Large variations in potential vorticity at small spatial scales in the upper ocean

Abstract

THE depth of the ocean's surface layer and stratification of the seasonal thermocline have long been thought to be controlled by heat exchange with the atmosphere and wind-induced mixing. Here we present observations which show that distortion by oceanic eddies also exerts an important influence on the structure of the upper ocean. Eddies induce horizontal shears, which are alternately cyclonic and anticyclonic on scales of 10–20 km. Conservation of potential vorticity then results in vertical motions which modify the stratification of the seasonal thermocline and can change its depth by 100 m or more in a few days. By subducting surface waters and transporting phytoplankton across the seasonal thermocline in a time comparable to that required to double the population, these motions are important to both the physics and the biology of the upper ocean.