Interannual sea level changes and associated mass transports in the tropical Pacific from TOPEX/Poseidon data and linear model results (1964-1999)

Abstract

The main modes of interannual climate variability in the tropical Pacific are identified using satellite-derived wind (ERS) and sea level (TOPEX/Poseidon) data for the 1993-1999 period and output from a linear model forced by observed Florida State University (FSU) wind for the 1964-1999 period. During the 1993-1999 period a dominant El Niño-Southern Oscillation (ENSO) signal showed up in the equatorial band as a zonal seesaw pattern in sea level around a fulcrum at the dateline, associated with an equatorial patch of zonal wind centered near the dateline. This oscillation is described in terms of long equatorial waves. A secondary ocean-atmosphere coupled mode of variability, mainly linked to the strong 1997-1998 El Niño event, is identified, consisting of a meridional seesaw pattern in sea level with its fulcrum at 5°N, associated with opposite zonal wind anomalies in the northwestern and southeastern parts of the tropical Pacific. Both modes of variability are well reproduced in the linear model forced by either the ERS wind during the 1993-1999 period or the FSU wind during the 1964-1999 period. The second mode of interannual variability, 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 enabled us to identify idealized zonal wind patterns as being responsible for the observed and modeled sea level zonal and meridional seesaw patterns. The role of horizontal (geostrophic and Ekman) mass advection on the sea level variations is then quantified in three selected boxes based on the sea level zonal and meridional seesaw patterns (the 15°-5°S, 156°E-80°W southern box, the 5°S-5°N, 136°E-80°W equatorial box, and the 5°-15°N, 136°E-80°W northern box). For both modes of variability, there is little mass advection across 15°N and 15°S. At the ENSO timescale (first mode of variability) the equatorial box fills or empties mainly through zonal geostrophic transport across its western boundary, while meridional transport partially counterbalances this mass budget. The second mode of variability accounts in large part for the buildup and depletion of the equatorial band, mainly through changes in the 5°N meridional geostrophic transport. Its modulation at decadal timescale could explain the shift from balanced El Niño/La Niña conditions before the 1982-1983 major El Niño event to prevailing El Niño conditions after.