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is increased, but the response pattern remains virtually unchanged (Latif
and Barnett (1996)). The response pattern is the weak Aleutian-low extreme of the "Pacific North American" (PN A) mode. The response shown
in Fig. 4 arose from an atmospheric model simulation in which tropical
SSTs were near their climatological norms, that is the tropics played a
minor role in the result. This was confirmed by an additional integration
in which SST anomalies south of 25° N were entirely neglected.
The associated changes in the net surface heat flux (Fig. 4b) are such
that they tend to reinforce the SST anomalies over the western North Pacific. Heat is anomalously pumped into the ocean in most of the region
where it is already warm and vice versa (compare Fig. 2b with Fig. 4b).
Changes in the latent and sensible heat fluxes contribute most to the net
surface heat flux anomaly. Furthermore, because the westerlies are weakened over the warm SST anomaly, the mean wind speed is reduced, leading
to reduced mixing in the ocean, which tends also to strengthen the initial
SST anomaly (Miller et al. (1994)). Thus, ocean and atmosphere form
a positive feedback system capable of amplifying an initial disturbance,
giving rise to instability of the coupled system. The growth is eventually
equilibrated by nonlinear processes and the phase switching mechanism
described below.
The changes in the wind stress curl (Fig. 4c) force characteristic changes
in the ocean, eventually reducing the strength of the subtropical gyre and
enhancing meridional SST gradients. The ocean has a memory to past
changes in the wind stress and is not in equilibrium with the atmosphere,
as described below. It is this transient response of the ocean to imposed
wind stress, expressed in terms of planetary wave propagation illustrated
in Fig. 3, that provides a mechanism to switch from one phase of the
decadal mode to another and, hence, enables the coupled system to oscillate. The oscillation can become easily irregular in the presence of high
frequency weather fluctuations. Elements of these ideas were put forward
some decades ago (Namias (1959 and 1969), (Bjerknes (1964), White and
Barnett (1972)).
3.4 Oceanic response experiments
As described above, a crucial part of the mechanism for the generation of
decadal climate variability in the North Pacific is the transient response
of the ocean circulation to low-frequency wind stress variations. In order
is increased, but the response pattern remains virtually unchanged (Latif
and Barnett (1996)). The response pattern is the weak Aleutian-low extreme of the "Pacific North American" (PN A) mode. The response shown
in Fig. 4 arose from an atmospheric model simulation in which tropical
SSTs were near their climatological norms, that is the tropics played a
minor role in the result. This was confirmed by an additional integration
in which SST anomalies south of 25° N were entirely neglected.
The associated changes in the net surface heat flux (Fig. 4b) are such
that they tend to reinforce the SST anomalies over the western North Pacific. Heat is anomalously pumped into the ocean in most of the region
where it is already warm and vice versa (compare Fig. 2b with Fig. 4b).
Changes in the latent and sensible heat fluxes contribute most to the net
surface heat flux anomaly. Furthermore, because the westerlies are weakened over the warm SST anomaly, the mean wind speed is reduced, leading
to reduced mixing in the ocean, which tends also to strengthen the initial
SST anomaly (Miller et al. (1994)). Thus, ocean and atmosphere form
a positive feedback system capable of amplifying an initial disturbance,
giving rise to instability of the coupled system. The growth is eventually
equilibrated by nonlinear processes and the phase switching mechanism
described below.
The changes in the wind stress curl (Fig. 4c) force characteristic changes
in the ocean, eventually reducing the strength of the subtropical gyre and
enhancing meridional SST gradients. The ocean has a memory to past
changes in the wind stress and is not in equilibrium with the atmosphere,
as described below. It is this transient response of the ocean to imposed
wind stress, expressed in terms of planetary wave propagation illustrated
in Fig. 3, that provides a mechanism to switch from one phase of the
decadal mode to another and, hence, enables the coupled system to oscillate. The oscillation can become easily irregular in the presence of high
frequency weather fluctuations. Elements of these ideas were put forward
some decades ago (Namias (1959 and 1969), (Bjerknes (1964), White and
Barnett (1972)).
3.4 Oceanic response experiments
As described above, a crucial part of the mechanism for the generation of
decadal climate variability in the North Pacific is the transient response
of the ocean circulation to low-frequency wind stress variations. In order