Two Limiting Types of Oceanic Finestructures

Abstract

Profiles from CTD surveys in the Northeastern Pacific are used to study temperature, salinity and density finestructures. Finestructures are defined here as those perturbations of the thermodynamic fields which are not due to internal wave advection and whose vertical extent is between 1 and 100 m. It is shown that two limiting types of finestructures can be distinctly identified: step-structures and intrusions. A step-structure could be the signature of a mixing event; the temperature versus salinity (T?S) relationship is almost linear and the vertical configuration shows a weakly gratified layer bounded above and below by highly stratified but relatively thin regions. An intrusion is the result of lateral interleaving when the displaced water is of different type than the intruding one and their densities are practically the same. The difference in water types could be so large that the intrusion is warmer and saltier, or colder and fresher than its neighbors above and below. In this limit, density increases smoothly with depth, and sudden changes of temperature and salinity occur simultaneously in a density compensating way. Therefore, the T?S relationship is nonlinear and shows cusps in the direction of isopycnals. These two process in a perfect correlation between the temperature and salinity contributions to the buoyancy frequency squared. It is +1 when only step-structures show up, and ?1 when only intrusions do. This note shows that in the area of the observations some vertical ranges are populated mainly by one or the other limiting type, implying a differential strength in the finestructure producing processes.