Comparison of Upper Ocean VACM and VMCM Observations in the Equatorial Pacific

Abstract

Measurements from a Savonius rotor and vane vector-averaging current meter (VACM) and dual orthogonal propeller vector-measuring current meter (VMCM) placed 1 m apart beneath a surface-following buoy moored on the Pacific equator for 6-month intervals are described. Experiments at 13, 98, 120 and 160 m depths were completed in light winds (≈5 m s?1), small surface wave heights (1?2 m), strong current speeds (monthly averaged speeds ≈65 cm s?1) and large current shears (0.0 1 s?1). Excellent coherence was found between each VACM-VMCM doublet's data. As expected, the near-surface VACM recorded slightly larger speeds (≈10% for monthly mean 15-min vector-averaged values) than the VMCM. The only kinetic energy density mismatch at the 95% confidence level was between the 13-14 m dyad and occurred above 1.3 cph. The most striking feature of them intercomparison tests was that the VACM and VMCM observations were virtually identical below the new-surface layer. Below 98 m the average rms amplitude difference within a decadal frequency interval was <1 cm s?1. The average maximum usable frequency and mean frequency of the 50% noise level were 1.8 and 1.5 cph, respectively.