Modeled and observed impacts of the 1997-1998 El Niño on nitrate and new production in the equatorial Pacific
Résumé
The impact of the strong 1997-1998 E1Nifio event on nitrate distribution and newproduction in the equatorial Pacific is investigated, using a combination of satellite and insitu observations, and an ocean circulation-biogeochemical model. The general circulationmodel is forced with realistic wind stresses deduced from ERS-1 and ERS-2 scatterometersover the 1993-1998 period. Its outputs are used to drive a biogeochemical model wherebiology is parameterized as a nitrate sink. We first show that the models capture the essentialcirculation and biogeochemical equatorial features along with their temporal evolution duringthe 1997-1998 event, although the modeled variability seems underestimated. In particular,the model fails to reproduce unusual bloom conditions. This is attributed to the simplicity ofthe biological model. An analysis of the physical mechanisms responsible for the dramaticdecrease of the biological equatorial production during E1 Nifio is then proposed. During thegrowth phase (November 1996 through June 1997), nitrate-poor waters of the western Pacificare advected eastward, and the vertical supply of nitrate is reduced due to nitraclinedeepening. These processes result in the invasion of the equatorial Pacific by nitrate-poorwaters during the mature phase (November 1997 through January 1998). At that time, thecentral Pacific is nitrate limited and experiences warm pool oligotrophic conditions. As aresult, the modeled new production over the equatorial Pacific drops by 40% compared to themean 1993-1996 values. Then, while E1Nifio conditions are still present at the surface, thenitracline shallows over most of the basin in early 1998. Therefore the strengthening of thetrade winds in May 1998 efficiently switches on the nitrate vertical supply over a large partof the equatorial Pacific, leading to a rapid return of high biological production conditions.Strong La Nifia conditions then develop, resulting in a biologically rich tongue extending asfar west as 160°E for several months.
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