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anomalies in the North Pacific and associated changes in the overlying
atmosphere. Jiang et al. (1995) attribute low-frequency climate variability
in mid-latitudes to the chaotic nature of the ocean that allows, for instance,
multiple equilibria of the wind-driven ocean circulation.
The observations show pronounced decadal variability in the North Atlantic also (e. g. Kushnir (1994)). Most studies attribute the decadal variability in the Atlantic to variations of the thermohaline circulation (e. g.
Yang and Neelin (1993), Delworth et al. (1993), Weisse et al. (1994)).
Such variations are forced by density anomalies at the sea surface which
modify the pattern and strength of oceanic convection, which in turn leads
to large-scale variations in the oceanic general circulation.
Here, we propose an alternative hypothesis for the generation of decadal
variability in both the North Pacific and North Atlantic Oceans which
involves variations in the wind-driven ocean circulation. Our hypothesis is based on unstable large-scale ocean-atmosphere interactions in midlatitudes leading to climate cycles in these regions. The possibility of unstable ocean-atmosphere interactions in mid-latitudes on seasonal and longer
time scales was originally hypothesized by Namias (for the North Pacific)
in a series of papers (e.g. Namias, 1959, and 1969) and by Bjerknes (1964)
(for the North Atlantic). N amias argued that SST anomalies in the North
Pacific can change the transient activity in the atmosphere, which in turn
changes the mean westerly flow reinforcing the initial SST anomalies. The
paper by Bjerknes is interesting, since it postulates a climate cycle with a
period of about 10 years in the Atlantic which involves interactions of the
westwind regime and the subtropical ocean gyre. As we shall see below,
the "early" ideas of both Namias' and Bjerknes' apply remarkably well to
the decadal climate variability observed in both oceans.
The decadal variability is studied by means of a hierarchy of numerical
models and a mechanism is postulated for the generation of decadal variability in mid-latitudes. We describe in section 3 a decadal mode in the
North Pacific from observations and as simulated in a multi-decadal integration with a coupled ocean-atmosphere general circulation model which
is described in section 2. A preliminary description of this mode and the
physics responsible for it were given by Latif and Barnett (1994). Suffice
to say, it has many of the features of the decadal variability observed in the
North Pacific. The results of uncoupled experiments with the individual
model components are also shown in section 3. These uncoupled experiments were performed, in order to get more insight into the nature of the
anomalies in the North Pacific and associated changes in the overlying
atmosphere. Jiang et al. (1995) attribute low-frequency climate variability
in mid-latitudes to the chaotic nature of the ocean that allows, for instance,
multiple equilibria of the wind-driven ocean circulation.
The observations show pronounced decadal variability in the North Atlantic also (e. g. Kushnir (1994)). Most studies attribute the decadal variability in the Atlantic to variations of the thermohaline circulation (e. g.
Yang and Neelin (1993), Delworth et al. (1993), Weisse et al. (1994)).
Such variations are forced by density anomalies at the sea surface which
modify the pattern and strength of oceanic convection, which in turn leads
to large-scale variations in the oceanic general circulation.
Here, we propose an alternative hypothesis for the generation of decadal
variability in both the North Pacific and North Atlantic Oceans which
involves variations in the wind-driven ocean circulation. Our hypothesis is based on unstable large-scale ocean-atmosphere interactions in midlatitudes leading to climate cycles in these regions. The possibility of unstable ocean-atmosphere interactions in mid-latitudes on seasonal and longer
time scales was originally hypothesized by Namias (for the North Pacific)
in a series of papers (e.g. Namias, 1959, and 1969) and by Bjerknes (1964)
(for the North Atlantic). N amias argued that SST anomalies in the North
Pacific can change the transient activity in the atmosphere, which in turn
changes the mean westerly flow reinforcing the initial SST anomalies. The
paper by Bjerknes is interesting, since it postulates a climate cycle with a
period of about 10 years in the Atlantic which involves interactions of the
westwind regime and the subtropical ocean gyre. As we shall see below,
the "early" ideas of both Namias' and Bjerknes' apply remarkably well to
the decadal climate variability observed in both oceans.
The decadal variability is studied by means of a hierarchy of numerical
models and a mechanism is postulated for the generation of decadal variability in mid-latitudes. We describe in section 3 a decadal mode in the
North Pacific from observations and as simulated in a multi-decadal integration with a coupled ocean-atmosphere general circulation model which
is described in section 2. A preliminary description of this mode and the
physics responsible for it were given by Latif and Barnett (1994). Suffice
to say, it has many of the features of the decadal variability observed in the
North Pacific. The results of uncoupled experiments with the individual
model components are also shown in section 3. These uncoupled experiments were performed, in order to get more insight into the nature of the