On the Slow Decay of El Niño in CMIP6 Models

The decay pace of El Niño can significantly modify its impacts on the Asian climate during the post-El Niño summer. Hence, accurately reproducing the observed decay pace in state-of-art coupled models is essential for realistic climate simulations. In the CMIP6 models, El Niño decays slower than observed. This slower decay can be attributed to weaker-than-observed air-sea coupling in the models that causes a weaker atmospheric convective response and smaller westerly anomalies along the equatorial Pacific during the El Niño life cycle. The smaller westerly anomalies result in a slower discharge of equatorial ocean heat, weaker negative/positive thermocline anomalies along/off the equator and thus a weaker meridional gradient of the thermocline anomalies. This weakens the easterly current anomalies, diminishes the zonal advection feedback, and ultimately slows the decay pace of El Niño in the models.