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Australia: The Land Where Time Began |
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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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |