The contribution of ENSO variability to the recent expansion of the tropical belt

Wednesday, July 29, 2015: 8:30 AM
Joy Monteiro1, John Michael Wallace2, Jai Sukhatme1 and Raghuram G Murtugudde3, (1)Indian Institute of Science, Centre for Atmospheric and Oceanic Sciences, Bangalore, India, (2)University of Washington, Seattle, WA, United States, (3)University of Maryland College Park, 3. Earth System Science Interdisciplinary Center (ESSIC), College Park, MD, United States
Abstract:
The expansion of the tropics from 1979 onward is examined based on
ERA Interim data, using as a metric the sea surface temperature and
OLR and 200 mb zonal wind changes from the 1979-1997 to the 2000-2013
reference period.

The patterns of the differences are qualitatively consistent with
published results and the longitudinally-dependent patterns are
suggestive of an imprint of the distinctive shift toward the cold
polarity of ENSO that took place around the year 1998.
This ENSO-related component of the change in the global circulation
is presumably a reflection of the internal variability of the coupled
atmosphere-ocean system.

We have attempted to quantify this ENSO-related component of the change
using linear regression and remove it to create a set of
“ENSO-residual” patterns. Because the spatial signature of ENSO is to some
degree frequency dependent and the record of atmospheric upper air observations
is not long enough to extensively sample the variability on the interdecadal
time scale, the results must be regarded as tentative. However, it appears
that the ENSO contribution to the changes in zonal wind and OLR over the past 35 years
are large enough to call into question whether there exists a robust,
equatorially-symmetric signature of the widening of the tropics,
exclusive of the ENSO contribution.

This finding may help to explain why the observed widening of the tropics during
recent decades is much more pronounced than the widening simulated in climate models
forced by increasing concentrations of greenhouse gases.

The attached figure shows our results for the 200 mb zonal wind and OLR anomalies. The first row shows the changes between the periods 1979-1997 and 2000-2013. The second row shows the contribution due to ENSO. The third row shows the ENSO-residual. All maps are accompanied by their zonal means to the right. The left half of the figure is the zonal winds and the right half is the OLR.

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