Equatorial and off‐equatorial fine‐scale and large‐scale shear variability at 140°W

Abstract

In April 1987, the mean vertical shear was unusually low in the upper 350 m at 0°N, 140°W. The surface winds, the equatorial undercurrent, and the 20–30 day instability waves were at seasonal minima, and the equatorial zone was recovering from an El Niño event which had suppressed the undercurrent earlier that year. Unexpectedly, fine‐scale fluctuations with a vertical wavelength of 40 m and a period of about 12 days produced shears up to 0.03s−1, in contrast to the 0.01 s−1 related to the undercurrent. This “shear wave” accounted for most of the shear between 40 and 120 m. Its space and time scales are consistent with the dispersion characteristics of Doppler‐shifted equatorial inertia‐gravity waves. Turbulence measurements indicate that the shear wave was strongly damped on a time scale of one wave period or less. These equatorial observations differ markedly from those made at the same place in November 1984, when surface winds, the undercurrent, and instability waves were strong and when there was no shear wave. Measurements taken in transit to and from the equator show an increase in internal wave energy and shear toward the equator that can be described by a Garrett and Munk spectrum modified to have constant spectral density and increasing bandwidth as the Coriolis parameter approaches zero.