Mountain Torques and the Equatorial Components of Global Angular Momentum

Abstract

The atmosphere induces changes of the earth?s angular momentum via orographic and frictional torques and, in particular, via torques exerted by the equatorial bulge. While the torques leading to fluctuations of the axial component of angular momentum and to corresponding changes of the length of the day have been investigated intensively in the past, this is not so for the torques linked to the equatorial components of global angular momentum. Here, the orographic torques that affect the equatorial components of global angular momentum and contribute to polar motion are discussed. The torques linked to the equatorial bulge are considered as well. European Centre for Medium-Range Weather Forecasts reanalysis data are used to investigate the torques both in an inertial and an earth fixed frame. The basic statistical characteristics of the mountain torques are derived for various regions on the globe. It is found that a large fraction of the torque exerted by the bulge is due to the presence of orography. Moreover, mountain torques are not negligible when compared to those exerted by the bulge. Examples of atmospheric flow patterns linked to large torques at the Tibetan plateau are given.