30–60 Day Atmospheric Oscillations: Composite Life Cycles of Convection and Circulation Anomalies

Abstract

Life cycles of the 30–60 day atmospheric oscillation were examined by compositing 30–60 day filtered NMC global wind analyses (250 mb and 850 mb) and outgoing longwave radiation (OLR) for the years 1979–84. Separate composite life cycles were constructed for the May–October and November–April seasons using empirical orthogonal function analysis of the large-scale divergent wind field (250 mb velocity potential) to define the oscillation's phase. Monte Carlo simulations were used to assess the statistical significance of the composite OLR and vector wind fields. Large-scale (wavenumber one) tropical divergent wind features propagate eastward around the globe throughout the seasonal cycle. The spatial relationships between these propagating circulation features and OLR are shown using sequences of composite maps. Good agreement exists between areas of upper-air divergence and areas of convection inferred from the OLR satellite data. Convection anomalies are smaller over tropical Africa and South America than over the Indian and western Pacific oceans. Anomalies of OLR are nearly negligible over cooler tropical sea surfaces. Fluctuations in summer monsoon region convection are influenced by the global-scale eastward-moving wave. The oscillation's vertical structure varies with latitude. In the tropics, upper-level and lower-level tropospheric wind anomalies are about 180° out of phase. Poleward of about 20°, there is no pronounced phase shift between levels. In tropical and subtropical latitudes, analysis of the nondivergent circulation composites at 250 mb (ψ₂₅₀) reveals cyclones to the east of the convection and anticyclones alongside or west of the convection. While convection anomalies are most pronounced in the summer hemisphere tropics, the tropical and subtropical ψ₂₅₀ features are most prominent in the winter hemisphere. There is some evidence of symmetry of cyclonic and anticyclonic circulations about the equator. A subset of the composite extratropical vector wind fields were statistically significant (95% level) at 850 and 250 mb in the winter hemisphere (25°–85° latitude), based upon a Monte Carlo simulation. During the November-April season, the East Asian jet is retracted toward Asia when positive 30–60 day convection anomalies are occurring over the equatorial Indian Ocean. The eastward shift of convection into the western and central Pacific is accompanied by a series of circulation features over northern Asia and an eastward extension of the East Asian jet. During the May-October season, the shift of large-scale tropical convection anomalies from the Indian Ocean and Indian monsoon regions to the tropical western Pacific is followed (10–15 days later) by the occurrence of strengthened westerlies over southern Australia. In contrast, the extratropical “response” in the summer hemisphere for both the May–October and November–April seasons was not statistically significant.