Decadal changes in cloudiness over the Amazon forests: Observations and potential causes

Changes in rainforest growth and mortality rates, especially in the deep and least perturbed forest areas, have been consistently observed across global tropics in recent decades. Previous studies have suggested that such a change may be caused by an elevated atmospheric CO2 and increasing surface temperature. However, the rainforests in wet tropical regions are mostly light limited and changes in atmospheric circulation and radiation balance over tropical oceans have been reported, although it is not clear how cloudiness has changed over tropical land. This study shows a decadal-scale decrease of seasonal mean convection, cloudiness and shortwave (SW) downwelling radiation during 1984 to 2007 over the Amazon basin. These changes are consistent with observed increase in surface temperature and reduction of moisture transport to the Amazon basin. Further analysis suggests that this observed change in cloudiness is linked to the expansion of the western Pacific warm pool during December-February season, to the Atlantic Multidecadal Oscillation during March-May season, and to the tropical Atlantic SST gradient during September-November season. Mechanisms that carry these oceanic influences on cloudiness over the Amazon will be discussed.