Modeled and observed impacts of the 1997-1998 El Nino on nitrate and new production in the equatorial Pacific

The impact of the strong 1997-1998 El Nino event on nitrate distribution an d new production in the equatorial Pacific is investigated, using a combina tion of satellite and in situ observations. and an ocean circulation-biogeo chemical model. The general circulation model is forced with realistic wind stresses deduced from ERS-1 and ERS-2 scatterometers over the 1993-1998 pe riod. Its outputs are used to drive a biogeochemical model where biology is parameterized as a nitrate sink. We first show that the models capture the essential circulation and biogeochemical equatorial features along with th eir temporal evolution during the 1997-1998 event, although the modeled var iability seems underestimated. In particular, the model fails to reproduce unusual bloom conditions. This is attributed to the simplicity of the biolo gical model. An analysis of the physical mechanisms responsible for the dra matic decrease of the biological equatorial production during El Nino is th en proposed. During the growth phase (November 1996 through June 1997), nit rate-poor waters of the western Pacific are advected eastward, and the vert ical supply of nitrate is reduced due to nitracline deepening. These proces ses result in the invasion of the equatorial Pacific by nitrate-poor waters during the mature phase (November 1997 through January 1998). At that time , the central Pacific is nitrate limited and experiences warm pool oligotro phic conditions. As a result, the modeled new production over the equatoria l Pacific drops by 40% compared to the mean 1993-1996 values. Then, while E l Nino conditions are still present at the surface, the nitracline shallows over most of the basin in early 1998. Therefore the strengthening of the t rade winds in May 1998 efficiently switches on the nitrate vertical supply over a large part of the equatorial Pacific, leading to a rapid return of h igh biological production conditions. Strong La Nina conditions then develo p, resulting in a biologically rich tongue extending as far west as 160 deg reesE for several months.