ATMOSPHERIC EXCITATION OF NONSEASONAL POLAR MOTION

Analysis of nonseasonal polar motion excitation and atmospheric mass equatorial angular momentum (EAM) over land for the period 1980-1989 reveals a clear pattern of high power and correlation during the northern hemisphere (NH) winter followed by low power and correlation during the NH summer. A special case of this pattern occurs for longer than 14 months (from January 1987 to March 1988) when the correlation throughout the NH summer remains statistically significant. During this epoch an average of 72% of the nonseasonal polar motion excitation power at frequencies between -30 and +12 cycles/yr is linearly related to atmospheric EAM over land. During the southern hemisphere winter there is significant correlation between the atmospheric EAM over midlatitude southern oceans and polar motion excitation indicating the existence of a dynamic atmosphere-ocean ocean excitation. The atmospheric excitation power is too small to explain the large correlation during the NH winter. The effects of winds probably account for die deficit in power. The implication of these results is that there are two main excitation sources each dominant at different seasons. Atmospheric mass redistribution over land forces polar motion during the NH winter, and a dynamic atmosphere-ocean response is important during the SH winter.