coast of tropical South America, subtropical latitudes of western North America, and southeastern America” (Intergovernmental Panel on Climate Change, 2012:155). The cold phase, known as La Niña, generally shows opposite anomalies. The effects of ENSO can also be felt in the other ocean basins and on all continents (Dai et al., 1998). Recent research has shown that different phases of ENSO are associated with different frequencies of short-term weather events such as heavy rainfall and extreme temperatures in the affected regions (Intergovernmental Panel on Climate Change, 2012).

These regularities in the effects of ENSO on climate events allow for skillful short-term climate predictions on time-scales from a season to a year by coupling atmospheric general circulation models with ocean general circulation models initialized with observations of the state of atmosphere and ocean. Forecast skill decreases away from the equator, however. This seasonal climate prediction has now become operational at many of the world’s major weather prediction centers. (NOAA, for example, offers seasonal outlooks online at http://www.cpc.ncep.noaa.gov/products/predictions/90day/ [accessed November 13, 2012].) Prediction on time-scales from years to decades is still very much in the research realm, so it is not yet possible to estimate the eventual skill that might be achieved in forecasting at these time-scales.

The fact that ENSO drives unusual weather patterns on several continents at the same time demonstrates that extreme weather patterns in different regions may not be independent in a statistical sense, suggesting the possibility that extreme climate events may cluster in time, a topic discussed more fully in the next section.

The question of whether and how anthropogenic climate change may be altering the ENSO cycle is being actively examined by climate scientists (Intergovernmental Panel on Climate Change, 2012). Dai et al. (1998) noted a shift in ENSO events in the mid-1970s toward more warm events, which coincided with record high global temperatures and drought anomalies that were greater than expected. There is some evidence linking these changes in ENSO events to intensified droughts in some drier regions since the 1970s, while the extent of wet areas has declined during this period. These changes are qualitatively consistent with the observed increases in greenhouse gases in the atmosphere, which act to enhance the hydrological cycle. The IPCC special report on extreme events (Intergovernmental Panel on Climate Change, 2012) describes systematic changes in ENSO behavior that have been observed over the past 50 to 100 years, but it concludes that it is not clear what role, if any, increased greenhouse gases have played in this phenomenon. All models used in that assessment predict continued ENSO interannual variability in the future no matter what the change in average background conditions. However, the changes in ENSO interannual variability differ from model to model based on subtle changes



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