Comparison of Geostrophically-Derived and ADCP Current Velocity in the Upper 600 Meters of the Equatorial Pacific

Abstract

Geostrophic balance, a fundamental concept describing the equilibrium between Coriolis forces and pressure gradients, influences current patterns in the ocean. However, at the equator, the absence of Coriolis forces impacts current predictions using standard equations. Measurements were taken on research cruise TN427 on the R/V Thomas G. Thompson, from 28 December 2023 to 11 January 2024. On a transect along 167°W, Conductivity, Temperature, and Depth (CTD) measurements are taken at degree intervals between -5°- 5° and in half degree intervals between -1°-1°. These measurements are supplemented with Underway CTD in quarter degree intervals between -2°-2° to have an increased resolution between these latitudes. Using the Thermodynamic Equation of Seawater, dynamic height is calculated using the dynamic height anomaly equation and used as a geostrophic streamfunction to predict velocities. Measurements from CTD casts are used to calculate the vertical profile of geostrophic velocity using the relative geostrophic velocity equation and compared to Acoustic Doppler Profiler (ADCP) measurements, which have been averaged at midpoint latitudes. These two vertical profiles are compared to determine the latitude-dependent variability in Root Mean Squared error values. Results indicate that as latitudes approach the equator, discrepancies between calculated geostrophic velocities and measured ADCP velocities increase, as evidenced by a latitudedependent rise in root mean square (RMS) error values. External factors such as Equatorial Counter Current (ECC) and North Equatorial Counter Current (NECC) contribute to these discrepancies. This paper addresses the lack of Coriolis fore at the Equator by comparing calculations and measured velocity profiles to determine the error between them, identifying the accuracy of predictions made using latitude while nearing the Equator.