DANDONNEAU: MONITORING SEA SURFACE CHLOROPHYLL CONCENTRATION 



Integrated Chlorophyll (0- 200m) 



.10 



.20 .30 .40 



.50 

 5SCC(mg.m-3) 



Figure 4.— Integrated chlorophyll (0-200 m)/SSCC relationship between 

 lat. 6°N and 9°N (data from the RV Ryofu Maru cruises at long. 137°E, 

 Anonymous 1972 to 1984). Points and continuous line: individual stations. 

 Crosses and dashed line: averaged values for each cruise. 



in this region. We can thus conclude that SSCC is 

 a reasonable index of the chlorophyll content in the 

 photic layer. The weakness of the Ryofu Maru data 

 series is that only 4-6 stations within 3 d are avail- 

 able for each El Nino episode. This sampling pat- 

 tern can describe the vertical structure of the ocean, 

 but it is not helpful in large-scale studies based on 

 chlorophyll, in which the signal to noise ratio is very 

 low (Dandonneau and Gohin 1984). 



Subtropical Zones 



At the start of the 1982-83 El Nino (and a possi- 

 ble cause of it?) strong southerly winds were re- 

 corded east of Australia in June and July 1982 

 (Harrison and Cane 1984). In the Coral and Tasman 

 Seas, a chlorophyll enrichment occurs in austral 

 winter between 22 °S and higher latitudes (Dandon- 

 neau and Gohin 1984). This chlorophyll enrichment 

 can be seen in austral autumn and winter of 1982 

 (Fig. 1), while it only appears in winter in 1983. 

 Moreover, SSCC higher than 0.15 mg-m -3 spread 

 northward to lat. 20 °S in July-September 1982 

 around long. 160°E, but only to 24°S in July-Sep- 

 tember 1983 at the same longitude. The long and 

 intense SSCC winter increase in this area in 1982 

 may be the result of advection of richer water from 

 the south after the strong wind anomaly. In the 

 Northern Hemisphere, a zone with high SSCC 



values is observed off North America during the fall 

 of 1982 (Fig. 1); this feature is especially noteworthy 

 since most regions of the Pacific (even those from 

 the same merchant ship voyage) show low SSCC 

 values. Like other El Ninos, the 1982-83 one re- 

 sulted in temperatures and sea levels higher than 

 normal along the California coast, and strong 

 westerly winds at about 30°N. One would not ex- 

 pect increased chlorophyll concentrations with 

 higher temperatures, and according to Chelton et 

 al. (1982), El Nino episodes are likely to diminish 

 advection of water from the north which generates 

 a higher biomass. However, our data points cor- 

 responding to the enriched zone were far offshore 

 (Fig. 1) and the thermal anomaly there did not great- 

 ly differ from zero. The high SSCC values off North 

 America during the fall of 1982 might then be 

 related to the severe wind conditions which pre- 

 vailed during this time, and probably induced ver- 

 tical mixing of deep nutrients. 



A few more features which appear on Figure 1 

 would be worthy of discussion, but conclusion is 

 hindered by the lack of accordance with a poorly 

 known field of oceanic properties and by the risk 

 of sampling or instrumental errors in SSCC 

 measurements. For instance the shape of the area 

 with SSCC >0.15 rng-m -3 centered slightly south 

 of the Equator at 165°W in July- September 1982 

 (Fig. 1), while the upwelling was collapsing, is sur- 



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