The Modulation of the Low-Latitude Radiation Budget by Cloud and Surface Forcing on Interannual Time Scales

Abstract

The source and forcing mechanisms of radiation budget variability were examined over tropical latitudes by separating the variations into cloud- and surface-forced components. A zonal harmonic analysis of emitted longwave radiation emphasizes that these variations are largely controlled at the planetary wave scale. Positive total and cloud-forced longwave (LW) anomalies embedded within this planetary-scale structure show eastward movement from the Indian Ocean toward the eastern Pacific together with the easterly displacement of negative anomalies from the western Pacific toward Africa during the period prior to and after the active phase of the 1982?83 ENSO. The overall effect leads to an approximately 50° per year propagation phase speed that is considerably slower than the oceanic Kelvin wave capable of driving east-west LW anomalies through sea surface temperature (SST) feedback. The oceanic Kelvin wave speed is about 60° per month over the Pacific basin in the course of an ENSO cycle. This suggests there are longer time scales of climatic signals in the tropical radiation budget. The examination of time-dependent radiative energetics over the tropics reveals that the aforementioned anomaly LW propagation is mainly due to cloud forcing associated with east-west circulation changes, although surface forcing contributes within the Pacific basin. Since cloud amount changes are directly linked to variations in latent heat release, diabatic heating associated with coupled ocean-atmosphere feedback appears to be largely responsible for the LW anomaly propagation. An examination of the complete radiation budget over the maritime continent and equatorial central Pacific during the 1982?83 ENSO event demonstrates that radiative forcing produces positive feedbacks in conjunction with the sea surface temperature anomalies that develop in both regions. Furthermore, surface forcing is found to be an important control on net radiation variability within this teleconnection. An examination of two additional tropical cast-west teleconnections shows that surface forcing is even more important than cloud forcing in controlling variations in the east-west net radiation gradients.