VARIABILITY IN A HOMOGENEOUS GLOBAL OCEAN FORCED BY BAROMETRIC-PRESSURE

The nature of the oceanic response to pressure loading is explored using a constant-density, shallow-water numerical model driven by atmospheric pressure fields from the European Centre for Medium Range Weather Forecasts. The model has realistic bottom topography and coastlines and is run for 1 year (1986) on a global domain. Meridional gradients in mean sea-level are generally large (10-20 cm over 20-30-degrees), particularly in high southern latitudes. Sea-level variability is strong in mid- and high latitudes (typical standard deviations of 10-15 cm), but weakens towards the equator. Results indicate a significant contribution of pressure-driven fluctuations to the observed large-scale sea-level variability in mid- and high latitudes, away from western boundary regions. Pressure-induced velocity signals are, in contrast, generally small compared with other types of variability. The validity of the inverted barometer approximation is found to be strongly dependent on frequency and geographical location. Globally, the approximation is not reliable for periods shorter than approximately 2 days, but failure at longer periods occurs over extensive regions (e.g. the tropical Atlantic and Pacific, and the Southern Ocean). Nonisostatic contributions to the sea-level variability are substantial in many areas, including the tropics, the high-latitude North Atlantic, the Gulf of Mexico, and several other boundary regions. The dynamical signals are partly associated with the excitation of several high-frequency normal modes. Some of these features have a spatial structure and period very similar to normal modes calculated by Platzman and collaborators. Their presence in the model indicates that atmospheric pressure forcing is a possible mechanism for normal mode excitation.