Low Pacific Secular Variation

Abstract

The historical record shows that secular variation at Hawaii is limited to a few degrees in the last 400 years, whereas in the Atlantic hemisphere it often exceeds 30°. Paleomagnetic measurements from Hawaii show virtually no change in declination during the last 5 kyr and only a slow, millenium-scale inclination change of less than 20°. The usual directional scatter analysis of paleomagnetic data cannot discriminate between the two time scales. The disparity of time scales and difference in activity suggest different physical mechanisms for secular variation in the two hemispheres. This could arise from thermal core-mantle interaction. Seismic models of the lower mantle give a pattern of lateral variations that is nearly symmetric about the Pacific rim: two slow regions centered beneath the Pacific and Atlantic separated by a fast ring below the Pacific rim. These seismic anomalies are thought to be caused mostly by temperature variations in the bottom 200 km of the mantle. It is difficult to see how such a symmetric pattern could lead to long-term hemispheric differences. We show here a convection solution in a rapidly rotating sphere with heat flux on the outer boundary determined from a seismic model. The convection is suppressed beneath the Pacific but the usual drifting convection (Busse) rolls remain beneath the Atlantic. This mode of periodic convection arises because the Pacific hot region extends east-west and is much larger than a single convection roll, whereas the Atlantic hot region is elongated north-south and is about the same east-west size as a convection roll. This could explain the absence of normal, century-long secular variation in the Pacific: hot mantle suppresses short wavelength phenomena at the century time scale but not the longer wavelengths at the millenium time scale.