461
35"40'
5°20'
5° 0'
4°20'
4° O·
3°40'
3°20'
Figure 5b: Q-vector (arrows) and its divergence associated with short scale fluctuations of the
gyre in the Alboran Sea (from Tintore et aI. , 1991).
The classical QG omega equation often has been applied to ocean fron ts of large
Rossby number, therefore violating the QG approximation, since in those cases higher
order dynamics would be required. In order to take into account. the higher order dynamics , Hoskins and Draghici (1977) established a semigeostrophic (SG ) omega equation,
which also allows the diagnostic of vertical velocity from density measurements alone.
The diagnostic performance of the SG omega equat ion has been evaluated with using
vertical velocities obtained from simulation of frontal instabilities in a primitive equation
model as a reference (Pinot et aI. , 1995b). The QG and SG formulations have been tested
and compared. Results have shown that both formulations correctly diagnose the vertical
velocity pattern and magnitude. However , results are improved when using the SG for -
mulation because it takes into account ageostrophic advection which plays a crucial role