Hui Su, J. David Neelin and Joyce E. Meyerson
J. Climate, 16, 2003.
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© Copyright 2003 by the American Meteorological Society.
Abstract.
During El Niño, there are substantial tropospheric temperature
anomalies across the entire tropical belt associated with the warming of
sea surface temperature (SST) in the central and eastern Pacific.
The quasi-equilibrium tropical circulation model (QTCM) is used to
investigate the mechanisms for tropical tropospheric temperature
response to SST forcing. In both observations and model simulations,
the tropical-averaged tropospheric temperature anomaly
is approximately linear with
tropical mean SST anomaly 
for
observed SST forcing. Regional SST anomaly experiments are used to
estimate regional sensitivity measures and quantify the degree of
nonlinearity. For instance, the SST anomalies from the 1997-1998 El
Niño give a nonlinear response about 20% greater than a linear
fit to small SST anomaly experiments would predict. Nonlinearity in
tropical-averaged tropospheric temperature response is modest even when
local precipitation response is highly nonlinear. While temperature
anomalies have large spatial scales, the main precipitation anomaly tends
to be local to the SST anomaly regions. The approximate linearity of the
response is due to two factors:
(1) the strong nonlinearities that occur locally tend to be associated
with the transport terms which become small in the large-area average;
and (2) the dependence on temperature of the top-of-atmosphere
and surface fluxes has only weak nonlinearity over the range of
variations. Analytical approximations
to the QTCM suggest that the direct impact of climatological SST, via flux terms,
contributes modestly to regional variations in the sensitivity
of
to .
Wind speed has a fairly strong effect on
but tends to oppose the direct effect of SST since cold SST regions often have stronger
climatological wind which would yield larger slopes.
A substantial contribution to regional variation in
comes from the different reaction of moisture to SST anomalies
in precipitating and non-precipitating regions. Although regions
over climatologically warm water have a slightly higher sensitivity,
subregions of El Niño SST anomalies even in the colder eastern Pacific
contribute substantially to tropospheric temperature anomalies.
Citation. Su, H., J. D. Neelin and J. E. Meyerson, 2003: Sensitivity of tropical tropospheric temperature to sea surface temperature forcing. J. Climate, 16, 1283-1301.
Acknowledgments.
This work was supported under National Science
Foundation Grant ATM-0082529,
National Oceanographic and Atmospheric Administration Grants
NA86GP2003, and National Aeronautics and Space
Administration Grant NAG5-9358.
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