Diurnal Cycle of Internal Wave Variability in the Equatorial Pacific Ocean: Results from Moored Observations

Abstract

A diurnal cycle in temperature and vertical displacement variance has been observed in the stratified region below the surface mixed layer using moored time-series data at 0°, 140°W for there periods: November 1984, April 1987, and May?June 1987. The November 1984 and April 1987 periods coincided with the TROPIC HEAT and TROPIC HEAT-2 experiments, during which direct measurements of turbulent dissipation rates were made near the mooring site. In May?June 1987, a special set of moored time series were collected between 30-m and 61-m depth with 1-minute temporal resolution in addition to standard measurements at 15-minute resolution. The high-resolution data indicated the existence of a diurnal cycle in variance that was most pronounced at frequencies of 10?30 cph and that was coherent over the 31-m extent of the vertical array. It is likely that this diurnal cycle in variance was due in part to internal waves remotely generated at the base of the nighttime mixed layer and that the appearance of internal waves in the thermocline at frequencies higher than the local Väisälä frequency (about 2?7 cph) in May?June 1987 was due to Doppler shifting by the Equatorial Undercurrent. It is also likely that part of the observed diurnal cycle in variance was due to local shear instabilities in the Equatorial Undercurrent that may have been triggered by the diurnally modulated internal wave field. More coarsely resolved 15-minute moored time-series data from November 1984, April 1987, and May?June 1987 indicated the presence of a diurnal cycle after averaging over at least 30 days of data. Largest diurnal ranges in temperature and vertical displacement variance were typically observed in November 1984 when wind speed, zonal wind stress, and mean vertical shear were largest and when the mean gradient Richardson number was smallest. The diurnal cycle in turbulent dissipation rate had a larger amplitude in November 1984 than in April 1987, consistent with a dynamical connection between internal wave variability and turbulence in the equatorial thermocline.