Volcanic aerosol plumes over south east Asia (SEAsia), and only over SEAsia, have always been the trigger and sustaining cause of: El Niño Southern Oscillation (ENSO) events which are the dominant mode of variability in the global climate; Australian and Indonesian droughts; increased global temperatures; and Indian Ocean Dipole (IOD) events. In recent decades this natural plume has been augmented by an anthropogenic plume which has intensified these events especially from September to November. Understanding the mechanism which enables aerosols over SEAsia, and only over SEAsia, to create ENSO events is crucial to understanding the global climate. I show that the SEAsian aerosol plume causes ENSO events by: reflecting/absorbing solar radiation which warms the upper troposphere; and reducing surface radiation which cools the surface under the plume. This inversion reduces convection in SEAsia thereby suppressing the Walker Circulation and the Trade Winds which causes the Sea Surface Temperature (SST) to rise in the central Pacific Ocean and creates convection there. This further weakens/reverses the Walker Circulation driving the climate into an ENSO state which is maintained until the SEAsian aerosols dissipate and the climate system relaxes into a non-ENSO state. Data from the Global Volcanism Program (151 years), the Last Millennium Ensemble (1,156 years), MERRA-2 (41 years) and NASA MODIS on Terra (21 years) demonstrates this connection with the Nino 3.4 and 1+2 SST, the Southern Oscillation Index, and three events commonly associated with ENSO: drought in south eastern Australia; the IOD and a warmer World.

The Historical Experiment (HE) data in the Intergovernmental Panel on Climate Change (IPCC) Climate Model Intercomparison Project six (CMIP6) should demonstrate that all submitted models accurately simulate the climate of the recent past. I show: none of nine models analysed accurately creates the known occurrence of ENSO events; no model agrees with any other; and average aerosol levels of the South East Asian Plume (SEAP), the South AMerican Plume (SAMP) and the West African Plume (WAP) are too low. Additionally, the SEAP and the SAMP cause the global temperature to rise in all nine models and the WAP in six models. Hence these models and all others which cannot accurately portray the known occurrence of ENSO events should be withdrawn from CMIP6 until they can and use of the IPCC Assessment Report six (AR6) should be paused until the effects of these aerosol plumes on the global temperature is re-evaluated.

The literature, Inter Governmental Panel on Climate Change (IPCC) and the USA Climate Change Science Program suggest that aerosols can affect the large-scale atmospheric circulation and hydrologic cycle I therefore examine the relationship between aerosols and Iberian droughts, floods in the UK/Ireland and higher European winter temperatures. Aerosols exist mainly as eight continental scale plumes which typically, but not exclusively, exist for a few months in the tropics at the end of the local dry season when biomass burning can occur. The anthropogenic West African aerosol Plume (WAP) exists from late December to early April and is located in the region which drives the northern regional Hadley Circulation towards Europe. It is therefore the prime candidate for investigation into European, winter climate variability. Using the Last Millennium Ensemble (1,156 years and 13,872 months of data) and the MERRA-2 reanalysis (40 years of data) I show that drought in the Iberian Peninsula, floods in the UK/Ireland and higher winter temperatures in northern Europe are created by extreme apparitions of the anthropogenic WAP. The WAP creates these effects by Aerosol Regional Dimming (ARD), which, by altering the surface radiation budget under the plume and warming the upper atmosphere, forces the regional Hadley Circulation into an abnormal seasonal position. These effects alter the regional atmospheric circulation systems and hydrologic cycle in Europe thereby causing drought, floods and higher temperatures and, as the WAP has intensified over recent decades, created climate change.

