Monthly period oscillations in the Pacific North Equatorial Countercurrent

Abstract

Monthly period oscillations in temperature and velocity in the vicinity of the North Equatorial Countercurrent along 140°W are examined using moored time series data collected during 1988–1989. These oscillations, which presumably derive their energy from instability of the large‐scale shear of the zonal equatorial current system, are found to be energetic across a broad band of periods of about 15–50 days. Along 7°N, wavelike motions at these periods propagate westward with a phase speed of about 30–40 cm s−1 and a zonal wavelength of 750–1150 km. The oscillations are approximately in geostrophic balance with the thermal field, with the velocity vector rotating anticyclonically around westward propagating high sea level centers. Zonal volume transport variations associated with these waves between 5° and 9°N at 20–180 m depth have a standard deviation of about 4.1 Sv, with peak‐to‐peak changes in some instances approaching 20 Sv. Maximum temperature variance along 140°W occurs between 5° and 7°N at 100–150 m depth in a region of strong mean vertical and meridional temperature gradient associated with a thermocline sloping upward to the north. At the surface, maximum temperature variability occurs from 2° to 5°N in the vicinity of the strong sea surface temperature front north of the equator. Analysis of the upper ocean temperature balance at 7°N, 140°W reveals that meridional advection is important both in the surface layer and in the thermocline. Vertical advection, estimated using vertical velocity calculated from a simplified linear vorticity balance, is also important in the thermocline, where it contributes a variance comparable to that of meridional advection. Zonal advection is likely to be significant in the upper 80 m, below which it is probably of secondary importance compared to meridional and vertical advection. One consequence of advection in the temperature balance is that variance is shifted toward lower frequencies in temperature vis‐a‐vis velocity spectra. This “red shift” may account for the apparent difference between periods for instability waves inferred from near‐equatorial meridional velocity spectra, and slightly longer periods reported for oscillations of the sea surface temperature front north of the equator based on satellite imagery.