Intraseasonal (30–60 Day) Fluctuations of Outgoing Longwave Radiation and 250 mb Streamfunction during Northern Winter

Abstract

Ten years of outgoing longwave radiation (OLR) and 250 mb circulation data are used in a statistical study which concentrates on 28?72 day fluctuations during Northern Hemisphere winter. The results of spectral and cross-spectral analyses show that 28?72 day planetary-scale oscillations of OLR and 250 mb circulation are statistically significant features of the entire 10-year dataset. The strongest OLR fluctuations at 28?72 day periods are located from the equator to 15°S and extend from about 60 to 160°E and in the vicinity of the South Pacific Convergence Zone (SPCZ). The streamfunction variance shows significant 28?72 day fluctuations over the subtropics of both hemispheres and over the extratropical North Atlantic. The OLR anomalies propagate from west to east between 60 and 160°E at about 5 m s?1. There are statistically significant relationships between the regions of (inferred) equatorial cloudiness and planetary-scale circulation features. Fluctuations in the windfield near the exit regions of the east Asian and North American jets are important components of the life cycle of 28?72 day oscillations during northern winter. The life cycle also includes a prominent wavenumber 1 evolution which manifests itself synoptically as an eccentric circumpolar vortex, expanded in regions of enhanced equatorial cloudiness and contracted in regions of suppressed equatorial cloudiness.