Mapping Ageostrophic Ekman Currents with Imagery and Altimetry

Three 7-year data sets of sea-surface velocities in the California Current are compared. The velocities are calculated from merged altimetry data, Lagrangian surface drifter data, and data extracted from thermal satellite imagery using the maximum cross-correlation (MCC) technique. The main goal of this analysis is to determine the characteristics of the velocities that the MCC method produces; mainly at what depth the current is being measured. This question will be answered by using the ageostophic signal that is evident in the MCC data set. The existence of the ageostrophic velocity component in the MCC data set can be demonstrated by a comparison of a gridded and time-averaged merged absolute altimetry product and a similarly gridded and averaged product from MCC. The MCC velocities are found to be consistently displaced to the right of the purely geostrophic altimetric currents. This demonstrates, consistent with Ekman theory, that an ageostrophic velocity component is being measured by the MCC method. Two time dependent ageostrophic velocity data sets will be created by subtracting a merged absolute geostrophic altimetry product from the MCC and drifter data sets. These data sets will be compared with an ageostrophic velocity data set calculated from scatterometer wind stress data using a physical model of a weakly stratified upper ocean in which the Richardson number remains near unity. The ageostrophic drifter data set, which has a known depth, will be used to estimate the gradient of vertical turbulent stress. Using this information we will estimate the depth of MCC velocity measurements by comparing the rotation of the ageostrophic MCC product with the depth dependent rotation of the wind-driven scatterometer velocities. A current hypothesis of MCC measurement depth is that the method measures the average flow above the thermocline. This will be tested by comparing the estimated MCC depth from the ageostrophic analysis with the depth given by XBT data. If this hypothesis is correct, the MCC product could be used to test the validity of the physical model used to calculate wind driven ocean currents and to better quantify the vertical exchange coefficient. The employment of a wide-swath altimeter would further this analysis by removing the intrinsic one dimensional measuring capabilities of the current altimeters. This would allow for a more reliable analysis of the time dependent characteristics of wind-driven ocean currents.