Global View of the intraseasonal Oscillation during Northern Winter

Abstract

In this study, SVD analysis is applied to a normalized 200-mb streamfunction/OLR covariance matrix to extract the most recurrent coupled pattern in the northern winter, after the removal of the climatological seasonal cycle and seasonal means for each winter. The first two singular vectors are on an intraseasonal timescale and the combination of the two forms an oscillation. It is characterized by eastward propagation in the Indian Ocean and the western Pacific, standing oscillations in both the Tropics and the extratropics, and the poleward propagation of a zonally symmetric structure. Although there exists an eastwardly propagating pattern, the phase relationship between geopotential, wind, and streamfunction fields is inconsistent with that of equatorial Kelvin waves. Eastward propagation is most evident in the Tropics of the Southern Hemisphere, while the signals in the Northern Hemisphere are characterized by standing oscillations. The distinct characteristics of the Northern and Southern Hemisphere can be attributed to the different properties of the mean flows, which load to distinct Rossby wave source distributions in the two hemispheres. Abrupt developments of regional circulation anomalies are found in the exit regions of the Pacific and Atlantic jet streams. The one in the Pacific resembles the Pacific/North American pattern and develops in a process similar to the optimal excitation of the normal mode, by extracting barotropic energy from the mean flow. Similar energy conversion also occurs in the Atlantic. Both analyses of energy conversion and Rossby wave source indicate the occurrence of rigorous extratropical activity in the Northern Hemisphere, that is affected indirectly by tropical beating. The propagating circulations in the Southern Hemisphere, that resemble equatorial Rossby waves, could be the direct response to the tropical heating, while the signals of standing oscillation are the mixed results of direct response to the tropical heating and internal dynamics in the extratropics. The results show the complexity of the intraseasonal oscillation, involving equatorial wave dynamics, tropical?extratropical interactions, and eddy?mean flow interactions. The phenomenon is global and it is inadequate to treat the problem as either a purely tropical phenomenon or a purely extratropical phenomenon.