Dynamical and thermodynamical signatures of Rossby waves in presence of mean flow and topography
University of Reading,
NCAS CGAM Earley Gate PO Box 243,
RG6 6BB Reading,
A major achievement of satellite altimetry has been to reveal the ubiquity of westward propagating planetary wave features at nearly all latitudes. Such signals have been interpreted as first mode baroclinic Rossby waves, but the standard theory for such waves appears to underestimate the speed of the observed signals by a factor of two to three at mid and high latitudes, and to overestimate it in the equatorial regions. This has prompted many theoretical investigations aimed at understanding the physical mechanisms affecting the dynamics of Rossby waves. Probably the two most important effects are the background mean flow and the topography. With regard to the mean flow effect, a puzzling result is that the net effect on the speed of the waves is the difference between two large terms which nearly cancel each other: the doppler shift effect on the one side, versus the modification of the background planetary vorticity gradient. Although the mean flow effect is often held to account for the observed ``too-fast'' propagation, the large uncertainties generally accompanying the estimation of the large-scale circulation raises questions as to the reality and statistical significance of the net speed-up found in the published literature. On the other hand, it can be shown that the topography always produce a Rossby wave mode that is systematically faster than the standard first baroclinic mode, and whose speed closely match the observed wave speeds. Furthermore, the topography is found to generically led to the split-up of Rossby wave modes, implying a scattering of wave energy near the top of ocean ridges which could potentially explain the observed increased wave activity westward of major topographic features in the ocean. Finally, an important property of oceanic Rossby waves is that they possess a thermodynamic signature in the surface temperature field, by which they may have some climatic impact. The thermodynamic signature of Rossby waves can be caused by several processes, including the advection by Rossby wave velocity field accross mean temperature gradients, and the coupling with the mixed layer. This paper will summarize the theoretical results achieved by our work over the past 10 years on the above issues.