• Energy Research

A multi-model average shows that 21st century warming over the eastern Indian Ocean (IO) is slower than that to the west, but with strong inter-model variations. Is the simulation of the Indian Ocean Dipole (IOD) relevant to the inter-model variations? We demonstrate that inter-model variations of this future warming are consistent with how well models simulate historical IOD properties; models with a stronger IOD amplitude systematically produce a slower eastern IO warming rate with greater future rainfall changes in IOD-affected regions. These models also produce a stronger Bjerknes-like positive feedback, involving sea surface temperatures (SSTs), winds and a shoaling thermocline in the eastern IO. As warming proceeds, models with a stronger positive feedback induce a greater response to warming-induced changes such as easterly trends associated with the Walker circulation, generating a smaller warming in the eastern IO. Simulating the present-day IOD properties is, therefore, a relevant criterion for selecting models for climate projections.

A multi-model average shows that 21st century warming over the eastern Indian Ocean (IO) is slower than that to the west, but with strong inter-model variations. Is the simulation of the Indian Ocean Dipole (IOD) relevant to the inter-model variations? We demonstrate that inter-model variations of this future warming are consistent with how well models simulate historical IOD properties; models with a stronger IOD amplitude systematically produce a slower eastern IO warming rate with greater future rainfall changes in IOD-affected regions. These models also produce a stronger Bjerknes-like positive feedback, involving sea surface temperatures (SSTs), winds and a shoaling thermocline in the eastern IO. As warming proceeds, models with a stronger positive feedback induce a greater response to warming-induced changes such as easterly trends associated with the Walker circulation, generating a smaller warming in the eastern IO. Simulating the present-day IOD properties is, therefore, a relevant criterion for selecting models for climate projections.

AbstractThe Australian decade-long “Millennium Drought” broke in the summer of 2010/11 and was considered the most severe drought since instrumental records began in the 1900s. A crucial question is whether climate change played a role in inducing the rainfall deficit. The climate modes in question include the Indian Ocean dipole (IOD), affecting southern Australia in winter and spring; the southern annular mode (SAM) with an opposing influence on southern Australia in winter to that in spring; and El Niño–Southern Oscillation, affecting northern and eastern Australia in most seasons and southeastern Australia in spring through its coherence with the IOD. Furthermore, the poleward edge of the Southern Hemisphere Hadley cell, which indicates the position of the subtropical dry zone, has possible implications for recent rainfall declines in autumn. Using observations and simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), it is shown that the drought over southwest Western Australia is partly attributable to a long-term upward SAM trend, which contributed to half of the winter rainfall reduction in this region. For southeast Australia, models simulate weak trends in the pertinent climate modes. In particular, they severely underestimate the observed poleward expansion of the subtropical dry zone and associated impacts. Thus, although climate models generally suggest that Australia’s Millennium Drought was mostly due to multidecadal variability, some late-twentieth-century changes in climate modes that influence regional rainfall are partially attributable to anthropogenic greenhouse warming.

AbstractThe Australian decade-long “Millennium Drought” broke in the summer of 2010/11 and was considered the most severe drought since instrumental records began in the 1900s. A crucial question is whether climate change played a role in inducing the rainfall deficit. The climate modes in question include the Indian Ocean dipole (IOD), affecting southern Australia in winter and spring; the southern annular mode (SAM) with an opposing influence on southern Australia in winter to that in spring; and El Niño–Southern Oscillation, affecting northern and eastern Australia in most seasons and southeastern Australia in spring through its coherence with the IOD. Furthermore, the poleward edge of the Southern Hemisphere Hadley cell, which indicates the position of the subtropical dry zone, has possible implications for recent rainfall declines in autumn. Using observations and simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), it is shown that the drought over southwest Western Australia is partly attributable to a long-term upward SAM trend, which contributed to half of the winter rainfall reduction in this region. For southeast Australia, models simulate weak trends in the pertinent climate modes. In particular, they severely underestimate the observed poleward expansion of the subtropical dry zone and associated impacts. Thus, although climate models generally suggest that Australia’s Millennium Drought was mostly due to multidecadal variability, some late-twentieth-century changes in climate modes that influence regional rainfall are partially attributable to anthropogenic greenhouse warming.

The relationship between the interannual changes in streamflow, precipitation and temperature of the Murray-Darling basin is investigated by using a two-parameter climate elasticity of streamflow approach. The non-linear relationship between streamflow and both precipitation and temperature indicates a greater streamflow sensitivity to precipitation than to temperature but a more significant impact of temperature change on streamflow than previously reported. The physical mechanisms producing high streamflow sensitivity to temperature change are not well understood, but may relate to concurrent changes in sub-annual precipitation characteristics such as seasonality, spatial distribution and intensity. Thus these characteristics need to be assessed and accounted for when attempting to project how streamflow, and hence water availability, may change in a future warmer world.

