STRATOSPHERIC SUDDEN WARMING IN 1989 FROM THE VIEWPOINT OF ATMOSPHERIC ANGULAR MOMENTUM

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A major wavenumber-2 stratospheric sudden warming occurred in mid-February of 1989. Using the U. S. National Meteorological Center (NMC) data, an analysis of this event was performed based on the angular momentum equation in the transformed Eulerian-mean (TEM) formalism. Although the warming took place abruptly, the atmospheric angular momentum (AAM) of the Northern Hemisphere (NH) stratosphere above 100hPa started decreasing gently about 4 weeks before the warming. The decreasing occurred in the whole region of the NH stratosphere. During the decreasing period of the northern stratospheric AAM prior to the sudden warming, the global AAM also decreased and hence the length of day (LOD) decreased. The contribution of the global stratospheric AAM to the decreasing of the LOD was about 40-50%. Using the TEM formalism, the AAM budget in the NH stratosphere was investigated. The eddy forcing which can be represented as the Eliassen-Palm (E-P) flux vectors generally acts to decrease the AAM, and the residual mean meridional circulation acts in the opposite way. Analysis of the relationship between the stratospheric sudden warming and the AAM budget shows that the eddy forcing was enhanced gradually during the 4 weeks before the warming and contributed to the decreasing of the AAM.
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