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(like the PN A) an eigenmode of the atmospheric circulation and characterized by a dipole pattern, with opposite changes in the Iceland low and
Azores high. The centers of action, however, are displaced equatorward
by about 10° in the coupled model simulation. All major features in the
canonical SST and 500 hPa height fields are statistically highly significant,
with explained variances up to 60% relative to the low-pass filtered values.
The mechanism behind the decadal variability in the North Atlantic is
similar to that for the North Pacific (see section 3). The variations in heat
content or subsurface temperature are qualitatively similar to those found
in the Pacific (Fig. 3) and show basically the same propagation characteristics (not shown). Thus, we belive that the decadal variability in the
North Atlantic is generated by the same mechanism as its counterpart in
the North Pacific. This would imply that the period of the North Atlantic
mode is about half the period ofthe North Pacific mode, since the basin size
of the North Atlantic is about half the basin size of the North Pacific. Our
results indicate, however, that the time scales of the two oscillations are
very close to each other. The reasons for this are still under investigation.
6 Summary and discussion
A hierarchy of numerical models was used to understand the decadal variability observed in the Northern Hemisphere, and we propose a mechanism for its generation. Our results suggest that a considerable part of
the decadal variability in both the North Pacific and North Atlantic can
be attributed to cycles with periods of approximately 15 to 20 years. The
decadal modes originate from unstable ocean-atmosphere interactions and
must therefore be regarded as inherently coupled phenomena. The existence of such cycles implies the potential for long-range climate forecasting
at decadal time scales over North America and Europe.
The scenario for the generation of the decadal mode is similar to that
developed by Bjerknes (1964) for the Atlantic Ocean. According to this
picture, the memory of the coupled system resides in the ocean. The ocean
adjusts slowly to past variations in the surface wind stress field, and these
slow variations in the wind-driven ocean circulation are crucial in setting
the time scale of the decadal mode. Wave and advective processes are
both found to be important in the ocean adjustment. The wave adjustment, however, appears to be the dominant process, as discussed by Latif
and Barnett (1996). The atmosphere responds passively to the changes
(like the PN A) an eigenmode of the atmospheric circulation and characterized by a dipole pattern, with opposite changes in the Iceland low and
Azores high. The centers of action, however, are displaced equatorward
by about 10° in the coupled model simulation. All major features in the
canonical SST and 500 hPa height fields are statistically highly significant,
with explained variances up to 60% relative to the low-pass filtered values.
The mechanism behind the decadal variability in the North Atlantic is
similar to that for the North Pacific (see section 3). The variations in heat
content or subsurface temperature are qualitatively similar to those found
in the Pacific (Fig. 3) and show basically the same propagation characteristics (not shown). Thus, we belive that the decadal variability in the
North Atlantic is generated by the same mechanism as its counterpart in
the North Pacific. This would imply that the period of the North Atlantic
mode is about half the period ofthe North Pacific mode, since the basin size
of the North Atlantic is about half the basin size of the North Pacific. Our
results indicate, however, that the time scales of the two oscillations are
very close to each other. The reasons for this are still under investigation.
6 Summary and discussion
A hierarchy of numerical models was used to understand the decadal variability observed in the Northern Hemisphere, and we propose a mechanism for its generation. Our results suggest that a considerable part of
the decadal variability in both the North Pacific and North Atlantic can
be attributed to cycles with periods of approximately 15 to 20 years. The
decadal modes originate from unstable ocean-atmosphere interactions and
must therefore be regarded as inherently coupled phenomena. The existence of such cycles implies the potential for long-range climate forecasting
at decadal time scales over North America and Europe.
The scenario for the generation of the decadal mode is similar to that
developed by Bjerknes (1964) for the Atlantic Ocean. According to this
picture, the memory of the coupled system resides in the ocean. The ocean
adjusts slowly to past variations in the surface wind stress field, and these
slow variations in the wind-driven ocean circulation are crucial in setting
the time scale of the decadal mode. Wave and advective processes are
both found to be important in the ocean adjustment. The wave adjustment, however, appears to be the dominant process, as discussed by Latif
and Barnett (1996). The atmosphere responds passively to the changes