Using inverted echo sounders to measure dynamic height in the eastern equatorial Pacific during the 1982–1983 El Niño

Abstract

We investigate the relationship between the dynamic height anomaly ( ΔD) and roundtrip acoustic travel time (τ) measured by inverted echo sounders (IES) in the eastern equatorial Pacific. Salinity variations in the upper 50 m have little effect on τ but do affect surface dynamic height, ΔD 0–2500. Consequently, in the eastern equatorial Pacific it is better to interpret τ as a record of dynamic height below 50 dbar, ΔD 50–2500. The slope o f the calibration, derived from a regression of ΔD 50–2500 with τ, is −60 dyn m s -1. This slopeis controlled mainly by the highly energetic El Niño signal, and is approximately 17% steeper than the simulated IES response to the first baroclinic mode. This difference may be explained by the significant second-vertical-mode component of El Niño, but is small since τ is relatively insensitive to the second vertical mode. As a result, purely first-mode signals (which are likely to dominate IES records because of the low second-mode sensitivity) will still be estimated will with the same calibration.