High‐wavenumber shear and temperature structure during the Patches Experiment

Abstract

Acoustic Doppler current profiler (ADCP) measurements made during the Patches Experiment (PATCHEX) in the Pacific Ocean are examined for the occurrence of high‐wavenumber shear (vertical scales of >2 m) which is presumed to lead to patches of small‐scale waves and turbulent mixing. Shear is found to be dominated by near‐inertial waves having upward energy propagation and ∼30‐m vertical wavelengths over the depth range ∼80–180 m. The predominance of upgoing waves is unexpected and supports the RiNo float measurements made by Kunze et al. (1990a) at 180 to 200‐m depth. Profiles of rms shear are qualitatively similar to those measured by Gregg and Sanford (1988) with the multi‐scale profiler (MSP); and the fraction of time shear exceeds a threshold based on Gregg's (1989) scaling is about 10%, similar to the fraction of active turbulence seen in MSP profiles of dissipation rate. Towed thermistor chain data show infrequent patches of short (∼15 m) waveforms which may be Kelvin‐Helmholtz shear instabilities (as assumed by Kunze et al.). The data support previous suggestions that near‐inertial waves are important sources of turbulent mixing in the thermocline.