Have Australian rainfall and cloudiness increased due to the remote effects of Asian anthropogenic aerosols?

Abstract

There is ample evidence that anthropogenic aerosols have important effects on climate in the Northern Hemisphere but little such evidence in the Southern Hemisphere. Observations of Australian rainfall and cloudiness since 1950 show increases over much of the continent. We show that including anthropogenic aerosol changes in 20th century simulations of a global climate model gives increasing rainfall and cloudiness over Australia during 1951–1996, whereas omitting this forcing gives decreasing rainfall and cloudiness. The pattern of increasing rainfall when aerosols are included is strongest over northwestern Australia, in agreement with the observed trends. The strong impact of aerosols is primarily due to the massive Asian aerosol haze, as confirmed by a sensitivity test in which only Asian anthropogenic aerosols are included. The Asian haze alters the meridional temperature and pressure gradients over the tropical Indian Ocean, thereby increasing the tendency of monsoonal winds to flow toward Australia. Anthropogenic aerosols also make the simulated pattern of surface‐temperature change in the tropical Pacific more like La Niña, since they induce a cooling of the surface waters in the extratropical North Pacific, which are then transported to the tropical eastern Pacific via the deep ocean. Transient climate model simulations forced only by increased greenhouse gases have generally not reproduced the observed rainfall increase over northwestern and central Australia. Our results suggest that a possible reason for this failure was the omission of forcing by Asian aerosols. Further research is essential to more accurately quantify the role of Asian aerosols in forcing Australian climate change.

Ocean circulation response to anthropogenic aerosol and greenhouse gas forcing in the CSIRO-Mk3.6 climate model

“…The magnitude of the AA-induced increase in the Mk36 Atlantic MOC is three times greater than that modelled in the Mk3A study (Cai et al 2006). This is a striking result, given that the overall AA forcing at the top of the atmosphere is only moderately stronger in Mk36 (-1.4 W m ; Rotstayn et al 2007). However, further examination shows that the increase in heat loss from the ocean surface of the northern hemisphere in Mk36 is about three times greater than in Mk3A during the period 1951-2000.…”

“…The bulk (~0.6 W m -2 ) of this difference can be attributed to indirect aerosol effects, since the direct AA forcing over the oceans of the northern hemisphere is -0.36 (-0.54) W m -2 in Mk36 (Mk3A). The relatively weaker indirect aerosol effect over oceans in Mk3A is at least partly related to a 'tuning' that was applied to reduce the magnitude of the effect (over oceans only) in that model; this was discussed by Rotstayn et al (2007; their section 2.1).…”

“…Cai et al (2006Cai et al ( , 2007 took a more global perspective regarding aerosols and ocean circulation. They examined AA-induced effects on ocean circulation in historical transient simulations with a low resolution version of the CSIRO GCM (Mk3A; Rotstayn et al 2007). Cai et al (2006) found that aerosols induce a strengthening of the Atlantic MOC and increase of northward cross-equatorial heat transport in the Atlantic and Pacific Oceans, with the majority of change taking place in the Atlantic Ocean.…”

A possible cause of decreasing summer rainfall in northeast Australia

“…The rainfall decrease in SWWA has been studied intensely in recent years (e.g. Smith et al, 2000;Cai and Watterson, 2002;IOCI, 2002;Pitman et al, 2004;Cai et al, 2005;Cai and Cowan, 2006;Hope et al, 2006;Meneghini et al, 2007;Feng et al, 2010aFeng et al, , 2010b, as has the increasing trend in NWA (Wardle and Smith, 2004;Rotstayn et al, 2006;Shi et al, 2008aShi et al, , 2008bTaschetto and England, 2008). In contrast, relatively little is known of the cause and mechanism of the decreased NEA rainfall.…”

Regionalizing global climate models

“…The deposition of black carbon on ice and snow has a large effect in terms of radiative forcing and this could accelerate regional warming and trigger important biophysical (albedo) and biogeochemical (release of soil and peat carbon) feedbacks in the climate system (Law and Stohl, 2007;Quinn et al, 2008). A second region that shows potential sensitivity to aerosols is Australia (Rotstayn et al, 2007) where increasing rainfall over northwest of the continent has been linked to Asian aerosol emissions. The Australian monsoon may also be affected by seasonality and severity of the continent's savannah fires (Lynch et al, 2007).…”