The relationship between the interannual changes in streamflow, precipitation and temperature of the Murray-Darling basin is investigated by using a two-parameter climate elasticity of streamflow approach. The non-linear relationship between streamflow and both precipitation and temperature indicates a greater streamflow sensitivity to precipitation than to temperature but a more significant impact of temperature change on streamflow than previously reported. The physical mechanisms producing high streamflow sensitivity to temperature change are not well understood, but may relate to concurrent changes in sub-annual precipitation characteristics such as seasonality, spatial distribution and intensity. Thus these characteristics need to be assessed and accounted for when attempting to project how streamflow, and hence water availability, may change in a future warmer world.

Abstract Severe rainfall deficiencies have plagued southern and eastern Australian regions over the past decades, where the long-term rainfall is projected to decrease. By contrast, there has been an increase over northwest Australia (NWA) in austral summer, which, if it continues, could be an important future water resource. If increasing anthropogenic aerosols contribute to the observed increase in summer rainfall, then, as anthropogenic aerosols are projected to decrease, what will the likely impact over NWA be? This study uses output from 24 climate models submitted to phase 3 of the Coupled Model Intercomparison Project (CMIP3) with a total of 75 experiments to provide a multimodel perspective. The authors find that none of the ensemble averages, either with both the direct and indirect anthropogenic aerosol effect (10 models, 32 experiments) or with the direct effect only (14 models, 43 experiments), simulate the observed NWA rainfall increase. Given this, it follows that a projected rainfall reduction is not due to a projected decline in future aerosol concentrations. The authors show that the projected NWA rainfall reduction is associated with an unrealistic and overly strong NWA rainfall teleconnection with the El Niño–Southern Oscillation (ENSO). The unrealistic teleconnection is primarily caused by a model equatorial Pacific cold tongue that extends too far into the western Pacific, with the ascending branch of the Walker circulation situated too far west, exerting an influence on rainfall over NWA rather than over northeast Australia. Models with a greater present-day ENSO amplitude produce a greater reduction in the Walker circulation and hence a greater reduction in NWA rainfall in a warming climate. Hence, the cold bias and its impact represent a source of uncertainty for climate projections.

Abstract Severe rainfall deficiencies have plagued southern and eastern Australian regions over the past decades, where the long-term rainfall is projected to decrease. By contrast, there has been an increase over northwest Australia (NWA) in austral summer, which, if it continues, could be an important future water resource. If increasing anthropogenic aerosols contribute to the observed increase in summer rainfall, then, as anthropogenic aerosols are projected to decrease, what will the likely impact over NWA be? This study uses output from 24 climate models submitted to phase 3 of the Coupled Model Intercomparison Project (CMIP3) with a total of 75 experiments to provide a multimodel perspective. The authors find that none of the ensemble averages, either with both the direct and indirect anthropogenic aerosol effect (10 models, 32 experiments) or with the direct effect only (14 models, 43 experiments), simulate the observed NWA rainfall increase. Given this, it follows that a projected rainfall reduction is not due to a projected decline in future aerosol concentrations. The authors show that the projected NWA rainfall reduction is associated with an unrealistic and overly strong NWA rainfall teleconnection with the El Niño–Southern Oscillation (ENSO). The unrealistic teleconnection is primarily caused by a model equatorial Pacific cold tongue that extends too far into the western Pacific, with the ascending branch of the Walker circulation situated too far west, exerting an influence on rainfall over NWA rather than over northeast Australia. Models with a greater present-day ENSO amplitude produce a greater reduction in the Walker circulation and hence a greater reduction in NWA rainfall in a warming climate. Hence, the cold bias and its impact represent a source of uncertainty for climate projections.

Abstract Since the 1950s annual rainfall over southeastern Australia (SEA) has decreased considerably with a maximum decline in the austral autumn season (March–May), particularly from 1980 onward. The understanding of SEA autumn rainfall variability, the causes, and associated mechanisms for the autumn reduction remain elusive. As such, a new plausible mechanism for SEA autumn rainfall variability is described, and the dynamics for the reduction are hypothesized. First, there is no recent coherence between SEA autumn rainfall and the southern annular mode, discounting it as a possible driver of the autumn rainfall reduction. Second, weak trends in the subtropical ridge intensity cannot explain the recent autumn rainfall reduction across SEA, even though a significant relationship exists between the ridge and rainfall in April and May. With a collapse in the relationship between the autumn subtropical ridge intensity and position in recent decades, a strengthening in the influence of the postmonsoonal winds from north of Australia has emerged, as evident by a strong post-1980 coherence with SEA mean sea level pressure and rainfall. From mid to late autumn, there has been a replacement of a relative wet climate in SEA with a drier climate from northern latitudes, representing a climate shift that has contributed to the rainfall reduction. The maximum baroclinicity, as indicated by Eady growth rates, has shifted poleward. An associated poleward shift of the dominant process controlling SEA autumn rainfall has further enhanced the reduction, particularly across southern SEA. This observed change over the past few decades is consistent with a poleward shift of the ocean and atmosphere circulation.