Working Group I: The Scientific Basis

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There are now a greater number of global coupled atmosphere-ocean models and a number of them have been run for multi-century time-scales. This has substantially improved the basis for estimating long time-scale natural unforced variability. There are still severe limitations in the ability of such models to represent the full complexity of observed variability and the conclusions drawn here about changes in variability must be viewed in the light of these shortcomings (Chapter 8).
Some new studies have reinforced results reported in the SAR. These are:

  • The future mean Pacific climate base state could more resemble an El Niño-like state (i.e., a slackened west to east SST gradient with associated eastward shifts of precipitation). Whilst this is shown in several studies, it is not true of all.
  • Enhanced interannual variability of daily precipitation in the Asian summer monsoon. The changes in monsoon strength depend on the details of the forcing scenario and model.

Some new results have challenged the conclusions drawn in earlier reports, such as:

  • Little change or a decrease in ENSO variability. More recently, increases in ENSO variability have been found in some models where it has been attributed to increases in the strength of the thermocline. Decadal and longer time-scale variability complicates assessment of future changes in individual ENSO event amplitude and frequency. Assessment of such possible changes remains quite difficult. The changes in both the mean and variability of ENSO are still model dependent.

Finally there are areas where there is no clear indication of possible changes or no consensus on model predictions:

  • Although many models show an El Niño-like change in the mean state of tropical Pacific SSTs, the cause is uncertain. In some models it has been related to changes in cloud forcing and/or changes in the evaporative damping of the east-west SST gradient, but the result remains model-dependent. For such an El Niño-like climate change, future seasonal precipitation extremes associated with a given ENSO would be more intense due to the warmer mean base state.
  • There is still a lack of consistency in the analysis techniques used for studying circulation statistics (such as the AO, NAO and AAO) and it is likely that this is part of the reason for the lack of consensus from the models in predictions of changes in such events.
  • The possibility that climate change may be expressed as a change in the frequency or structure of naturally occuring modes of low-frequency variability has been raised. If true, this implies that GCMs must be able to simulate such regime transitions to accurately predict the response of the system to climate forcing. This capability has not yet been widely tested in climate models. A few studies have shown increasingly positive trends in the indices of the NAO/AO or the AAO in simulations with increased greenhouse gases, although this is not true in all models, and the magnitude and character of the changes varies across models.

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