Observations of Atmosphere-Ocean coupling in
the North Atlantic
(A. Czaja and J. Marshall, MIT)
Quat. J. of the Royal Meteor. Soc., 2001, 127, 1893-1916
Abstract:
An index of sea surface temperature (SST) variability - DT
- is introduced that measures the difference in SST across the
separated Gulf Stream in late winter.
By analyzing a long observational record of SST and
sea level pressure (SLP), we show that DT exhibits damped
oscillations of decadal period and covaries with the strength of a
dipolar sea level pressure anomaly reminiscent of the North Atlantic
Oscillation (NAO). Analysis in the frequency domain shows a broad band `peak'
at 10-20 yr in DT, with a continuous decrease of power
on longer timescales. Similar spectral signatures
are found in the northern part of the SLP dipole (the Greenland - Icelandic
Low region) but not in its southern part (the subtropical High region),
which keeps its power increasing on timescales longer than about 25 yrs.
The observations are
interpreted in the framework of a delayed-oscillator model in which
the ocean circulation introduces the delay, and modulates
DT on decadal timescales. The decrease of
power seen on long timescales (> 25 yrs) in the DT index
is well reproduced by a model including wind driven ocean circulation,
and arises primarily as a passive
response of the latter to the NAO forcing.
Variability of the ocean's meridional overturning
circulation could also play a role in modulating DT on
decadal timescales.
If a small feedback of DT
on the NAO pattern is introduced, our simple model can
also reproduce the spectral structures seen
in SLP anomaly in the Greenland - Iceland region.
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