Séminaire

1. Robust twenty-first-century projections of El Niño and related precipitation variability.

The El Niño–Southern Oscillation (ENSO) drives substantial variability in rainfall, severe weather, agricultural production, ecosystems and disease in many parts of the world. Given that further human-forced changes in the Earth’s climate system seem inevitable, the possibility exists that the character of ENSO and its impacts might change over the coming century. Although this issue has been investigated many times during the past 20 years, there has been very little consensus on future changes in ENSO, apart from an expectation that ENSO will continue to be a dominant source of year-to-year variability. Here we show that there are in fact robust projected changes in the spatial patterns of year-to-year ENSO-driven variability in both surface temperature and precipitation. These changes are evident in the two most recent generations of climate models, using four different scenarios for CO2 and other radiatively active gases. By the mid- to late twenty-first century, the projections include an intensification of both El-Niño-driven drying in the western Pacific Ocean and rainfall increases in the central and eastern equatorial Pacific. Experiments with an Atmospheric General Circulation Model reveal that robust projected changes in precipitation anomalies during El Niño years are primarily determined by a nonlinear response to surface global warming. Uncertain projected changes in the amplitude of ENSO-driven surface temperature variability have only a secondary role. Projected changes in key characteristics of ENSO are consequently much clearer than previously realized.

2. Consensus on 21st century rainfall projections in climate models more widespread than previously thought.

We then examine 21st century projections of precipitation. We will address a question that is very important for adaptation, but is rarely asked or addressed by physical climate scientists: what won’t change in response to anthropogenic forcing? The last IPCC Report (2007) concluded that increases in precipitation in association with global warming over the 21st century are very likely in high latitudes and near major convergence zones in the tropics in some seasons, while decreases are likely in many subtropical land areas. However, in many areas fewer than 66% of models agreed on the sign of projected change. This can occur, for example, if some models project a “large” increase while other models project a “large” decrease, making projected changes in such regions very uncertain. Christensen et al. (2007) concluded that: “for some regions, there are grounds for stating that the projected precipitation changes are likely or very likely. For other regions, “confidence in the projected change remains weak”. While some of this uncertainty stems from known model deficiencies and imperfect simulations of observed regional climate, much of this uncertainty arises because the models do not exhibit a consensus on the sign of projected change in these regions. Large uncertainty is very undesirable because it impedes the ability of the scientific community to provide guidance on future climate in the affected regions. This, in turn, limits the ability of decision-makers in the wider community to optimise their mitigation and adaptation plans. We will examine projected precipitation changes in the late 21st century WCRP/CMIP3 and CMIP5 climate model integrations forced using several emissions scenarios. We will highlight (i) the importance of clarifying where projected changes are small relative to variability rather than necessarily uncertain, and, as a result, (ii) a stronger consensus among climate model on 21st century precipitation projections in the Pacific than previously appreciated.

References

Power, S., F. Delage, C. Chung, G. Kociuba, K. Keay, 2013: Robust twenty-first-century projections of El Niño and related precipitation variability. Nature, 502, 541–545, http://dx.doi.org/10.1038/nature12580.

Power, S.B., F. Delage, R. Colman, and A. Moise, 2011: Consensus on 21st century rainfall projections in climate models more widespread than previously thought. J. Climate, 25, 3792-3809, DOI: 10.1175/JCLI-D-11-00354.1.

Scott Power : s.power@bom.gov.au