Mountain Torques Caused by Normal-Mode Global Rossby Waves, and the Impact on Atmospheric Angular Momentum

Harald Lejenäs Department of Meteorology, Stockholm University, Stockholm, Sweden

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Roland A. Madden National Center for Atmospheric Research*, Boulder, Colorado

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Abstract

Planetary-scale free Rossby waves present in the earth’s atmosphere propagate toward the west. Pressure torques varying in time then arise as a consequence of unequal pressure on the eastern and western sides of mountains and small-scale topographic features. These torques, referred to as mountain torques, have an influence on the exchange of angular momentum between the atmosphere 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 momentum for each wave.

Comparing contributions from individual waves, the authors found that for the average wave amplitudes the maximum torque for a particular wave is around 2 Hadleys, and that considering all meridional wavenumbers, zonal wavenumber 2 causes the largest global torques. Changes in angular momentum depend on both the amplitude of the changing torque and on its period. As a result zonal wavenumbers 1 and 2 cause the largest angular momentum anomalies with peak-to-trough amplitudes of 2–5 × 1023 kg m2 s−1. The 16-day wave produces the largest amplitude, 4.9 × 1023 kg m2 s−1. These values refer to average amplitudes reported in the literature. Individual waves may cause anomalies five times as big.

Corresponding author address: Dr. Harald Lejenäs, Department of Meteorology, Stockholm University, 106 91 Stockholm, Sweden.

Email: harald@misu.su.se

Abstract

Planetary-scale free Rossby waves present in the earth’s atmosphere propagate toward the west. Pressure torques varying in time then arise as a consequence of unequal pressure on the eastern and western sides of mountains and small-scale topographic features. These torques, referred to as mountain torques, have an influence on the exchange of angular momentum between the atmosphere 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 momentum for each wave.

Comparing contributions from individual waves, the authors found that for the average wave amplitudes the maximum torque for a particular wave is around 2 Hadleys, and that considering all meridional wavenumbers, zonal wavenumber 2 causes the largest global torques. Changes in angular momentum depend on both the amplitude of the changing torque and on its period. As a result zonal wavenumbers 1 and 2 cause the largest angular momentum anomalies with peak-to-trough amplitudes of 2–5 × 1023 kg m2 s−1. The 16-day wave produces the largest amplitude, 4.9 × 1023 kg m2 s−1. These values refer to average amplitudes reported in the literature. Individual waves may cause anomalies five times as big.

Corresponding author address: Dr. Harald Lejenäs, Department of Meteorology, Stockholm University, 106 91 Stockholm, Sweden.

Email: harald@misu.su.se

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