Using an Army of Inverted Echo Sounders to Measure Dynamic Height and Geostrophic Current in the North Pacific Subtropical Gyre

Abstract

In 1991, an array of five inverted echo sounders (IESs) was deployed about 100 km north of the island of Oahu, Hawaii, as part of the World Ocean Circulation Experiment deep-water station. IESs record the round-trip acoustic travel time from the seafloor to the sea surface, which is known to correlate well with dynamic height. CTD (conductivity-temperature-depth) casts made during monthly cruises to the deep-water station are used to calibrate the IES; the conversion factor from travel time to dynamic height is found to be ?52.8 ± 0.9 dyn m s?1. The time series from the IESs show that there are fluctuations in dynamic height having timescales from weeks to months, which are not well resolved with the CTD sampling. The array of dynamic height can be used to compute geostrophic currents, which are shown to be only reasonably well correlated with acoustic Doppler current profiler (ADCP) measurements. Differences between the IES- and ADCP-derived currents probably exist because the velocity field has some horizontal structure on scales smaller than the IES array.