Interannual Sea Level Changes and Associated Mass Transports in the Tropical Pacific From Topex-Poseidon Data and Model Results (1964-1999)

The main modes of interannual climate variability in the tropical Pacific are identified using satellite-derived wind (ERS) and sea level (Topex-Poseidon) data from 1993 to 1999, and output from a linear model forced by observed FSU wind from 1964 to 1999. During the 1993-1999 period, the dominant ENSO signal consists on a zonal seesaw pattern in sea level around the date-line, associated with an equatorial patch of zonal wind centered on the date-line. This oscillation is described in terms of equatorial waves. A second mode of variability, mainly linked to the strong 1997-1998 El Niño event, depicts a meridional seesaw pattern in sea level around 5\deg N, associated with opposite zonal wind anomalies in the north-western and south-eastern tropical Pacific. Both modes are well reproduced in the linear model forced by either the ERS (1993-1999) or the FSU (1964-1999) wind. The second mode, particularly active during the very strong 1982-1983 and 1997-1998 El Niño events, is reminiscent of the recharge oscillator theory. However, it shows no equatorial symmetry and its temporal function seems to include a decadal component. Sensitivity studies showed simple wind patterns are responsible for the sea level zonal and meridional seesaw patterns. The role of horizontal mass advection on the sea level variations is then quantified in the 15\deg S-5\deg S, 156\deg E-80\deg W southern box, the 5\deg S-5\deg N, 136\deg E-80\deg W equatorial box and the 5\deg N-15\deg N, 136\deg E-80\deg W northern box. For both modes of variability, there is little mass advection across 15\deg N and 15\deg S. At the ENSO time-scale, the equatorial box fills (El Niño) and empties (La Niña) mainly through zonal geostrophic transport across its western boundary, and counteracting meridional transport mitigates this mass budget. The second mode of variability contributes to the buildup and depletion of the equatorial band mainly through changes in the 5\deg N meridional geostrophic transport. Its modulation at decadal time scale could explain the shift from prevailing La Niña conditions before the 1982-1983 major El Niño event to prevailing El Niño conditions after. Complementary results based on 1948-1999 outputs from an OGCM are discussed.