Atmospheric effects of volcanic eruptions Introduction Satellite tracking of stratospheric clouds arising from selected volcanic eruptions since 1980 has led to the recognition of a connection between volcanic eruptions and regional extreme weather events. Although every eruption differs in terms of climatic impact, the difference seen may be explained mainly by their geological characteristics, timing, and geographic location. Atmospheric effects of volcanic eruptions Atmospheric changes may take place in a number of ways including: (a) Air circulation changes including a possible role in atmospheric rivers. (b) Input of particulates. (c) Input of gases including sulfur dioxide and water vapor. (d) Ozone destruction. A simplified model of the atmospheric effects of a thermal plume arising from a volcanic eruption is shown in Fig. 1. Table 2 shows a list of events following the 1982 El Chichõn eruption including the position of the stratospheric cloud and meteorological observations made by the Royal Observatory Hong Kong. Date Event April 4, 1982 Most violent of four eruptions since March 28, 1982 April 16, 1982 Lowest daily mean relative humidity of 57% at ground level was recorded for the month April 18, 1982 Stratospheric volcanic cloud tracked by satellites to have reached above Hong Kong from the east (Robock & Matson 1983) April 19, 1982 Easterly winds freshened and rain fell for the first time after 14 days April 20-22, 1982 Mean pressure at MSL dropped from 14.1 to 8.9 mb April 21-22, 1982 Global solar radiation dropped from 20.52 MJ/m2 to 5.21 MJ/m2 April 22-30, 1982 Active trough of low pressure brought disturbed weather including floods and landslides April 22, 1982 Thunderstorms and violent showers 48.5 mm/hour during evening April 25, 1982 Stratospheric volcanic cloud tracked by satellites to have circled the globe (Robock & Matson 1983) April 27, 1982 A maximum instantaneous rainfall intensity of 186 mm/hour was recorded April 1982 Mean surface relative humidity of 80% is 5th lowest on record for April and is supportive of the stratospheric influence; monthly rainfall of 310 mm was 139.4% above normal and the 9th wettest April on record May 1982 Monthly rainfall of 767.4 mm was 257.4% above normal and the 5th wettest May on record; 730 reports of floods and 213 reports of landslides Fig. 4 Rainfall during May 11-20 and May 21-30, 2008 and as a % of normal in South Africa. The easterly spread of the rainfall is consistent with the spread of the eruption cloud from the Chaitén volcano which erupted in Chile on May 2, 2008. The abnormally heavy rainfall during June 2008 in Australia (Fig. 5) may also be attributed to the continuous easterly spread of the eruption cloud. This is supported by the identification of stratospheric aerosols by Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) over southeastern Australia (Carn et al. 2009). Table 2 Selected events following the 1982 El Chichõn eruption including the tracking of volcanic cloud by satellites and meteorological observations by Royal Observatory Hong Kong. Pinatubo, Phillippines VEI = 6 The 1991 Mount Pinatubo eruption was the largest this century with a maximum cloud height of 55 km above sea level. Before the main eruption on June 15, the first sign of the eruption was in March 1991. The main eruption occurred when a rapidly evolving typhoon named Yunya was passing Luzon at the time (Fig. 3). Because of the transfer of large amounts of water vapour from the troposphere and the volcano into the stratosphere, 1991 was a year of below normal global rainfall probably also assisted by the Cerro Hudson eruption (VEI = 5) in Chile the same year. Hong Kong’s annual rainfall of 1639.1 mm in 1991 is the eleventh driest on record. Fig. 1 Simplified model of the atmospheric effects of a thermal plume arising from a volcanic eruption. Selected regional extreme weather events identified by satellite tracking to be related to volcanic eruptions since 1980 are shown in Table 1. Volcanic eruption Date of main eruption Extreme weather event El Chichõn, Mexico 4/4/1982 Heavy rainfall in southern China during late April/May including the second wettest year in Hong Kong since record began in 1884 Pinatubo, Philippines 15/6/1991 Post-eruption drought conditions in southern China; heavy snowfall in eastern Mediterranean* Chaitén, Chile 2/5/2008 Wet May in South Africa; wet June over much of the Australian continent; wettest June in Hong Kong since record began in 1884 including an 1-in-1,100 year rainstorm on 7/6/2008 causing >2400 landslides on Lantau Island Soufrière Hills, Montserrat 11/2/2010 East Atlantic frontal activity storms affecting Madeira (20/2/2010) and western Europe (Xynthia 26 to 28/2/2010) Eyjafjallajökull, Iceland 14/4/2010 Frontal activity storms affecting central Europe including the wettest year in Slovakia since record began in 1881 Table 1 Selected regional extreme weather events identified by satellite tracking to be related to volcanic eruptions since 1980. El Chichõn, Mexico VEI = 5 Fig. 2 shows the distribution of the stratospheric volcanic cloud after the April 4, 1982 eruption. The stratospheric cloud was over Hong Kong on April 18, 1982 and took three weeks to circle the globe (Robock & Matson 1983). Hong Kong’s annual rainfall of 3247.5 mm in 1982 is the second wettest on record. Fig. 3 Satellite image of the Pinatubo eruption ash cloud and Typhoon Yunya cloud about 3 hours before the main eruption. Yunya dissipated into a tropical depression within 2 days transferring large amounts of water vapor into the stratosphere. Fig. 5 Average rainfall June 1961-1990 and June 2008 rainfall in Australia. The wet June during 2008 was caused by the May 2, 2008 eruption of the Chaitén volcano in Chile. This is supported by CALIOP which detected stratospheric aerosol over southeastern Australia (Carn et al. 2009). Chaitén, Chile VEI = 4 The eruption of the Chaitén volcano in Chile on May 2, 2008 has led to the recognition of a connection between the spread of the volcanic cloud across the globe and the timing of regional extreme rainfall. Fig. 4 shows the rainfall in South Africa during the period May 11-20 and May 21-30, 2008. The rainfall can be seen to have spread from the Atlantic Ocean eastward across the country. A further connection is the record monthly rainfall in Hong Kong during June 2008. The total rainfall of 1346.1 mm or 346.8% above average was the wettest since record began. This included an 1-in-1100 year rainstorm (Fig. 6) causing severe flooding and over 2400 landslides on Lantau Island (Fig. 7). Fig. 2 Global maps made by combining information from the geostationary GOES East and GEOS West satellites and the polar-orbiting NOAA 7 satellite showing the position of the El Chichõn volcanic cloud during the three weeks after the April 4, 1982 eruption.

