DYNAMICS OF THERMAL-CONVECTION WITH NEWTONIAN TEMPERATURE-DEPENDENT VISCOSITY AT HIGH RAYLEIGH NUMBER

Two-dimensional, time-dependent convection with a Newtonian temperatur e-dependent viscosity has been investigated in wide aspect-ratio boxes . Large-scale circulations have been found in the regime with volumetr ically averaged Rayleigh numbers, Ra-v ranging between O(10(5)) and O( 10(8)). Viscosity contrasts up to 1000 have been examined. The horizon tally averaged temperature in the interior, [T](1/2), does not vary mu ch within the above range of Ra-v for large-aspect-ratio boxes. The ho rizontal Fourier spectra of the viscosity field show a decrease of mag nitude in the viscosity with shorter wavelength. The viscosity spectra decay differently with decreasing wavelength than the corresponding t hermal spectra. Lithospheric processes, such as lubrication of subduct ion and lithospheric thinning, are facilitated by high Ra-v. The verti cal correlation function for the temperature anomalies used recently f or comparing convection calculations with tomographic models is found to be strongly time-dependent for Ra-v O(10(6)). The temporal fluctuat ions of the vertical correlation function decrease for higher Rayleigh numbers. Correlation functions for low Ra are broad and smooth, while those for high Ra, greater than 10(7), are narrow and fragmented.