CLIMATE VARIABILITY IN A LOW-ORDER COUPLED ATMOSPHERE-OCEAN MODEL

The dynamical behavior of the climate system is investigated through t he use of a low-order coupled atmosphere-ocean general circulation mod el. The goal is to gain some qualitative understanding of how non-line ar interactions between the individual system components may affect th e climate. Both the atmosphere and ocean models are fully dynamic: the former is defined by 3 ordinary differential equations derived from a truncated Fourier series expansion of the mean and perturbation compo nents of the quasi-geostrophic potential vorticity equation, while the latter is specified by 6 ordinary differential equations representing the time-dependent variations of ocean temperature and salinity in a 3-box model of the North Atlantic. Despite the existence of 2 basic eq uilibrium ocean model responses to perpetual atmospheric conditions, e quilibrium states are never attained in the coupled system within 1000 0 years of integration; the deep ocean now continually adjusts to the atmospheric regime changes associated with particular ocean circulatio ns, which leads to new circulations and new atmospheric regimes. Low-f requency quasi-periodic oscillations about a single state of the therm ohaline circulation result from an advective-diffusive process, modula ted by the correlation of the atmospheric behavior with the phase of t he ocean cycle. The climate is strongly effected by interactions with the ocean, leading to distinct atmospheric patterns for different phas es in the oscillations, and a conversion of some of the high-frequency atmospheric signal to lower frequencies. This conversion also results in a measurable ocean response at high Frequencies. Furthermore, owin g to the richness of the atmospheric response to small modifications i n the meridional and zonal gradients in diabatic heating, even modest adjustments in the ocean circulation resulting from interactions with the high-frequency atmospheric component can also lead to climate chan ge over relatively short time periods. The results of the model are ap plied to recent deductions of climate variability in the North Atlanti c, obtained From Greenland ice-cores.