ON THE ORIGIN OF THE INVERTED-BAROMETER EFFECT AT SUBINERTIAL FREQUENCIES

An analytical model, simulating the frictionless response of the sea c ontained in a rotating, rectangular channel of arbitrary width to air pressure waves travelling at varying directions, is developed. Since p lanetary atmospheric waves are Of Primary interest as forcing agents, a solution is found for subinertial frequencies. For an atmospheric wa ve travelling,along a channel whose width is close to the Rossby defor mation radius, the model predicts sea levels and currents organized in two coastal waves and a geostrophic current system prevailing in mid- basin. The right-hand coastal wave is more pronounced than the left-ha nd wave. The structure is coupled to the atmospheric wave, and is reso nantly driven when the phase velocity of the forcing wave approaches t he Kelvin wave velocity. Along the coasts a quasi-static adjustment oc curs under off-resonant conditions. When the atmospheric wave is movin g across the channel at a sharp angle, the response of the sea is enha nced for the apparent along-channel velocities below those of free sha llow-water waves, due to reflections at channel boundaries. For the at mospheric wave that travels at right angle across the channel, the res onance is not possible, and the sea level undershoots a simple inverte d-barometer response. Both travelling and standing waves appear in the channel. In the narrow-channel limit only a standing wave remains, wi th a nodal line in the middle of the channel. In the central part of t he channel the currents are almost geostrophic at very low frequencies . The model is used to interpret some aspects of the response of the M editerranean Sea to planetary-scale atmospheric forcing. In particular , it is shown that resonant transfer of energy from the atmosphere to the sea is most unlikely, since planetary atmospheric waves are rather slow and they travel along the main axis of the Mediterranean basin.