IPCC AR5 WGI Chapter 14 - A Review
Climate Phenomena and their Relevance for Future Regional Climate Change
El Niño-Southern Oscillation (ENSO)
The El Niño-Southern Oscillation (ENSO) is a coupled atmosphere-ocean phenomenon occurring on time scales of 2-7 years that leads to basin-wide warming and cooling of the tropical Pacific Ocean. These oceanic temperature changes are associated with fluctuation of the Southern Oscillation, a global-scale surface pressure pattern in the tropics and subtropics. The prevailing trade winds weaken during an ENSO event, altering ocean currents and upwelling, which causes sea surface temperatures to warm. This event strongly impacts wind, temperature, and, precipitation patterns in the tropical Pacific, and can have far-reaching effects on global climate. Certain aspects of this phenomenon are expected to change under a warming climate (Fig. 14.12).
Some studies suggest that the amplitude of ENSO has been modified by changes in mean climatic conditions in the tropical Pacific. An increasing trend in ENSO amplitude has been observed in the past century, though the dataset used to produce this trend remains limited. However, long runtime global circulation model simulations suggest that the changes in ENSO amplitude on the decadal to centennial scale can be produced without any change in external forcing (i.e. without climate change) due to variability in the tropical climatic system. Thus, there is little consensus as to whether recent (last few decades) trends in ENSO amplitude (Fig. 14.13) are a result of anthropogenic effects or natural variability in the climate system. CMIP5 models do a better job of simulating ENSO amplitudes than previous model iterations, though future changes in ENSO intensity remain model dependent and often indistinguishable from the effects of natural variability. However, there is high confidence that ENSO will remain the dominant mode of natural climate variability in the 21st century – the main conclusion from the IPCC report for this chapter.
Figure 14.13 | Intensities of El Niño and La Niña events for the last 60 years in the eastern equatorial Pacific (Niño3 region) and in the central equatorial Pacific (Niño4 region), and the estimated linear trends, obtained from Extended Reconstructed Sea Surface Temperature v3 (ERSSTv3).
Summary
ENSO shows considerable variations in amplitude on decadal timescales, but this modulation cannot be directly ascribed to the effects of anthropogenic climate change due to the large potential changes from natural variability alone. There is high confidence that ENSO will remain the most important mode of climate variability in the 21st century, but low confidence in changes in its intensity and spatial extent in a warmer climate.
Figure 14.12 | Idealized schematic showing atmospheric and oceanic conditions of the tropical Pacific region and their interactions during normal conditions, El Niño conditions, and in a warmer world. (a) Mean climate conditions in the tropical Pacific, indicating sea surface temperatures (SSTs), surface wind stress and associated Walker Circulation, the mean position of convection and the mean upwelling and position of the thermocline. (b) Typical conditions during an El Niño event. SSTs are anomalously warm in the east; convection moves into the central Pacific; the trade winds weaken in the east and the Walker Circulation is disrupted; the thermocline flattens and the upwelling is reduced. (c) The likely mean conditions under climate change derived from observations, theory and coupled General Circulation Models (GCMs). The trade winds weaken; the thermocline flattens and shoals; the upwelling is reduced although the mean vertical temperature gradient is increased; and SSTs (shown as anomalies with respect to the mean tropical-wide warming) increase more on the equator than off. Diagrams with absolute SST fields are shown on the left, diagrams with SST anomalies are shown on the right. For the climate change fields, anomalies are expressed with respect to the basin average temperature change so that blue colours indicate a warming smaller than the basin mean, not a cooling (Collins et al., 2010).
