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Abstract:
Analyses indicate that the Atlantic Ocean SST was considerably colder at
the beginning than in the middle of the century. Parallel to this a systematic
change of the North Atlantic SLP pattern was observed. To find out whether
the analyzed SST and SLP changes are consistent, which would indicate that the
SST change was real and not an instrumental artifact, a response experiment
with a low resolution (T21) atmospheric GCM was performed. Two perpetual
January simulations were conducted which differ solely in Atlantic Ocean (40$
- 60N) SST: the ”cold” simulation utilizes the SSTs for the period 1904-13, the
”warm” simulation uses the SST’s for the period 1951—60. Also a ”control” run,
with the model’s standard SST somewhat between the ”cold” and ”warm” SST,
was made. For the response analysis a rigorous statistical approach was taken:
First the null hypothesis of identical horizontal distributions was subjected to
a multivariate significance test. Second, the level of recurrence was estimated.
The multivariate statistical approaches are based on hierarchies of test models.
We examined three different hierarchies: a scale dependent hierarchy based on
spherical harmonics (S), and two physically motivated ones, one based on the
barotropic normal modes of the mean 300 hPa flow (B) and one based on the
eigenmodes of the advection diffusion operator at 1000 hPa (A).
The intercomparison of the ”cold” and ”warm” experiments indicates a signal
in the geostrophic stream function that is in the S-hierarchy significantly nonzero
and highly recurrent. In the A—hierarchy, the low level temperature field is identi—
fied as being significantly and recurrently affected by the altered SST distribution.
The SLP signal is reasonably similar to the observed SLP change. Unexpectedly
the upper level streamfunction signal does not appear significantly nonzero in the
B—hierarchy. If, however the pairs of experiments ”warm vs. control” and ”cold
vs. control” are examined in the B—hierarchy, a highly significant and recurrent
signal emerges. We conclude that the ”cold vs. warm” response is not a ”small
disturbance” which would allow the signal to be described by eigenmodes of the
linearized system.
An analysis of the three—dimensional structure of the signal leads to the hy—
pothesis that two different mechanisms are acting to modify the model’s mean
state. At low levels, the local heating and advection are dominant, but at upper
levels the extratropical signal is a remote response to modifications of the tropical
convection.
