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  1. Where is ENSO stress balanced? Authors: Matthias Münnich and David Neelin First Published: 20th November, 2003 Published online: 14th April, 2004 Abstract: The zonal surface torque budget associated with the tropical wind stress anomalies during El Niño/Southern Oscillation is analyzed. Mountain and surface stress torques over South America are found to play a prominent role. Local momentum change is negligible for 6 month averages allowing the balance among regional contributions to the torque anomalies to be compared. During El Niño, eastward torque anomalies over the central equatorial Pacific are largely compensated by westward anomalies elsewhere in the equatorial band, notably over South America. Torque anomalies over South America and the Pacific in latitude bands north and south of the equator are both westward and are not compensated within the band, implying an export of eastward momentum to higher latitudes. Copyright © 2003 Royal Meteorological Society. Published by Elsevier Ltd. All rights reserved. Link to full paper:
  2. Annual atmospheric torques: Processes and regional contributions Authors: Olivier de Viron, Jean O. Dickey and Steven L. Marcus Published: 12th April, 2002 Abstract: All three components of annual atmospheric torque are analyzed with a focus on understanding the contributions from various sources and the physical interactions involved. The annual variations of the equatorial component are dominated by the torque on Earth's ellipticity, with the X component mainly due to an anomaly over the Himalayas, and the Y component associated with pressure anomalies over the North Pacific Ocean. The axial annual component is due to the combined effect of friction and mountain torque, whose amplitudes are at the same order of magnitude with the friction term being larger. Partial cancellation of the mountain torque over Asia and North America is effected by the out‐of phase contribution of the Andes (South America having the opposite seasonal cycle to Asia and North America). Link to full paper:
  3. The Dynamics of Intraseasonal Atmospheric Angular Momentum Oscillations Authors: Dr Klaus M. Weickmann, George N. Kiladis and Prashant D. Sardeshmuykh Published: 17th October, 1996 Abstract: The global and zonal atmospheric angular momentum (AAM) budget is computed from seven years of National Centers for Environmental Prediction data and a composite budget of intraseasonal (30–70 day) variations during northern winter is constructed. Regressions on the global AAM tendency are used to produce maps of outgoing longwave radiation, 200-hPa wind, surface stress, and sea level pressure during the composite AAM cycle. The primary synoptic features and surface torques that contribute to the AAM changes are described. In the global budget, the friction and mountain torques contribute about equally to the AAM tendency. The friction torque peaks in phase with subtropical surface easterly wind anomalies in both hemispheres. The mountain torque peaks when anomalies in the midlatitude Northern Hemisphere and subtropical Southern Hemisphere are weak but of the same sign. The picture is different for the zonal mean budget, in which the meridional convergence of the northward relative angular momentum transport and the friction torque are the dominant terms. During the global AAM cycle, zonal AAM anomalies move poleward from the equator to the subtropics primarily in response to momentum transports. These transports are associated with the spatial covariance of the filtered (30–70 day) perturbations with the climatological upper-tropospheric flow. The zonally asymmetric portion of these perturbations develop when convection begins over the Indian Ocean and maximize when convection weakens over the western Pacific Ocean. The 30–70-day zonal mean friction torque results from 1) the surface winds induced by the upper-tropospheric momentum sources and sinks and 2) the direct surface wind response to warm pool convection anomalies. The signal in relative AAM is complemented by one in “Earth” AAM associated with meridional redistributions of atmospheric mass. This meridional redistribution occurs preferentially over the Asian land mass and is linked with the 30–70-day eastward moving convective signal. It is preceded by a surface Kelvin-like wave in the equatorial Pacific atmosphere that propagates eastward from the western Pacific region to the South American topography and then moves poleward as an edge wave along the Andes. This produces a mountain torque on the Andes, which also causes the regional and global AAM to change. Link to full paper:<1445%3ATDOIAA>2.0.CO%3B2
  4. Regional Sources of Mountain Torque Variability and High-Frequency Fluctuations in Atmospheric Angular Momentum Authors: Haig Iskenderian and David A. Salstein First Published: 23rd May, 1997 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. Link to full paper:<1681%3ARSOMTV>2.0.CO%3B2
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