correlation functions were calculated for SST and chlorophyll. In general, changes in chlorophyll lagged changes in SST by one to two days. This was observed for both cyclonic and anticyclonic eddies and supports the hypothesis that the phytoplankton response to changes in the physical environment (and presumably the light/nutrient regime) may be governed by a “shift-up” response. Decorrelation time scales were calculated based on the first zero-crossing of the autocorrelation function, and the biological scales were compared with the physical scales. Decorrelation time scales for most variables increased as the drifters moved from nearshore to offshore. Nearshore (defined as the region within 200 km of the coast) decorrelation time scales were two days for both sea surface temperature (SST) and chlorophyll. In the region between 200 and 400 km offshore, the decorrelation scales were six days for SST and four days for chlorophyll. In the region more than 400 km offshore, the SST decorrelation scale was seven days and decreased to 2.5 days for chlorophyll. This pattern of decorrelation time scales suggests that the processes regulating the distribution of temperature and chlorophyll are similar in the nearshore region and significantly different offshore. Similar calculations were made for solarinduced fluorescence/chlorophyll, and the corresponding decorrelation time scale increased steadily from less than one day nearshore to two days offshore. The rapid change of fluorescence nearshore, relative to changes in pigment concentration, supports the notion that phytoplankton have adopted different strategies for growth in the nearshore versus the offshore region.