Intraseasonal Oscillations in the Earth-Atmosphere System

摘要

Intraseasonal fluctuations in the earth's angular momentum budget are studiedusing 12-year overlapping records of length-of-day (LOD) variations from the Jet Propulsion Laboratory and atmospheric angular momentum (AAM) data from the National Meteorological Center (NMC), as well as the result of three-year (with mountains) and on-year (without mountains) perpetual January simulations of the global atmosphere, performed with the UCLA General Circulation Model (GCM). Two distinct oscillations, with periods of 50 and 40 days, are found in the observed series of LOD and global AAM. Regional analysis of the NMC data shows that the stronger 50-day oscillations arises from zonal wind variations in the tropics, presumably associated with the Madden-Julian (MJ) oscillation, while the 40-day oscillation originates in the Northern Hemisphere (NH) extratropics. Evidence of tropical-extratropical interaction in the 36-60 day band, especially prominent in a case study of the strong global oscillations occurring in early 1988, is derived from auto-spectral and cross-spectral analysis of the AAM data by latitude. A 42-day AAM oscillation arises in the NH extratropics of the three-year GCM run, characterized by a 300-day amplitude fluctuation cycle which strongly resembles that found in the the same region from the NMC data. A barotropic, zonally-symmetric oscillation occurs in the model tropics, driven by mass exchange with the NH extratropics; localized tropical-extratropical teleconnections are also found. A standing, wavenumber-two oscillation occurring at 500 mb in the NH extratropics was found to undergo tilted-trough vacillation in conjunction with the model's AAM cycle. During the extreme phases of the AAM oscillation, the height anomalies tilt southwest-northeast, generating little net mountain torque, while during the phases of rapid AAM change, they tilt northwest-southeast, producing strong mountain torques over the Rockies and Himalayas.