Eyjafallajökull, Iceland VEI = 4 Soufriére Hills, Montserrat VEI = 3 Fig. 9 Aftermath of the February 20, 2010 flood in downtown Funchal, Madeira. Because Madeira has an elevation of 1862 m above sea level the orographic influence contributed to the torrential rainfall during the passage of the frontal activity storm. Fig. 6 4-hour rainfall distribution in mm over Hong Kong during the June 7, 2008 rainstorm. The return period of the rainstorm is 1-in-1100 years based on the Civil Engineering Development Department, Hong Kong SAR Government. Fig. 12 Annual precipitation in Slovakia during 2010, the annual precipitation as a % of 1961-90 normal and the May 2010 and August 2010 precipitation as a % of 1961-90 normal from Pecho et al. (2010). Fig. 10 Coastal flooding in La Faute-sur-Mer, southwestern France on March 2, 2010 caused by the passage of the frontal activity storm Xynthia. Fig. 7 Some of the 2400+ landslides on Lantau Island in Hong Kong triggered by the June 7, 2008 rainstorm. Eyjafallajökull, Iceland VEI = 4 The eruption of the Eyjafjallajökull volcano in Iceland (Fig. 11) on April 14, 2010 has resulted in torrential rainfall in central Europe. In Slovakia the 2010 rainfall was the wettest since record began (Figs. 12-13). This is attributed to the atmospheric effects of the eruption cloud intensifying a succession of frontal activity storms which penetrated central Europe. Soufriére Hills, Montserrat VEI = 3 Two disastrous east Atlantic storms inflicting severe damage were exacerbated by the eruption of the Soufriére Hills volcano in Montserrat on February 11, 2010 (deWind 2010) (Fig. 8). On February 20, a severe rainstorm associated with widespread landslides and severe flooding passed over the island of Madeira causing over 48 fatalities and damage exceeding 1.4 billion Euros (Fig. 9). On February 26-28, another violent storm named ‘Xynthia’ with maximum wind gust of 241 km per hour and torrential rainfall crossed Western Europe causing over 63 fatalities and damage exceeding 3 billion Euros. Over 1 million homes were left without power and French cities such as La Faute-sur-Mer (Fig. 10) were severely flooded. Both storms were associated with active cold fronts and low-pressure areas in the southern part of the North Atlantic Ocean. Fig. 13 Annual mean temperature and annual mean precipitation in Hurbanovo, Slovakia 1881-2010 after Pecho (2011). 2010 was the wettest year since record began and was at least partially a product of the Eyjafjallajökull eruption in Iceland on April 14, 2010. Conclusion Volcanic eruptions have been found to a contributing factor to extreme regional weather events.   Acknowledgements Thanks are due to Judy Huang, Terence Lam and Jozef Pecho for their assistance. References Carn SA et al. (2009) The unexpected awakening of Chaitén volcano, Chile. EOS Transactions 90/24: 205-206. deWind A (2010) Volcanoes and storms. Geoscientist 20/6: 11-12. Pecho J et al. (2010) Extreme precipitation totals in Slovakia in spring and early summer of 2010. Meteorological J. 13: Slovak Hydrometeorological Institute Bratislava 2010, ISSN1335-339X. Pecho J (2011) Annual Bulletin – Slovakia (state of climate 2010). Rampino MR & Self S (1984) The atmospheric effects of El Chichõn. Scientific American 250/1: 34-43. Robock A & Matson M (1983) Circumglobal transport of the El Chichõn volcanic dust cloud. Science 221: 195-197. Yim WW-S (2011) Climatic effects of atmospheric water vapour distribution through volcanic eruptions and human activities. Imperial Engineer Spring 2011: p10. Fig. 11 Satellite analysis of weather conditions over Europe on April 29, 2010. Successive frontal activity storms associated with heavy rainfall migrated from Iceland towards Slovakia in central Europe. Fig. 8 Meteosat SEVIRI Channel 7 (8.3-9.1m infrared) image of the southern part of the North Atlantic Ocean on February 12, 2010 showing the Soufriére Hills eruption cloud caught up within the warm sector of the frontal system spreading in a northeasterly direction towards western Europe. Source: www.sat.dundee.ac.uk.