Westward Intensified and Topographically Modified Planetary Modes

Abstract

The process study presented in this note originates from the interest in investigating the existence of barotropic planetary Rossby modes in Mediterranean subbasins that have relatively small length scales and include extensive topographic slopes. First, the flat-bottom case is considered, then the effect of idealized bathymetries is analyzed. In the flat-bottom case an analytical equilibrium solution of the divergent quasigeostrophic equation forced by a periodic wind stress curl in a circular domain is recovered numerically in the framework of a primitive equation model. The initial spinup phase leading to the establishment of the lowest normal mode, the mode in equilibrium with the wind, and the free mode are then considered in a square domain. The equilibrium response to fluctuating winds reveals a westward intensification (absent in the inviscid theory as well as for free oscillations) that, unlike for steady western boundary currents, increases with increasing lateral friction. A dynamical explanation of this anomalous behavior in terms of vorticity balance is proposed. These westward intensified equilibrium solutions can be seen as the oscillating counterpart of the steady western boundary currents. The effect of idealized bathymetries is then analyzed. The presence of shelf and slope topographies representing larger and larger regions of intense topographic ? effect, while leaving the forced response westward intensified, leads to a progressive shift to lower frequencies of the ground state because of the reduced effective length scale of the basin. Such shift is also accompanied by a remarkable reduction of the amplitude at resonance. For large topographic gradients covering more than 90% of the total basin area (a typical situation for the central Ionian and the Tyrrhenian Seas), the existence of planetary modes and even any sign of westward propagation is prevented by the overwhelming action of the topographic steering. This property, and that for flat-bottom or for less extensive topographies planetary resonances do occur, suggests that the choice of Mediterranean topographies in circulation models should not contain regions of unrealistic flat-bottom, otherwise spurious Rossby resonances could contaminate the model response.