CoAuthors

James Richman (Naval Research Laboratory, United States)

Mesoscale circulation is an important driver of ocean frontogenesis as eddy features force confluence of different buoyancy water masses into surrounding fronts. A secondary ageostrophic circulation associated with the frontogenesis results in upwelling and downwelling leading to a thinning or thickening of the mixed layer. An experiment was set up in July 2013 across the Gulf Stream to determine the predictability of frontogenesis by deploying about 250 airborne expendable bathythermograph (AXBT) instruments from four aircraft surveys to develop a synoptic picture of the ocean temperature structure and to detect changes in mixed layer depth around the fronts of eddies. These results are used to validate the forecasts of a 3 km resolution ocean model assimilating the satellite sea surface height observations. When the mesoscale eddies are located correctly, the numerical model is able to represent the processes leading to the frontogenesis and thinned mixed layers wrapping around the eddy circulation. Anticyclones on the south side of the Gulf Stream contain deep mixed layer even in July when mixed layers typically are shallow. Both model results and observations show a local minima in mixed layer depth on the periphery of the mesoscale eddies.

The forcing mechanism for frontogenesis is the omega vector or total derivative of horizontal buoyancy gradient, which measures the rate different buoyancy water masses are forced together. The omega vector from the model aligns well with thinned mixed layers as expected from theory. Prior results of Observation System Experiments (OSEs) show conditional predictability of frontogenesis with the condition of accurate mesoscale eddy positions. The OSEs are performed using permutations of four operating altimeter satellites to demonstrate the increasing accuracy of properties associated with frontogenesis including the omega vector, surface divergence and mixed layer depth. The accuracy of these properties increasesas additional altimeter data streams are used in the OSEs even though the data streams provide no direct observations of these properties. The increased accuracy is due to more precise positioning of the mesoscale eddy field. However these prior experiments show skill only relative to a single model control experiment. The aircraft results from the Gulf Stream experiment extend the prior work to the real world and demonstrate predictability in ocean frontogenesis.