Estimating Sea Level Slope on the Equator

Abstract

A new method for diagnosing the zonal gradient of sea level on the equator from in situ data is proposed and validated using satellite altimetry. A no-turbulent-stress-level boundary condition in the Pacific Equatorial Undercurrent core constrains the average flux of zonal momentum above, uniquely determining the zonal sea level gradient. The method is applied to simultaneous data from Tropical Atmosphere?Ocean Array moorings spanning 154°?165°E and ADCP moorings in the vicinity of 157.5°E from 6 October 1992 to 21 December 1993. An independent estimate of sea level slope is obtained from contemporaneous daily equatorial crossings of the first European Remote Sensing Satellite (ERS-1) and Ocean Topography Experiment (TOPEX)/Poseidon altimeters. The slope is also estimated by assuming no pressure gradient at 500-m depth. When accelerations and the pressure gradient can be estimated at the same location, 5-day averages of the no-stress-level estimate explain 59% of the variance in the altimetry data (correlation 0.77) with an rms difference of 2 ? 10?8 and no significant mean offset. The variance explained improves to 66% with 7-day averages. In the absence of velocity and salinity gradient observations, 58%?75% of the signal can be captured with a bias of 0.5 ? 10?8 on timescales greater than 10 days. Assuming no pressure gradient at 500 m cannot explain more than one-half of the variance on any timescale and is biased by ?3 ? 10?8 to 2 ? 10?8, depending on the time period and timescale. Estimating the salinity gradient using temperature?salinity relations worsens the results. Error analyses of the observed data indicate that the daily sea level slope can be accurately determined to within less than 1.5 ? 10?8 when comprehensive in situ data are available. Daily altimetry estimates from equatorial crossings determine the slope to within 2 ? 10?8. Both methods are superior to assuming no pressure gradient at depth and improve upon previous comparisons between remotely sensed and in situ observations of sea level that have succeeded only for monthly and longer timescales.