Asymmetrical Interannual Variation in Aerosol Optical Depth over the Tropics in Terms of Aerosol-Cloud Interaction

Abstract

We statistically investigated an interannual co-variation among aerosol optical depth (AOD), cloud effective radius (CER), and precipitation, focusing on aerosol-cloud interaction over the tropics. A three-month composite analysis for AOD, CER, and precipitation for 2000-2012 based on El Niño-Southern Oscillation phases during September-October-November (SON) and December-January-February shows that an increase (decrease) in AOD in the El Niño (La Niña) years was associated with a decrease (increase) in precipitation, particularly in SON over the Maritime Continent. Additionally, CER decreased in the El Niño years over the same region, which implies that CER was associated with interannual variation in aerosol burden; these results were statistically significant. Interannual variation in AOD and CER in SON in the Maritime Continent was asymmetrical, which can be explained by stronger aerosol-cloud interactions under drier conditions. Specifically, large amounts of aerosols suppressed cloud and precipitation formation, which leads to decreases in wet deposition and increases in emission under warmer and drier surface conditions. This feedback results in asymmetrical variation. Furthermore, the asymmetrical interannual variation was confirmed statistically.