Scripps Institution of Oceanography

Repeated horizontal sections at tidal and seasonal timescales are analyzed and show the two-dimensional (x-z) spatial patterns of internal waves and thermohaline structure. These spatial patterns give information about internal wave properties on tidal timescales, and the history of stirring and mixing on seasonal timescales.

The across-ridge structure of tidal internal waves at Kauai Channel, Hawaii was observed using 300 km SeaSoar and Doppler sonar sections perpendicular to the Hawaiian ridge. M2 tidal beams and their reflection off of the surface were observed in velocity and displacement variance and several covariances. Covariances were consistent with internal wave generation at the ridge flanks and corresponded to internal wave propagation away from the ridge. Energy flux was found to exceed dissipation almost everywhere across the ridge. Interactions between upward and downward tidal beams are shown to cause momentum flux divergences and mean flows.

The seasonal evolution of upper ocean thermohaline structure was studied using observations of a North Pacific section taken in winter, spring, and summer with 3-14 km horizontal resolution. In the mixed layer, seasonal modulations in the size of horizontal temperature and salinity differences as well as their tendency to compensate in density were observed. Water left behind after the mixed layer restratified inherited its thermohaline structure from the winter mixed layer, was most similar to the mixed layer in spring, and to the region just below in summer. Spectra of isopycnal depth and temperature showed seasonal variations at 30-370 km wavelengths. Effective isopycnal diffusivity was estimated by treating salinity anomalies as a tracer.

Thermohaline structure at 5-1300 km scales in the upper 1000 m was analyzed along a section north of Hawaii observed 16 times with autonomous gliders over 2.5 years. A spatial modulation of thermohaline fluctuations along isopycnals was observed that persisted over the observation period and reflects the history of horizontal stirring and mixing. Temporal modulations in thermohaline structure were observed in the mixed layer, including the subtropical frontal regions, and the water left behind after restratificaiton, where perturbation magnitude decayed with time once isolated from the mixed layer.