Interannual Variability of the Tropical Radiation Balance and the Role of Extended Cloud Systems

Abstract

The tropical radiation balance is investigated on an interannual time scale using a five-year(1979?83) dataset obtained from the Nimbus-7 Earth Radiation Budget (ERB) experiment. The study emphasizes the separate contributions to interannual fluctuations in the global radiation balance by the tropics and extratropics. An attempt is made to Identify source regions within the tropics that give rise to the fluctuations and to quantify the effect of the fluctuations on zonal heat transport. Superimposed on the five-year global trend pattern of net radiation are large amplitude nonseasonal variations largely confined to tropical latitudes. The significant regions are the Southwest?East Asian (SW?EA) monsoon and two regions associated with the ascent and descent branches of the Pacific Walker Cell. A ?cloud reciprocity index? is formulated in order to examine the degree to which extended cloud systems over the oceanic tropics can induce these interannual fluctuations in the radiation balance. The SW?EA monsoon and the eastern Pacific exhibit low-index patterns, suggesting that these are the two dominant sources of the anomalies. The impact of the fluctuations is examined in terms of external entropy exchange (EEE). Paltridge's theory that climate fluctuations are controlled by a minimum EEE constraint is partially supported. The impact of tropical fluctuations on zonal heat transport is examined. The amplitudes in the year-to-year tropical transport residuals are found to be as high as 50% of, and generally out of phase with, the total global residual. The SW?EA monsoon and the eastern Pacific can explain a large portion of the total tropical residual during specific years. Simultaneous and lagged spatial correlation analyses are used to determine the degree to which the radiative anomalies associated with the SW?EA monsoon region are coupled to other centers of variability. The simultaneous correlations with net radiation are dissimilar to those found with the albedo and outgoing longwave radiation, particularly in terms of seasonal forcing. The organization of lagged albedo anomaly correlation patterns suggest that predictive indicators of the SW?EA monsoon behavior may be found in the tropical ocean basins.