Even though several groups of scientists hold different opinions on the prediction of the ENSO’s frequency in the future, they all agree that the predictions depend on many other complicated processes such as cloud feedback, etc. Therefore, so far no one can give a comprehensive answer to the future ENSO’s frequency. In this post, I will explain two extreme theories of the prediction.
Figure 1. Predicted temperature anomaly along the Pacific Ocean. The east shows a large increase of temperature, while the west warms up relatively in a small degree.
Timmermann et al. demonstrate that El Niño’s frequency would possibly increase due to the increasing carbon dioxide concentration (Timmermann et al., 1999). The model, developed by Roeckner in 1996, shows sea surface temperature is characterized by a strong warming in the east equatorial Pacific, with westerly winds blowing to the east. This procedure creates a condition that is similar to the current El Niño events (Figure 1). In the present climate system, west Pacific is so warm that even a small rise of temperature will result in cloud shielding effect, which will develop high cirrus clouds and reduce the solar radiation coming in to the surface; on the other hand, east Pacific receives more solar radiation. As a result, a large temperature difference will exist between west and east Pacific (Ramanathan et al. 1991). The model also predicts that the thermocline will become sharpening, with more frequent cold La Niño events (Stone et al., 1998).
Figure 2. Vertical temperature anomaly at tropical Pacific. The positive anomaly happens at the surface and below the thermocline.
Meehl et al., on the other hand, give the opposite prediction that the amplitude of El Niño will decrease because of global warming (Meehl et al. 2001). Instead of considering horizontal transport of heat, Meehl’s model illustrates the heat conduction vertically. One of his models shows that the temperature gets warmer at both upper surface and below the thermocline (Figure 2). This change of the water column temperature eventually weakens the circulation of the subtropical cells (STC). As a result, thermocline becomes more diffuse and deeper, which contributes to reduce El Niño’s frequency (McPhaden et Zhang, 2002).
McPhaden MJ, Zhang D. (2002) Slowdown of the meridional overturning circulation in the upper Pacific Ocean. Nature 415:603–608
Meehl, G. A., H. Teng, G. Branstator, 2006, Future changes of El Niño in two global cou- pled climate models. Climate Dynamic. no. 26, pp. 549-566
Ramanathan, V. & Collins, W. (1991) Thermodynamic regulation of ocean warming by cirrus clouds deduced from observations of the 1987 El NinÄo. Nature 351, 27-32
Roeckner, E., Oberhuber, J. M., Bacher, A., Christoph, M. & Kirchner, I. (1996) ENSO variability and atmospheric response in a global atmosphere-ocean GCM. Clim. Dynam. 12, 737-754
Timmermann, A., M. Latif, A. Bacher, J. Oberhuber, E. Roeckner, 1999, Increased El Nin ̃o frequency in a climate model forced by future greenhouse warming. Nature, no. 398, pp. 694–696