DECADAL VARIABILITY IN AN IDEALIZED OCEAN MODEL AND ITS SENSITIVITY TO SURFACE BOUNDARY-CONDITIONS

Variability in a three-dimensional ocean model of idealized geometry i s analyzed. The variability is induced in the model by adding a stocha stic component to the surface buoyancy forcing. The influence of the s urface thermal forcing on the model variability is investigated under conditions in which the surface freshwater flux is specified. The ther mal boundary conditions that have been considered include restoring bo undary conditions with different restoring times, fixed surface heat f lux, and boundary conditions derived by assuming an energy balance mod el for the atmosphere. It is found that the ocean model response varie s considerably with the thermal boundary conditions used, given the sp ecific ratio of thermal to haline forcing chosen for these calculation s. A behavior characterized by sudden transitions between states of st rong overturning and states of much weaker overturning dominates the m odel's response when a strong restoring is used, while quasi-regular o scillations at a period of approximately 24 years are found with bound ary conditions that allow the sea surface temperature to respond to ch anges in the oceanic heat transport. The spatial pattern of the stocha stic forcing is considered here as a variable of the problem, and the model's response to different spatial patterns is analyzed. The same d ecadal signal is found for all spatial patterns, suggesting that the v ariability at this timescale can be considered as an internal mode of the system and not associated with some characteristics of the forcing . However, different special patterns can be more or less effective in exciting the oceanic mode. Large-scale forcing directly contributing to the east-west pressure gradient appears to produce the largest resp onse.