Geophysical Fluid Dynamics and Physical Oceanography.
Our research concentrates on geophysical fluid dynamics (GFD) applied to the ocean circulation. Particular interests have included abyssal circulation, Southern Ocean dynamics, thermobaricity, interannual variability in ocean circulation, and energetics of large scale (balanced) flow. The last of these is described in more detail below.
Models of ocean circulation invariably include parameterisations of unresolved processes. One approach in modelling is to tune the relevant coefficients (within reasonable limits) in a way that gives the best possible fit to observed data. Ultimately, however, this is unsatisfactory since it makes the science non-deductive. There has been a recent interest, therefore, in the sensitivity of simple models (especially of the wind driven circulation) to the precise values of tunable parameters. What one hopes is that the statistics of the solution become insensitive to eddy viscosities and the like in the limit that these become small (or, equivalently, as the Reynolds number becomes large). Unfortunately, this has not proved to be the case. Part of my research has related this to energetics and in particular, to the reverse cascade of geostrophic turbulence. We argue that model convergence in the limit of sm#mce_temp_url#all dissipation parameters should be expected only if i) the wind power input goes to zero or ii) there is a transfer of energy from geostrophic modes to (forward cascading) ageostrophic modes. (We are exploring both possibilities.) Below, evidence for the first is shown. Shown is the reduction in the wind power source that occurs when one takes into account that the wind stress acting on the ocean surface is (albeit weakly) dependent on the surface ocean velocity. In model simulations, accounting for this dependence reduces the power source by about 35%. As seen from the figure, most of this is associated with a midlatitude jet. Also shown is a snapshot of the vorticity forcing. At large scales, forcing is cyclonic in the northern half of the domain and anticyclonic in the southern half. At smaller scales, the forcing is intimately linked to the surface currents themselves.
Some recent publications
Duhaut, T.H.A. and D. N. Straub, 2005. Wind stress dependence on ocean surface velocity: implications for wind power input to the geostrophic circulation. J. Phys. Oceanogr, (in press).
Ngan, K., D. N. Straub and P. Bartello Three-dimensionalization of freely-decaying two-dimensional turbulence, Physics of Fluids, Vol. 16, #8, 2004, pp. 2918-1932.
Dupont, F. and D. N. Straub. Effects of a wavy wall on the single gyre Munk Problem., Tellus, Series A, Vol. 56, Issue 4, 2004, pp. 387-399.
Danielson, R.E., J.R. Gyakum and D.N. Straub. Downstream baroclinic development among forty- one cold-season Eastern North Pacific cyclones, Atmos.-Ocean. 2004,, 42, 235-250..
Dupont, F., D.N. Straub and C.A. Lin, 2003: Influence of a step-like coastline on the basin scale vorticity budget of mid-latitude gyre models. Tellus, Series A. vol. 55, 3, pp.255-272.
Straub, D. N. 2003: Instability of 2D flows to hydrostatic 3D perturbations, J. Atmos. Sciences vol. 60, pp. 79-102.