Saline intrusions in the intermediate waters north of Oahu, Hawaii

The Hawaii Ocean Time-series (HOT) hydrographic observations at station ALOHA (22°45′N, 158°W) reveal that the region is sometimes the site of intrusive interleaving of different water types near the salinity minimum of the North Pacific Intermediate Waters. This interleaving is associated with saline laminae, or lenses, 5–30 km long (submesoscale) and 5–100 m thick (finestructure). The temporal variability of these intermediate waters is investigated statistically, and a background water type is defined against which to measure the strength of intrusions. The salinity minimum shows a relative maximum in variability, caused mainly by the episodic appearance of the saline, finestructure intrusions, which are correlated with low oxygen and silicate concentrations. The intermediate salinity minimum has its southern boundary near the latitude of the Hawaiian Islands, where salinity increases towards the south and oxygen gradually decreases. Thus, stirring processes local to the Hawaiian Ridge may explain the observations. Possible mechanisms are suggested, including fluctuations of the subtropical gyre southern boundary and a hydrodynamically unstable flow at intermediate depths along the ridge. Either process could transport relatively saline water into the ALOHA region, and would lead to interleaving, driven by double diffusion on scales of the order of 10 m in the vertical.