Seasonality: The anthropogenic South East Asian aerosol Plume (SEAP) occurs from late August to mid November each year and is probably more variable inter annually than any of the other seven plumes. The natural SEAP derives from volcanic tephra which can, of course, occur at any time during the year.
Sources Anthropogenic: The SEAP comprises aerosols derived from: biomass burning in land clearing and agriculture, gas flares in the oil production industry; and transport.The satellite data from the NASA Nimbus 7, Earth Probe and Terra satellites shows the anthropogenic SEAP increased in intensity from September 1979 to September 1997 (an exceptionally extreme year) by 3,499% and by 687% from Sep 1979 to Sep 2000 (a non-extreme year). It is worth noting that the level of carbon dioxide in the atmosphere increased by only 9.9% in the same period (1979 – 2000). The satellite data and the Representation Concentration Pathways (rcp’s) for CMIP 5 show no evidence the anthropogenic SEAP existed in its current form prior to 1950.
Trajectory: The anthropogenic SEAP flows from south east Asia to:
- The south east across the Pacific Ocean where it joins other aerosols from southern Africa and South America over the southern ocean;
- The south west into the southern Indian Ocean.
Sources Natural: South east Asia is one of the most tectonically active region of the World with about 23% of the global volcanic eruptions and 5 of the 20 largest earthquakes since 1900 occurring here – 4 of them since 2000. Hence the aerosol loading of the region has been driven by volcanic tephra for eons and varies significantly at inter annual and decadal time scales. In the decade from 2000 to 2009 the total level of tephra emitted in the region was about treble the decadal average for the 20th century.
Most Extreme: October 2006 and 2015 are the most extreme apparitions of the SEAP in the Terra data. The October 2006 apparition is shown in this NASA image.
Effects: The SEAP is unique because of its location which coincides with the convective arm of the thermally direct Walker Circulation which drives the Trade Wind flow across the Pacific Ocean and because it, more than any other plume, has a huge natural component which can occur at any time through the year. My paper presented at the AGU Fall Meeting in 2015 shows how the SEAP simultaneously causes El Nino and Southern Oscillation (ENSO) events and drought in south eastern Australia.
Changes: The graph below shows the changes in the NASA Aerosol Index (AI) and Aerosol Optical Depth (AOD) together with the Mauna Loa carbon dioxide level in the atmosphere since 1979. The CO2 levels increased by 9.9% from September 1979 to the same month in 2000 whilst the AI levels increased by 687%.