Comparison of measurements from pressure-recording inverted echo sounders, moored ADCPs, and ocean reanalysis products in the North Equatorial Current region of the western Pacific

Abstract. With recent advances in data-assimilating ocean reanalysis products, mesoscale oceanic processes can be more realistically represented, increasing the need for regional performance assessments. Here, two widely used products, the Global Ocean Physics Reanalysis (GLORYS) and the Hybrid Coordinate Ocean Model reanalysis (HYCOM), are compared against two years of in situ observations from four pressure-recording inverted echo sounders (PIESs) and two acoustic Doppler current profilers (ADCPs) in the North Equatorial Current (NEC) region. While both products reproduce sea surface height (SSH) reasonably well, GLORYS shows closer agreement with the observations. However, their correlations are lower at the southern than at the northern sites located approximately 419 km apart, ranging from 0.84 (0.79) at the northernmost PIES site to 0.76 (0.76) at the southernmost site for GLORYS (HYCOM), consistent with the known degradation of satellite-measured SSH toward lower latitudes. The same trend also appears in the zonal current fields, with depth-averaged correlations (60–400 m) dropping far more sharply, from 0.75 (0.65) at the northern ADCP station to 0.36 (0.39) at the southern station. This degradation is largely associated with the reduced accuracy of satellite altimetry toward lower latitudes; assimilation of these errors can distort reanalysis SSH gradients and, consequently, zonal geostrophic currents. The relatively poor representation of intermediate-layer zonal currents at the southern ADCP station, particularly in GLORYS, may also reflect contributions from inaccuracies in the upper-ocean density structure. Although the accumulated density structure errors are comparable between the northern and southern NEC regions, differences in the relative contributions of the surface and baroclinic geostrophic components appear to explain the greater influence of baroclinic errors on the intermediate current in the southern region. In contrast, HYCOM shows vertically coherent anomalous flows throughout the water column, suggesting that surface-geostrophic variability is projected too deeply. Overall, GLORYS effectively reproduces the variability of the NEC region, though care should be taken of its limitations in interpreting intermediate-layer currents and variability affected by satellite altimetry errors, particularly in the southern NEC region.