Geostrophic current estimation using altimeter data at ground track crossovers in the northwest Pacific Ocean

Abstract

Geostrophic current comprises a large portion of the ocean current, which plays an important role in global climate change. Based on classic oceanography, geostrophic current can be derived from pressure gradient. Assuming water density to be constant, we can estimate geostrophic current from Absolute Dynamic Topography (ADT). In this paper, we use ADT data obtained from multi-satellite altimeters to extract sea surface tilts along-track at crossover points. The calculated tilts along these two tracks can be converted into orthogonal directions and are used to estimate geostrophic current. In northwest Pacific, computed geostrophic current velocities are evaluated with Argos data. In total, 771 pairs of temporally and spatially consistent Argos measurements along with estimated geostrophic velocity datasets are used for validation. In this study, the effect of different cut-off wavelengths of the low pass filter applied to ADT is discussed. Our results show that a cut-off wavelength of 75 km is the most suitable choice for the study area. The estimated geostrophic velocity and the Argos measurement are in good agreement with each other, with a correlation coefficient of 0.867 for zonal component, and 0.734 for meridional one. Furthermore, an empirical relationship between the estimated geostrophic velocity and Argos measurement is derived, providing us a favorable and convenient approach to estimate sea surface flow velocity from the geostrophic velocity derived from altimeter data. The experimental application of the derived method on Kuroshio reveals reasonable results compared with previous studies.