Abstract:
Any consideration of the response of the tropospheric circulation, to
increasing greenhouse gases, to changes in sea-surface temperatures, or
to forced changes in the stratosphere such as Antarctic ozone depletion,
needs to consider the coupled effects of eddies and mean flow. The
Fluctuation-Dissipation Theorem (FDT) is one theoretical tool available
to predict the tropospheric response to forcing. The FDT provides an
estimate of the linear operator relating forcing to response, based only
on the statistics of the unforced tropospheric circulation, calculated
from a suitable time series. The simplest prediction of the FDT, already
exploited in the context of response to stratospheric forcing, is that
response to forcing will be proportional to the longest correlation
timescale in the unforced circulation.
Potentially the FDT can provide more precise information on the
structure and magnitude of the response to an arbitrary forcing. However
the usefulness is limited by (a) sampling issues (i.e. the accuracy of
the prediction is limited by the length of the time series of the
unforced circulation) and (b) the gaussianity assumption in the
traditional form of the FDT. This presentation will discuss these and
other aspects of the FDT as a predictor of changes in the tropospheric
circulation. (This is joint work with Fenwick Cooper, now at University
of Oxford.)