New observations of ageostrophic (Ekman and barotropic) current in the ocean
Most global circulation models reproduce density fields reasonably well, but show poor performance in ocean circulation fields. It indicates that the numerical models do not represent dynamics of the ageostrophic current, mainly Ekman current and/or barotropic current.
Using 30-year observed current at 15 m from the satellite-tracked drifter data in the Northeastern Pacific (135°– 180°E, 40°– 50°N), Lee and Centurioni (2018) showed that the Ekman current at 15 m is forced by friction velocity () rather than wind stress () for most value ranges of less than 1.5 cm s-1 (equivalent to wind speed of about 12 m·s-1). But Polton et al. (2013) found that observations are consistent with Ekman’s theory with constant eddy viscosity in the layer from 28 m to 100 m based on observation from shipboard acoustic Doppler current profilers in Drake Passage. The observations of Ekman current imply that the wind-driven current in the near-surface layer and in the deep layer is possibly governed by different set of eddy diffusivity. With depth-varying eddy diffusivity, this study solves the Ekman equation,
numerically using Matlab differential equation solver. Various profiles that reproduce observations are being tried and will be discussed.
After referencing geostrophic current to the measured currents at 4000 m, the mass transport in the Kuroshio Extension and Gulf stream were found to be 31 Sv and 47 Sv to the west, respectively (Niiler et al., 1985). After applying the topostress that is induced by eddy-topography interactions, to the numerical model, Wang et al. (2008) showed that the model skill for current has improved markedly from 0.4 to 0.9. Their model reproduces the currents at a depth of 1000 m in the area of Gulf Stream and of around 30°N are to the west similar to the observations from Argo floats. Including the topostress to the vorticity equation in the East Sea, it reproduces observed seasonal variation of the barotropic flow in the East Sea, i.e. the cyclonic flow around the northern East Sea in the summer and the double gyre in the winter.
Lee, D.-K. and L. Centurioni, (2018). Water following characteristics of Global Drifter Program drifters with and without subsurface float. Deep-Sea Research Part I, 137, 20-29.
Niiler, P., W. Schmitz and D.-K. Lee, (1985). Geostrophic volume transport in high eddy-energy areas of the Kuroshio extension and Gluf Stream. J. Phys. Oceanogr., 15, 825-843.
Polton et al., (2013).Can Drake Passage Observations Match Ekman’s Classic Theory? J. of Physical Oceanography, 43, 1733 – 1740.
Wang et al. (2008). http://goapp.ocean.dal.ca/docs/kelowna/Wang_CMOS_2008.pdf