Aw. Robertson et al., Interdecadal changes in atmospheric low-frequency variability with and without boundary forcing, J ATMOS SCI, 57(8), 2000, pp. 1132-1140
The response of the Max Planck Institute's ECHAM3 atmospheric general circu
lation model to a prescribed decade-long positive anomaly in sea surface te
mperatures (SSTs) over the North Atlantic is investigated. Two 10-yr realiz
ations of the anomaly experiment are compared against a 100-yr control run
of the model with seasonally varying climatological SST using a model spati
al resolution of T42. In addition to the: time-mean response. particular at
tention is paid to changes in intraseasonal variability, expressed in terms
of North Atlantic-European weather regimes. The model regimes are quits re
alistic.
Substantial differences are found in the 700-mb geopotential height held re
sponse between the two decadal realizations. The rime-mean response in the
first sample decade is characterized by the positive (zonal) phase of the N
orth Atlantic oscillation (NAO); this response con be identified with chang
es in the frequency of occurrence of certain weather regimes by about one s
tandard deviation, (Preliminary results of this numerical experiment were r
eported at thr: Atlantic Climate Variability Workshop held at the Lamont-Do
herty Earth Observatory of Columbia University, Palisades, New York, 24-26
September 1997.) By contrast. the second SST anomaly decade shows a localiz
ed trough centered over the British Isles; it projects less strongly onto t
he model's intrinsic weather regimes. The control run itself exhibits prono
unced decade-to-decade variations in the weather regimes' frequency of occu
rrence as well as in its NAO index. The two 10-yr anomaly experiments are i
nsufficient; in length and number, to identify a robust SST response above
this level of intrinsic variability.