Regional Sources of Mountain Torque Variability and High-Frequency Fluctuations in Atmospheric Angular Momentum

Abstract

The sources of high-frequency (?14 day) fluctuations in global atmospheric angular momentum (AAM) are investigated using several years of surface torque and AAM data. The midlatitude mountain torque associated with the Rockies, Himalayas, and Andes is found to be responsible for much of the high-frequency fluctuations in AAM, whereas the mountain torque in the Tropics and polar regions as well as the friction torque play a much lesser role on these timescales. A maximum in the high-frequency mountain torque variance occurs during the cool season of each hemisphere, though the Northern Hemisphere maximum substantially exceeds that of the Southern. This relationship indicates the seasonal role played by each hemisphere in the high-frequency fluctuations of global AAM. A case study reveals that surface pressure changes near the Rockies and Himalayas produced by mobile synoptic-scale systems as they traversed these mountains contributed to a large fluctuation in mountain torque and a notable high-frequency change in global AAM in mid-March 1996. This event was also marked by a rapid fluctuation in length of day (LOD), independently verifying the direct transfer of angular momentum from the atmosphere to solid earth below. A composite study of the surface pressure patterns present during episodes of high-frequency fluctuations in AAM reveals considerable meridional elongation of the surface pressure systems along the mountain ranges, thus establishing an extensive cross-mountain surface pressure gradient and producing a large torque. The considerable along-mountain extent of these surface pressure systems may help to explain the ability of individual synoptic-scale systems to affect the global AAM. Furthermore, midlatitude synoptic-scale systems tend to be most frequent in the cool season of each hemisphere, consistent with the contemporary maximum in hemispheric high-frequency mountain torque variance.