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The South East Asian Aerosol Plume: The Cause of All El Niño Events
  • Keith Potts
Keith Potts

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ENSO events are the most significant interannual perturbation of the climate system. Previous attempts to link ENSO with volcanic eruptions failed because only large eruptions across the globe, which typically eject tephra into the stratosphere, were considered. I have analysed all volcanic eruptions in South Eastern (SE) Asia, about 10ºS to 10ºN and 90ºE to 160ºE (4d), the most volcanically active area in the world with over 23% of all eruptions in the Global Volcanism Program database since 1500 occurring here and with 5 volcanoes stated in the literature to have erupted nearly continuously for 30 years. SE Asia is also the region where the convective arm of the thermally direct Walker Circulation occurs driven by the intense equatorial solar radiation which creates the high surface temperature. The volcanic tephra plume intercepts some of the solar radiation by absorption/reflection which cools the surface and heats the atmosphere creating a temperature inversion compared to periods without the plume. This reduces convection and causes the Walker Circulation and Trade Winds to weaken. This reduced wind speed causes the central Pacific Ocean to warm creating convection there and further weakening the Walker Circulation. With the reduced wind stress the western Pacific warm pool migrates east. This creates an ENSO event which continues until the tephra plume reduces, typically when the SE Asian monsoon commences, and convection is re-established over SE Asia and the Pacific warm pool migrates back to the west. Correlations of SE Asian tephra and the ENSO indices are typically over 0.80 at ρ < 0.02 at 5c below. In recent decades the anthropogenic SE Asian aerosol Plume (SEAP) has intensified the volcanic plume in some years from September to November (SON). Using NASA satellite data and the NASA MERRA-2 reanalysis dataset I show correlation coefficients typically over 0.70 and up to 0.99 at ρ < 0.01 between the aerosol optical depth (AOD) or aerosol index (AI) and the ENSO indices on a detrended basis in SON at 5a. If two events A and B correlate 5 options are possible: (1) A causes B; (2) B causes A; (3) C, another event, causes A & B simultaneously; (4) It’s a coincidence; and (5) The relationship is complex with feedback. The volcanic results: only allow options 1 or 4 as ENSO cannot cause volcanic eruptions; and are backed up by 4 independent satellite datasets and NASA’s MERRA-2 reanalysis which assimilates aerosol observations. I conclude volcanic and anthropogenic aerosols over SE Asia are the sole cause of all ENSO events.