Amaya, D., N. Siler, S.-P. Xie and A. J. Miller, 2017:
The interplay of internal and forced modes of Hadley
Cell expansion: Lessons from the global warming
hiatus
Climate Dynamics, 51, 305-319.
Abstract.
The poleward branches of the Hadley Cells and
the edge of the tropics show a robust poleward shift during
the satellite era, leading to concerns over the possible
encroachment of the globe’s subtropical dry zones into currently
temperate climates. The extent to which this trend is
caused by anthropogenic forcing versus internal variability
remains the subject of considerable debate. In this study, we
use a Joint EOF method to identify two distinct modes of
tropical width variability: (1) an anthropogenically-forced
mode, which we identify using a 20-member simulation of
the historical climate, and (2) an internal mode, which we
identify using a 1000-year pre-industrial control simulation.
The forced mode is found to be closely related to the top
of the atmosphere radiative imbalance and exhibits a longterm
trend since 1860, while the internal mode is essentially
indistinguishable from the El Niño Southern Oscillation.
Together these two modes explain an average of 70% of the
interannual variability seen in model “edge indices” over the
historical period. Since 1980, the superposition of forced
and internal modes has resulted in a period of accelerated
Hadley Cell expansion and decelerated global warming (i.e.,
the “hiatus”). A comparison of the change in these modes
since 1980 indicates that by 2013 the signal has emerged
above the noise of internal variability in the Southern Hemisphere,
but not in the Northern Hemisphere, with the latter
also exhibiting strong zonal asymmetry, particularly in the
North Atlantic. Our results highlight the important interplay
of internal and forced modes of tropical width change and
improve our understanding of the interannual variability and
long-term trend seen in observations.
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