DANDONNEAU: MONITORING SEA SURFACE CHLOROPHYLL CONCENTRATION 



higher than 0.15 mg-m~ 3 reappeared at the 

 Equator (Fig.l). 



Western Pacific Around Lat. 7°N. 



Under normal conditions (see Figure 1: January 

 to March 1982, July to December 1983) the equa- 

 torial upwelling also drives a chlorophyll-rich zone 

 west of 180°. This does not appear on the map of 

 Koblentz-Mishke et al. (1970) on the primary pro- 

 duction in the world ocean, but is described as an 

 episodic feature by Oudot and Wauthy (1976). The 

 area with SSCC >0.15 mg-m~ 3 which appears 

 north of the Equator, centered at about 7°N from 

 October 1982 to March 1983 (Fig. 1) has nothing to 

 do with the equatorial upwelling. Based on approx- 

 imately 100 SSCC data points obtained by three dif- 

 ferent merchant ships, this chlorophyll-rich area can 

 hardly be thought to result from measurement 

 errors. It rather may be related to the eastward 

 draining of warm water from the western tropical 

 Pacific and consequent thinning of the surface mixed 

 layer and drop of the sea level (Wyrtki 1985). A 



simultaneous cooling of the sea surface by 1°C oc- 

 curred in this region during El Nino, which can be 

 explained by advection of cooler water, and also by 

 other potentially important processes which are 

 more difficult to quantify (Meyers and Donguy 

 1984). The observed SSCC increase supports the 

 hypothesis that vertical mixing of cooler nutrient- 

 rich deep water might be one of these processes. 

 Even if vertical mixing is unlikely, the 50 m rise of 

 the thermocline which has been observed at lat. 7°N 

 between January 1982 and January 1983 (Meyers 

 and Donguy 1984) allows more light to penetrate 

 to the deep chlorophyll maximum. This hypothesis 

 is supported by the shift which occurred between 

 January 1982 and January 1983 in the nitrate-tem- 

 perature relationship (Fig. 2; data collected by the 

 Japan Meteorological Agency along long. 137°E 

 aboard RV Ryofu Maru; Anonymous 1972-84). The 

 nitrate concentration at a given temperature (which 

 we assume to represent a given water mass) 

 dropped by about 2 ^moles-L -1 . Shifts in the 

 nitrate-temperature relationship provide informa- 

 tion on the consumption of nitrate by the phyto- 



25 



20- 



TCC) 



N03 (yumole.1" 1 ) 



(♦) January 1982 



(♦) January 1972 



January 1973 



•ITCC) 



Figure 2.— Nutrient-temperature relationships between lat. 6°N and 9°N. Crosses: observations before an El Nino; open circles: 

 observations after an El Nino. (Data from the RV Ryofu Maru cruises at long. 137°E, Anonymous 1972 to 1984). 



691 



