On the skewness of vorticity in the upper ocean

Abstract

Conservation of potential vorticity suggests that when the magnitude of relative vorticity approaches the planetary vorticity, cyclonic vorticity should be preferred. Small‐scale, 3‐km relative vorticity is examined using shipboard Acoustic Doppler Current Profiler (ADCP) velocity measurements and P‐code Global Positioning System (GPS) navigation from a cruise in the North Pacific. The skewness of relative vorticity in the mixed layer is positive, consistent with theory. The probability density function of vorticity reveals that positive skewness occurs because large vorticities are more likely to be cyclonic than anticyclonic. The skewness is shown to be significant relative to measurement and random errors. In contrast, skewness in the thermocline has no preferred sign. The skewness peaks at the surface and decays to zero at the base of the mixed layer.