How Does the Atmosphere Respond to Extratropical SST Anomalies?
- the Role of Transient Eddy Feedbacks
NOAA - CIRES Climage Diagnostics Center, University of Colorado
Much progress has been made over the past decade on the dynamics
determining the atmospheric responses to extratropical SST anomalies.
Most importantly, GCM studies suggest that transient eddy feedbacks
play a determining role in defining the extratropical SST-induced
responses. Hence, the nature of the simulated responses often
depends sensitively on background intrinsic variability associated
with different seasons and models.
The role of eddy feedbacks is examined here in particular in
determining the NAO response to the North Atlantic SST tripole. Large
(100-member) ensembles of GCM experiments reveal that the SST tripole
induces a significant NAO response in late winter (Feb-Apr) when the
model's intrinsic variability projects strongly on the NAO. The
tripole-induced NAO response exhibits also an asymmetry about the sign
of the SST anomaly, with a weaker response in the positive case than
in the negative case. Mechanisms for developing and maintaining the
GCM responses are elucidated through diagnostic experiments using a
linear baroclinic model and a statistical storm track model based on
GCM intrinsic variability.
The NAO-like symmetric response
is primarily maintained by a dipolar anomalous eddy forcing that
results from interactions between the heating-forced anomalous flow
and the Atlantic storm track, as expected from an eddy-feedback
mechanism. To account for the asymmetry of the responses about the
sign of the SST tripole, a nonlinear eddy-feedback mechanism is
considered. The model results demonstrate that, due to its nonlinear
self-interaction, the tripole heating induces a weaker response in the
positive phase than in the negative phase. Interactions of these
nonlinear heating-forced anomalous flows with the Atlantic storm track
result in symmetric eddy vorticity forcings that in turn sustain
asymmetric eddy-forced anomalous flows in the two cases.
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