Seasonally modulated tropical drought induced by volcanic aerosol

Major volcanic events with a high loading of stratospheric aerosol have long been known to cause cooling, but its impact on precipitation has only recently been emphasized, especially, as an analog for geoengineering of climate. Here we use a coupled atmosphere-ocean-land-vegetation model in conjunction with observations to study the effects of volcanic aerosol on the tropical and subtropical precipitation. The small internal variability in the model enables a clear identification of the volcanic impact, which is broadly supported by observations, especially for the large Pinatubo event. Area averaged rainfall over land between 40šS and 40šN decreases by about 0.15 mm day-1, 4-5 months after the height of a major volcanic aerosol loading such as from Pinatubo, with regional changes as large as 0.6 mm day-1 or higher such as over the Amazon and equatorial Africa. These anomalies migrate seasonally, following the movement of monsoon rainfall. This is because the low heat capacity of the land leads to rapid response of rainfall there, due to the energy imbalance caused by volcanic aerosol cooling. In contrast, precipitation response over ocean is much slower and considerably damped because of the much larger heat capacity. In addition, the difference in heat capacities over land and over ocean leads to an anomalous land-sea thermal contrast which could further contribute to the reduction of rainfall over land. The volcano-induced drought may have significant impact for the ecosystem, agriculture and the carbon cycle, especially in the monsoon regions.