Further analysis of the relationship between circulation anomaly patterns and tropical convection

Abstract

Tropical‐extratropical connections are investigated for 250‐mbar stream function fluctuations in both intraseasonal and interannual bands. The principal modes of anomalous 250‐mbar stream function are computed and related to anomalous outgoing longwave radiation (OLR), which is used as a proxy for deep tropical convection. The leading mode for both boreal winter and summer displays a large zonally symmetric component, which appears in both the intraseasonal (IS) and interannual (IA) bands. In the IS band, the dominant mode is an oscillation with a period of 48 days. Correlations between the principal component of this mode and OLR anomalies indicate that this pattern is directly related to convection in the tropical Pacific. The second mode for the boreal winter consists of a wave train from the Pacific Ocean east‐northeastward to western North America and then east‐southeast ward to the subtropical Atlantic Ocean. This mode also exists in both IA and IS bands. Simultaneous and lagged correlations between the amplitude time series of this mode and OLR anomalies indicate that extreme amplitudes of this wave train are preceded by extremes in the OLR anomalies (cloud band activity) in the subtropical North Pacific. Enhanced cloud band activity in the vicinity of Hawaii, associated with an anomalous cyclonic 250‐mbar circulation in the same region precedes the maximum development of a 250‐mbar ridge south of the Aleutian Islands, trough over western North America and another ridge along the east coast of the United States. The second mode for boreal summer (June‐August) is associated with variations of the Asian monsoon.