Abstract
This study analyzes the observed monthly deseasonalized and detrended variability of the tropical radiation budget and suggests that variations of the lower‐tropospheric stability and of the spatial organization of deep convection both strongly contribute to this variability. Satellite observations show that on average over the tropical belt, when deep convection is more aggregated, the free troposphere is drier, the deep convective cloud coverage is less extensive, and the emission of heat to space is increased; an enhanced aggregation of deep convection is thus associated with a radiative cooling of the tropics. An increase of the tropical‐mean lower‐tropospheric stability is also coincident with a radiative cooling of the tropics, primarily because it is associated with more marine low clouds and an enhanced reflection of solar radiation, although the free‐tropospheric drying also contributes to the cooling. The contributions of convective aggregation and lower‐tropospheric stability to the modulation of the radiation budget are complementary, largely independent of each other, and equally strong. Together, they account for more than sixty percent of the variance of the tropical radiation budget. Satellite observations are thus consistent with the suggestion from modeling studies that the spatial organization of deep convection substantially influences the radiative balance of the Earth. This emphasizes the importance of understanding the factors that control convective organization and lower‐tropospheric stability variations, and the need to monitor their changes as the climate warms.
Highlights
How well do we understand the factors that modulate the tropical radiation budget? This understanding has long been recognized as a path toward interpreting the long‐term stability of tropical temperatures over the past million years (Herbert et al, 2010) and estimating the sensitivity of the climate system to current and future increases of greenhouse gases in the atmosphere (Pierrehumbert, 1995)
By analyzing long time series of satellite observations, we show that monthly variations of lower‐atmospheric stability and convective clustering are both strongly correlated with variations of the radiative cooling of the tropics and that their contributions to the modulation of the radiation budget are complementary and important
It remains an open issue as to whether the radiative influence of changes in convective aggregation is significant compared to that of other well‐established controlling factors such as the lower tropospheric stability. We address these questions by analyzing observed variations of convective organization, tropospheric stability, and top‐of‐atmosphere (TOA) radiation budget over the tropical belt (30°S to 30°N) at the monthly time scale
Summary
In this Supporting Information, we provide a table comparing the relationships obtained between the convective organization index and the radiation budget components for two different methods of computation of the organization index (Table S1), a table comparing the linear correlation coefficients of Iorg and EIS with low-level, mid-level and high-level cloud amount derived from satellite observations (Table S2), a table comparing the linear correlation coefficients between Iorg or EIS and the different components of the radiation budget computed by considering all months, by considering just ENSO events, or by excluding ENSO events (Table S3). Weather Review, 142 1697-1715, 2014) (b) the daily Iorg index (deseasonalized and detrended anomalies) of deep convective organization from GridSat data, (c) the monthly Southern Oscillation Index (SOI) from NOAA, (d) the daily lower-tropospheric stability EIS (deseasonalized and detrended anomalies) from ERA interim reanalyses and (e) the daily tropically-averaged net radiation budget from CERES data (deseasonalized and detrended).
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