The Reduction in Precipitation due to the Effects of Volcanic Aerosols

Major volcanic eruptions inject stratospheric aerosols into the atmosphere, which causes a cooling effect. This phenomenon has been greatly studied, however its impact on precipitation lacks substantial analysis. The authors employ a coupled atmosphere-ocean-land-vegetation model paired with observations to study the effects of volcanic aerosol on precipitation in tropical and subtropical regions. They focused particularly on three large volcanic eruptions that occurred in the late twentieth century: Pinatubo, El Chichon, and Agung. The authors conclude that average precipitation over land decreases faster than the average precipitation over ocean. They state that volcano-induced droughts could greatly impact the ecosystem, agriculture, and the carbon cycle, especially in the monsoon regions. –Michela Isono

Joseph, R., Zeng, N., 2011. Seasonally Modulated Tropical Drought Induced by Volcanic Aerosol. American Meteorological Society, 2045–2060.

            Volcanic eruptions inject sulfer dioxie gas into the stratosphere, which then combines with water and oxygen to form aerosol particles that block incoming solar radiation to the earth. This process has a cooling effect. For this reason, aerosols have been proposed as a geoengineering technique to mitigate global warming. However, aerosols can also affect the hydrologic cycle, particularly precipitation in the tropics. Precipitation in the tropics has strong seasonal movement, where the response to volcanic forcing should differ between land and ocean.
The paper discusses the effect of volcanic aerosols on hydrologic processes. The tropical and subtropical land regions are the locations of focus because agriculture depends on the seasonal migration of monsoon rainfall.
            Methods:A coupled atmosphere-ocean-land-vegetation model is used to simulate a realistic seasonal climate compared to observations in the tropics and midlatitudes. The data used as reference for sea surface temperature (SST) is the Hadley Center’s sea ice and SST analysis. The analyses spans years 1870–2005, but the study focuses on years 1960–2000. The volcanic aerosols are injected uniformly across all latitudes. The three volcanic events used all occurred during El Niños. All model results are averages.
            Results: The time series of precipitation and temperature in the model and observations in relation to the volcanic events averaged between 40 degrees N and 40 degrees S, showed that the Pinatubo event was the largest and the model responded with a greater reduction in SAT and precipitation. The magnitude of reduction was approximately 0.15 mm day–1 in the model is similar to the GPCP observational data. The SAT and SST observational data showed a significant trend of 0.5 K per year, but a decrease in temperature of 0.35 K was seen for all three volcanic events in the SAT for both model and observations.
            The composites of precipitation anomalies of the three volcanic events for the model and CRU observations for one year during the peak response period to volcanic aerosol forcing showed a decrease in precipitation over equatorial Africa, the northern part of South America, northern Australia, and the Indian subcontinent. Differences between observations and model in the tropics in the African and the South American continent were also shown. Overall, the precipitation in the observations is less standardized compared to the model; however, when the average of the entire region was calculated they had a comparable decrease in precipitation of about 0.15 mm day–1.
            A seasonal shift in precipitation was observed when the spatial plots of the austral and boreal summer plots were analyzed. In the austral summer (January-March), there was a decrease in precipitation over South Africa, northern Australia, and South America in both observations and model. In the Africa model, the decrease in precipitation spread further south. In the boreal summer (July-September), there was a decrease in precipitation over the Indian and Asian subcontinent in both observations and model.
            The 1-year composite of SAT over land and SST for model and observations with CRU SAT over land and HADISSTs over the ocean was examined. There was a decrease in temperature over land and ocean in the tropics in both observations and model. Significant cooling was shown in the subtropics over the dry arid desert over land in both observation and model. A distinct land-sea thermal contrast was observed in both observations and model outside of regions with decreased precipitation: North Africa-Mediterranean and East Asia.
            The Hovmoller plots of optical depth and the model precipitation and temperature anomalies over land and ocean for the Pinatubo event showed more cooling over land than in the nearby oceans in the summer hemisphere. Once the zonal mean optical depth reached 0.02 in the tropics, the seasonal migration of the precipitation anomalies was observed. There was about a 10% decrease in precipitation over land that lasted for approximately three years. Slightly larger cooling occurred over land than oceans by about 0.1 K. Cooling for SST took longer, lasted longer, and was weaker than the land SAT at the same latitude. After initial inconsistent responses, the precipitation anomalies followed the same seasonal cycle of climatological precipitation. The optical depth of the EL Chichon and Agung volcanoes peaked in July but the Pinatubo volcano peaked in November. The most significant decrease in precipitation was in the Southern Hemisphere in the austral summer.
            The total moisture convergence overlaid with vertically integrated moisture transport for the austral and boreal summer time periods showed a distinct reversal in the expected wind directions during monsoon seasons. For the Australian and the South African monsoon, the winds were reversed in the austral summer. For the East Asian, North American, and East African monsoon, the winds were reversed in the boreal summer. In both observations and model, there was a seasonal decrease in anomalous precipitation over land in regions where the total precipitation was at maximum.
            The time evolution of the volcanic events was also compared. The oceanic SST response to Pinatubo was slow and reached a peak value at about –0.3 K after one year of the time marker (October 1991). Evaporation decreased peaks at one month after SST and precipitation followed after another month. In contrast, land had a different result in response to Pinatubo: the land precipitation response was fast and reached a peak value of –0.15 mm day–1 after about 5 months of the time marker, whereas evaporation lagged and surface temperature followed precipitation by four months. Overall, over the ocean SST response to volcanic which was followed by evaporation and then precipitation, whereas land precipitation responded fast to the forcing which was followed by evaporation, as surface temperature responded the slowest.
            The response of the land-atmosphere carbon flux, net primary production, heterotrophic respiration for the one-year period for the three volcanic effects was a reduction in the net flux, indicating that the cooling effect on respiration was stronger than the precipitation effect on productivity. However, in northern Australia and western South America, the net primary production response was different from most of the other regions. This showed that the competing effects of cooling and drying on the net primary production and heterotrophic respiration (the release of CO2 during the decomposition process of organic matter in soil) are not always dictated by heterotrophic respiration.  
Conclusion: The model used alongside observational data was employed to better understand the mechanisms that dictate precipitation decrease in response to volcanic aerosols. The study showed that precipitation responses over land to volcanic aerosol followed seasonal patterns of monsoons. The reduction in precipitation and cooling of the planet has also been shown to have significant implications for the ecosystem and carbon cycle over the tropics. In conclusion, the results demonstrates the affects that stratospheric aerosol geoengineering could have on the hydrological cycle. 

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