Anomalies of SST and Chlorophyll-a in the California Current

Mati Kahru,


·        Monthly anomaly images were created using Wimsoft ( software and monthly satellite date described in wam_series for was used for subsetting the global data and wam_anomaly for calculating the anomalies. The monthly anomalies are calculated relative to the respective monthly means. A tutorial for calculating anomalies is available here.

·        The limits for the area were: 20-40 N; 128-105 W.

·         The chlorophyll-a anomalies were composed from 7 years of SeaWiFS data using standard Version 4 Level-3 monthly data (Standard Mapped Images). The nominal pixel resolution is 9 km. The chlorophyll anomalies are calculated from a ratio of a monthly composite to the respective monthly mean. The full range for the ratio is from 0.1 to 10.

·        The SST anomalies were composed from almost 20 years (1985-2004) of AVHRR Pathfinder Version 5 monthly ascending (daytime) data. The nominal pixel resolution is 4 km. The SST anomalies are calculated by subtracting the respective monthly mean from a monthly composite. The full range for the difference is from -5.0 to +5.0.

·        The color scheme to visualize the anomalies uses blue tones for negative anomalies and red tones for positive anomalies. The intensity (saturation) shows the intensity of the anomaly. Values close to the mean are shown in pale colors and mean and no valid data are both shown as white.

·        The following example of SST anomalies in January, 1998 shows the effects of the strong 1997/1998 El Niño: most of the area shown has positive (red) SST anomaly. The positive SST anomaly is strongest in the central Gulf of California and offshore of the Baja California peninsula. The SST anomaly becomes weaker and even slightly negative offshore.



·        The corresponding chlorophyll-a anomaly image for January, 1998 shows that most of the California Current had lower than normal surface chl-a concentration with the exception of the warm waters off Baja California where a mysterious high-chlorophyll anomaly was detected (Kahru and Mitchell, 2000, 2002). It appears that such high-chl anomaly in warm waters was also present in the 1983-1984 El Niño (CZCS data). The mechanisms for this increased surface chl-a are still not clear.




·        An animated GIF showing the 7-year chl-a anomalies is shown at link given below. Please be patient as this is a 2 MB file that takes time to download! .

·        An animated GIF showing the nearly 20-year SST anomalies is shown at a link given below. Please be patient as this is a 20 MB file that takes time to download! .     




Kahru, M., B.G. Mitchell, Influence of the 1997-98 El Niño on the surface chlorophyll in the California Current, Geophys. Res. Let., 27(18): 2937-2940, 2000.

Kahru, M., B.G. Mitchell, Influence of the El Niño – La Niña cycle on satellite-derived primary production in the California Current. Geophys. Res. Let., 29(17), doi: 10.1029/2002GL014963, 2002.