Australia: The Land Where Time Began

A biography of the Australian continent 

El Niño – Amplification by Cloud Long-Wave Coupling to Circulation of the Atmosphere  

The dominant mode of inter-annual variability is the E Niño/Southern Oscillation (ENSO), which has major impacts on social and ecological systems through the influence it has on extreme weather (Nicholls et al., 1996). An understanding of the underlying physical mechanisms driving it is required to have the ability to forecast El Niño, as well as to anticipate how it will change with warming. Of these, the role of atmospheric processes has remained poorly understood (Guilyardi et al., 2004; Sun et al., 2006; Dommenget, 2010; Lloyd, Guilyardi & Weller, 2012; Chen, Yu & Sun, 2013; Bellenger et al., 2014; Chen et al., 2015; Chen & Wallace; 2015). In this paper Rädel et al. present the results of numerical experiments using an Earth system model, both with and without cloud radiative effects to circulation, which suggest that the variability of ENSO is enhanced by clouds by a factor of 2 or more. Heating in the mid and upper troposphere associated with enhanced high-level cloudiness (Bretherton & Sobel; 2002) is induced by clouds over the El Niño region, and the lower troposphere is cooled by low-level clouds in the surrounding regions (Muller & Held, 2012). These combined effects enhance the coupling of the atmospheric circulation to El Niño surface temperature anomalies, which therefore strengthens the Bjerknes feedback mechanism (Bjerknes, 1969) between zonal wind stress and sea surface temperature gradients in the west Pacific. Other global climate models and satellite observations robustly report behaviour that is consistent with the proposed mechanism. The response of ENSO to climate change is suggested by the mechanism to be determined, in part, by a balance between increasing levels of cloud long-wave feedback and a possible area covered by the upper level clouds.

Sources & Further reading

1. Rädel, G., T. Mauritsen, B. Stevens, D. Dommenget, D. Matei, K. Bellomo and A. Clement (2016). "Amplification of El Nino by cloud longwave coupling to atmospheric circulation." Nature Geosci 9(2): 106-110.