15.05.2017: FB1-Seminar

11 Uhr, Hörsaal West, Düsternbrooker Weg 20

Abstract:

El Niño/Southern Oscillation (ENSO) is the most dominant climate variability on interannual time scales and has large socio-economical impacts as it causes extreme weather events like floods and droughts. Despite improvements have been made in simulating ENSO in the last decades, realistic representation of ENSO and its projection under global warming remains highly uncertain, as it varies amongst current climate models of the 5th phase of the Coupled Model Intercomparison Project (CMIP5). The ENSO atmospheric feedbacks, namely the positive (amplifying) zonal wind feedback and the negative (damping) heat flux feedback, have been discussed as a major contributor to the diversity of simulated ENSO, as they are strongly underestimated in many state-of-the-art climate models.

Here we show in a multi model ensemble of CMIP5 and a perturbed physics ensemble of the Kiel Climate Model (KCM), that the ENSO atmospheric feedbacks strongly depend on the mean state, while the direct effect of biases in the model physics plays a minor role. More in detail, the underestimated atmospheric feedbacks are caused by a La Nina-like atmospheric mean state due to a sea surface temperature (SST) cold bias in the western Pacific, which is a common problem in climate models. It shifts the Walker Circulation to the west (up to 30° in longitude) and weakens ENSO atmospheric feedbacks. As the position of the Walker Circulation determines the strength of both ENSO atmospheric feedbacks, this explains the compensating effect between these two feedbacks seen in the climate models. Further, a too westward position of the Walker Circulation in climate models changes ocean-atmosphere coupling in the tropical Pacific from a predominantly wind-driven to a shortwave-driven mode and causes different ENSO dynamics as observed. This means that many climate models have an ENSO variability for the wrong reasons. In contrast climate models with a reduced cold SST bias show enhanced ENSO atmospheric feedbacks that lead to a substantial improvement of important ENSO properties such as seasonal ENSO phase locking and asymmetry between El Niño and La Niña. Our results suggest that differences in the mean state are a major source of ENSO diversity in current climate models.

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