Abstract
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 GM precipitation in one hemisphere is enhanced significantly by the remote volcanic forcing occurring in the other hemisphere. This remote volcanic forcing-induced intensification is mainly through circulation change rather than moisture content change. In addition, the NH volcanic eruptions are more efficient in reducing the NH monsoon precipitation than the equatorial ones, and so do the SH eruptions in weakening the SH monsoon, because the equatorial eruptions, despite reducing moisture content, have weaker effects in weakening the off-equatorial monsoon circulation than the subtropical-extratropical volcanoes do.
Highlights
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 (Fig. 8a–c)
Based on the long-term reconstruction of PDSI and precipitation, the Asian monsoon in the NH is found to be enhanced by the SH eruptions, while it is weakened by the NH and the equatorial eruptions (Fig. 3)
Our new finding shows that the GM changes significantly due to the volcanic forcing, and the three types of volcanoes, namely, the NH, the SH and the equatorial volcanoes, have different effects on the NH monsoon and the SH monsoon. These results are based on the superposed epoch analysis that has been widely used to study the climate responses to large volcanic eruptions[34,38,39]
Summary
We classify the volcanoes into three types based on the dataset in Gao et al.[19], because aerosols from different volcanoes have distinctive meridional distributions. Since the volcanoes erupt in different months and have maximum aerosol column density in the fifth month after the eruption (Fig. 2c), we assume that the volcanos erupting in the monsoon season have negligible effect on the current monsoon system itself. Since the millennium simulation is forced by the volcanic forcing based on the reconstruction of Gao et al.[19], this volcano dataset is used to study the observed monsoon responses; in this way, the volcano stratification can be consistent in both simulation and observation analyses. These uncertainties, do not change the conclusion obtained from the sensitivity experiment
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