Mountain torques caused by normal-mode global Rossby waves, and the impacton atmospheric angular momentum

Planetary-scale free Rossby waves present in the earth's atmosphere propaga te toward the west. Pressure torques varying in time then arise as a conseq uence of unequal pressure on the eastern and western sides of mountains and small-scale topographic features. These torques, referred to as mountain t orques, have an influence on the exchange of angular momentum between the a tmosphere and the earth. The authors investigated the impact of all identified planetary-scale free Rossby waves on atmospheric angular momentum by computing the contribution from mountain torques to the rate of change of total atmospheric angular mo mentum for each wave. Comparing contributions from individual waves, the authors found that for t he average wave amplitudes the maximum torque for a particular wave is arou nd 2 Hadleys, and that considering all meridional wavenumbers, zonal wavenu mber 2 causes the largest global torques. Changes in angular momentum depen d on both the amplitude of the changing torque and on its period. As a resu lt zonal wavenumbers 1 and 2 cause the largess angular momentum anomalies w ith peak-to-trough amplitudes of 2-5 x 10(23) kg m(2) s(-1). The 16-day wav e produces the largest amplitude, 4.9 x 10(23) kg m(2) s(-1). These values refer to average amplitudes reported in the literature. Individual waves ma y cause anomalies five times as big.