Defining the transition layer based on ocean mooring observations

An important property for climate studies is the depth of the surface mixed layer, the upper ocean layer well mixed in ocean properties. Wind stress forcing causes a spiraling Ekman flow in the rotating ocean that decays with depth and transports momentum into the ocean. Conventional viewpoint suggests that the wind-driven flow does not penetrate deeper than the mixed layer, but observations show the wind-driven momentum and shear penetrate deeper into the transition layer, the region where mixing rates transit from high values in the mixed layer to low values in the interior. This deeper momentum transfer into the poorly understood transition layer has significant effects on ocean dynamics.

In this study, transition layer is researched by using pre-existing mooring data (from D. Rudnick, Scrippts Institution of Oceanography and Upper Ocean Process Data Archives, Woods Hole Oceanographic Institute), which includes surface flux, current, temperature and salinity data at multiple geographic locations in different dynamical regimes. Wind stress penetration depth, mixed layer depth, shear, stratification, and Richardson number time series are calculated. Previous research by Plueddemann and Weller (1999) and Rudnick (2003) indicates the mixed layer has low shear, while the transition layer is an area of high shear. The transition layer also transits from the non-stratified mixed layer to the highly stratified interior and hence should be a region with high stratification.

The transition layer is calculated in several ways: the depth difference between Ekman depth and mixed layer depth, layer of high shear, layer of high Richardson number, and layer below the mixed layer with high stratification. Preliminary results indicate that for regions near the equator the Ekman depth shallows and is an unreliable measure of transition layer. In at least one region away from the equator the depth between the mixed layer base and the Ekman depth correlates to the high shear band, correlates to the high stratification band and has a high Richardson number. The various definitions of transition layer will be compared to derive a more comprehensive definition of the transition layer depth and to provide characterization of this layer. An inter-comparison of transition layer characteristics from four to six separate moorings in different geographic regions is planned. The transition layer characteristics from moored data will be compared to the results of the characteristics of transition layer derived from SeaSoar data as analyzed by Johnston and Rudnick (2007).