The Millennium Drought (Drought) from about 1997 to 2008 was one of if not the most severe drought in recorded Australian history. In 2006 River Murray inflows were 40% below the previous low. Nothing of this severity occurs without a cause, which must have the same time span, and the obvious challenge is to identify the culprit and explain how it caused the Drought. CAWCR Technical Report 26 discussed ENSO; the North West Shelf Sea Surface Temperature (SST); MSLP SE Australia; the SAM; and the Neighbouring Tasman Sea SST as possible causes and found that: local MSLP is the major influence; and ENSO’s influence is created by “large-scale circulation changes”. The literature and the Bureau of Meteorology (BoM) also link the positive phase of the Indian Ocean Dipole (IOD) to drought in Australia. Many researchers have tried to connect ENSO events to volcanic eruptions without success as their focus was on large eruptions which injected gases and tephra into the stratosphere. I include all eruptions in a restricted area, south east Asia (SE Asia), and show how they create ENSO events by reducing convection over the Maritime Continent and forcing the Trade Winds to relax. Simultaneously the volcanic plume moves the regional southern Hadley Cell south and creates the anomalous, persistent high pressure over south eastern Australia. Finally by intercepting the solar radiation by absorption and/or reflection, the volcanic plume cools the sea surface beneath it - the region where moisture which falls as rain in SE Australia evaporates - thereby reducing the available moisture in SE Australia and also creates IOD events. I show that SE Asian tectonic activity is highly variable and the level of volcanic tephra ejected during the Drought was treble the average for the 20th century and this was exacerbated by the anthropogenic aerosols in the same region which increased by 687% from 1979 to 2000 in September. Thus the Millennium Drought was caused by the increased levels of volcanic and anthropogenic aerosols over south east Asia which simultaneously: forced the circulation changes which created the anomalous high pressure over SE Australia; ENSO; and IOD events

Volcanic aerosols over south east Asia have always been the trigger and sustaining cause of ENSO events. In recent decades this natural plume has been augmented by the anthropogenic plume which has intensified ENSO events especially in SON. Data from the Last Millennium Ensemble (13,972 months), and Large Ensemble (3,012 months) demonstrate this connection with three ENSO indices and aerosol data derived from the same datasets correlating at 1.00 (LME), 0.97 and 0.99 magnitude (segmented and averaged). ENSO events are the dominant mode of variability in the global climate responsible for Australian, Indian and Indonesian droughts, American floods and increased global temperatures. Understanding the mechanism which enables aerosols over SE Asia and only over SE Asia to create ENSO events is crucial to understanding the global climate. I show that the South East Asian aerosol Plume causes ENSO events by: reflecting/absorbing solar radiation which warms the upper troposphere; and reducing surface radiation which cools the surface under the plume. This inversion reduces convection in the region thereby suppressing the Walker Circulation and the Trade Winds which causes the SST to rise in the central Pacific Ocean and creates convection there. This further weakens/reverses the Walker Circulation driving the climate into an ENSO state which is maintained until the aerosols dissipate and the climate system relaxes into a non-ENSO state. Measured aerosol data from four NASA satellites, estimates of volcanic tephra from the Global Volcanism Program (GVP) for over 100 years and the NASA MERRA-2 reanalysis dataset all confirm this analysis.

The Last Millennium Ensemble, Large Ensemble, MERRA-2, four satellite data sets and the Global Volcanism Program database all show independently that drought in south eastern Australia (SEAus) is created by apparitions of the natural and anthropogenic aerosol plume over south east Asia which simultaneously create ENSO and IOD events. From 1997 to 2008 SEAus endured an exceptionally severe drought - the Millennium Drought. The River Murray, the major waterway in the region, experienced inflows at record low levels in 2006-07 which were more than 40% below the previous low. As the literature, Inter Governmental Panel on Climate Change (IPCC) and the USA Climate Change Science Program suggest that aerosols can affect the large-scale atmospheric circulation and hydrologic cycle I examine the relationship between aerosols and Australian droughts. The global aerosol coverage is highly inhomogeneous and variable at daily, monthly, annual and decadal scales. I show that the aerosol optical depth (AOD) and aerosol index (AI) of the South East Asian Plume (SEAP) and the volume of aerosols ejected by volcanoes (tephra) in south east Asia correlate with drought in Australia and conclude that the SEAP causes drought in Australia by Aerosol Regional Dimming (ARD), which, by altering the surface radiation budget under the plume and warming the upper atmosphere, forces the regional Inter Tropical Convergence Zone and Hadley Cells into abnormal seasonal positions. These effects alter the regional atmospheric circulation systems and hydrologic cycle thereby causing drought and, as the SEAP has intensified over time, created climate change.

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.