164 Mojib Latif, Axei Timmermann, Anselm Gr6tzner, Christian Eckert, Reinhard Voss
a) CCA 1 SST Corr:O.83 18%
85N~~~~=,=~~~~~~~ 80N~
b) CCA 1 SLP Corr:O.83 52%
85N -r---====-r...,...,=",..,----,.....,......,
80N . :,
15N '--'-~'~ "~ ,._ .. .;,. .. , _ .. _ .. ,....; . _ .. .-;. ... _ .. ~ .. .;..: ~ '~ .. ...; .. ..l. ' ...:. .. ..;:. .•. ~ . '.:.j
80W lOW 60W sow 40W JOW 20W 10W o
Fig.9.11 a) North Atlantic SST anomaly and b) SLP anomaly fields as derived from the
leading CCA mode. These pattems are characteristic of the quasi-decadal mode. From
Timmermann (1996).
by Gratzner et al. (1998) studying the quasi-decadal variability in another coupled
model. Thus, the time scale of the oscillation is probably set by a combination of
advective and wave time scales, since both time scales are of the same order. The
importance of advection in the quasi-decadal variability was also highlighted by
Sutton and Allen (1997) who investigated North Atlantic SST observations for the
period 1945-1989.
In summary, the quasi-decadal variability in the North Atlantic region may be
regarded as an eigenmode of the coupled ocean-atmosphere system, so that the
interactions of the ocean and the atmosphere are crucial to the existence of the
quasi-decadal mode. Ocean dynamics appear to be crucial in producing the oscillatory behaviour at the quasi-decadal time scale. We would like to point out however,
that the quasi-decadal mode is strongly damped (as it is in the observations) and
that the bulk of the quasi-decadal variability may well be explained by the stochastic climate model scenario.
9.4 Interdecadal variability in the North Atlantic
On longer time scales, we expect that variations in the thermohaline circulation
become more and more important. The adjustment time ofthe wind-driven gyres is
of the order of one to two decades only (depending on the ocean basin considered),
while the adjustment time of the thermohaline circulation ranges from decades to
many centuries (depending on which portion of the global thermohaline circulation
is considered). As can be inferred from Fig. 9.10, enhanced variability in both the
a) CCA 1 SST Corr:O.83 18%
85N~~~~=,=~~~~~~~ 80N~
b) CCA 1 SLP Corr:O.83 52%
85N -r---====-r...,...,=",..,----,.....,......,
80N . :,
15N '--'-~'~ "~ ,._ .. .;,. .. , _ .. _ .. ,....; . _ .. .-;. ... _ .. ~ .. .;..: ~ '~ .. ...; .. ..l. ' ...:. .. ..;:. .•. ~ . '.:.j
80W lOW 60W sow 40W JOW 20W 10W o
Fig.9.11 a) North Atlantic SST anomaly and b) SLP anomaly fields as derived from the
leading CCA mode. These pattems are characteristic of the quasi-decadal mode. From
Timmermann (1996).
by Gratzner et al. (1998) studying the quasi-decadal variability in another coupled
model. Thus, the time scale of the oscillation is probably set by a combination of
advective and wave time scales, since both time scales are of the same order. The
importance of advection in the quasi-decadal variability was also highlighted by
Sutton and Allen (1997) who investigated North Atlantic SST observations for the
period 1945-1989.
In summary, the quasi-decadal variability in the North Atlantic region may be
regarded as an eigenmode of the coupled ocean-atmosphere system, so that the
interactions of the ocean and the atmosphere are crucial to the existence of the
quasi-decadal mode. Ocean dynamics appear to be crucial in producing the oscillatory behaviour at the quasi-decadal time scale. We would like to point out however,
that the quasi-decadal mode is strongly damped (as it is in the observations) and
that the bulk of the quasi-decadal variability may well be explained by the stochastic climate model scenario.
9.4 Interdecadal variability in the North Atlantic
On longer time scales, we expect that variations in the thermohaline circulation
become more and more important. The adjustment time ofthe wind-driven gyres is
of the order of one to two decades only (depending on the ocean basin considered),
while the adjustment time of the thermohaline circulation ranges from decades to
many centuries (depending on which portion of the global thermohaline circulation
is considered). As can be inferred from Fig. 9.10, enhanced variability in both the