264
changes at the air-sea interface reinforce the initial SST anomaly, so that ocean and
atmosphere act as a positive feedback system. The atmospheric response, however,
consists also of a wind stress curl anomaly which spins down the subtropical ocean
gyre, thereby reducing the poleward heat transport and the initial SST anomaly. The
ocean adjusts with some time lag to the change in the wind stress curl, and it is this
transient ocean response that allows continuous oscillations. The existence of such
decadal cycles provides the basis of long-range climate forecasting at decadal time
scales.
1 Introduction
The study of decadal climate variability is not only of scientific but also
of enormous social and economical interest. Civilization can cope with
extreme climate variations that last briefly, say a particularly severe winter. Climate variations with time scales of centuries are beyond a single
human's or even a single political system's lifetime and so elicit little but
intellectual interest. But the impact of variations in a key climate variable, say precipitation, that last a decade can generally not be avoided by
society, e.g. the extended droughts in California, Australia, or the Sahel.
We concentrate in this paper on the decadal variability in the North
Pacific and North Atlantic Oceans and describe one particular mechanism
that explains many of the aspects of the decadal variability observed in
these regions. Different competing hypotheses to explain the decadal variability observed in the North Pacific were put forward. Both Trenberth and
Hurrell (1994) and Graham (1994) argue that the decadal variations in the
North Pacific are forced by changes in the equatorial Pacific sea surface
temperature (SST) and subsequent changes in the atmospheric circulation
over the North Pacific. However, both studies disagree about the nature of
the decadal variability. While Graham (1994) and some other studies (e.
g. Nitta and Yamada (1989)) explain the changes in the mid-1970s as a
discrete "shift", Trenberth and Hurrel (1994) view the decadal variability
as more oscillatory.
A completely different hypothesis was offered by Jacobs et al. (1994).
They argue that the recent decadal variation in the North Pacific is caused
by planetary waves which were excited during the strong 1982/1983 El
Nino/Southern Oscillation (ENSO) warm extreme (see Philander (1990)
for a comprehensive review on ENSO). The planetary waves propagate
slowly westward across the Pacific basin at off-equatorial latitudes and
interact eventually with the Kuroshio current, leading 10 large-scale SST
changes at the air-sea interface reinforce the initial SST anomaly, so that ocean and
atmosphere act as a positive feedback system. The atmospheric response, however,
consists also of a wind stress curl anomaly which spins down the subtropical ocean
gyre, thereby reducing the poleward heat transport and the initial SST anomaly. The
ocean adjusts with some time lag to the change in the wind stress curl, and it is this
transient ocean response that allows continuous oscillations. The existence of such
decadal cycles provides the basis of long-range climate forecasting at decadal time
scales.
1 Introduction
The study of decadal climate variability is not only of scientific but also
of enormous social and economical interest. Civilization can cope with
extreme climate variations that last briefly, say a particularly severe winter. Climate variations with time scales of centuries are beyond a single
human's or even a single political system's lifetime and so elicit little but
intellectual interest. But the impact of variations in a key climate variable, say precipitation, that last a decade can generally not be avoided by
society, e.g. the extended droughts in California, Australia, or the Sahel.
We concentrate in this paper on the decadal variability in the North
Pacific and North Atlantic Oceans and describe one particular mechanism
that explains many of the aspects of the decadal variability observed in
these regions. Different competing hypotheses to explain the decadal variability observed in the North Pacific were put forward. Both Trenberth and
Hurrell (1994) and Graham (1994) argue that the decadal variations in the
North Pacific are forced by changes in the equatorial Pacific sea surface
temperature (SST) and subsequent changes in the atmospheric circulation
over the North Pacific. However, both studies disagree about the nature of
the decadal variability. While Graham (1994) and some other studies (e.
g. Nitta and Yamada (1989)) explain the changes in the mid-1970s as a
discrete "shift", Trenberth and Hurrel (1994) view the decadal variability
as more oscillatory.
A completely different hypothesis was offered by Jacobs et al. (1994).
They argue that the recent decadal variation in the North Pacific is caused
by planetary waves which were excited during the strong 1982/1983 El
Nino/Southern Oscillation (ENSO) warm extreme (see Philander (1990)
for a comprehensive review on ENSO). The planetary waves propagate
slowly westward across the Pacific basin at off-equatorial latitudes and
interact eventually with the Kuroshio current, leading 10 large-scale SST