Meridional and seasonal variations of the sea surface chlorophyll concentration in the southwestern tropical Pacific (14 to 32°S, 160 to 175°E)

Abstract Sea surface chlorophyll concentrations (SSCC) (3369) have been measured from January 1978 to December 1982 in the area 14 to 32°S. 160 to 175°E, on samples taken by voluntary observing ships. A log-transformation is first applied to the data to stabilize the variance; a function of time and latitude is obtained which accounts for 16% of the variance of the logs. It is then subtracted from the logs, resulting in a field with a zero mean and a constant and uncorrelated error, in which the longitude effect is small and will be neglected. The data are distributed into a one-month, two-degree latitude grid; the subgrid noise variance is 0.472 and corresponds to a variation coefficient of 78% after return to the antilog. Objective analysis is applied to obtain optimal estimates of the SSCC in the limits of the study. The most striking feature is an enrichment in austral winter, developing northward to about 20 to 22°S, with SSCC in the range 0.15 to 0.40 mg m−3. A positive correlation between SSCC and integrated (0 to 150 m) chlorophyll (log transformation, r = 0.714, n = 150) is found in the same region using existing data from oceanographic cruises; the results from SSCC data can thus cautiously be applied to the whole euphotic layer. The data from oceanographic cruises support the hypothesis that the winter enrichment results from an enhanced vertical mixing after the cooling of the sea surface. This is confirmed by the direct relationship between the frequency of strong winds and the importance of the winter enrichment. There is some indication that the combination of available light and mixed-layer depth is limiting south of 28°S in June and July, resulting in two maxima in May and August. The wind effect is inverse in the northern part where Trichodesmium spp. patches form during the calms; these patches are only surface features and are not indicative of a biomass increase at deeper levels. The interannual variability is reflected by the ratio of the range of the standard year to that of the monthly anomalies which varies from 0.5 at 15°S to 1.11 at 29°S.