Global Monsoon Precipitation Responses To Large Volcanic Eruptions

Climate variation of global monsoon (GM) precipitation involves both internal feedback and external forcing. Here, we focus on strong volcanic forcing since large eruptions are known to be a dominant mechanism in natural climate change. It is not known whether large volcanoes erupted at different latitudes have distinctive effects on the monsoon in the Northern Hemisphere (NH) and the Southern Hemisphere(SH). We address this issue using a 1500-year volcanic sensitivity simulation by the Community Earth System Model version 1.0 (CESM1). Volcanoes are classified into three types based on their meridional aerosol distributions: NH volcanoes, SH volcanoes and equatorial volcanoes.Using the model simulation, we discover that the explosive volcanoes tend to reduce GM precipitation in a few years after the eruptions,the NH monsoon and the SH monsoon have different responses to extratropical explosive volcanoes because of the asymmetric aerosol distributions. The NH eruptions are found to be more efficient in reducing the NH monsoon precipitation than the equatorial eruptions,and the SH eruptions are also more efficient in reducing the SH monsoon precipitation than the equatorial eruptions,because the extratropical eruptions tend to reduce moisture heavily and cause strong divergence anomalies, while the equatorial volcanoes only act to reduce the moisture and do not affect the circulation field much. The extratropical volcanism also reveals a new process to affect the NH monsoon or the SH monsoon precipitation. Different from the GM precipitation intensification induced by increasing moisture associated with the global warming caused by strong greenhouse-gas concentration,the SH eruptions produce asymmetry temperature anomalies characterized by strong SH cooling and weak NH temperature change; thus, the enhanced hemispheric thermal contrast generates meridional pressure gradient that drives low-level cross-equatorial flows from the SH, which converges into the monsoon trough regions in the NH. These convergence anomalies induced by the SH eruptions tend to enhance the NH monsoon precipitation dramatically. The NH volcanoes also enhance the SH monsoon precipitation in a similar way.Finally, we briefly discussed the changes in global monsoon precipitation in different external forces. The results show that the global monsoon region in each single forcing do not have much differences. The GMP change for per degree Celsius is largest under the volcanic forcing but smallest under GHG forcing. This is due to volcanic forcing that tends to excite both circulation and moisture strength.And under other three simulations, the moisture tends to be strengthen but the circulation is weakened.