19
variability within a particular segment of a climate record will happen to
assume the form of a linear trend within a particular period of record
like a century, as is sometimes presumed in studies of global change. For
example, in Figs. 16 and 17 of chapter 2 there is little indication of a
linear trend in Northern Hemisphere surface air temperature during the
period 1945-90, yet there is a distinct 'U shaped' signature of interdecadal
variability during this period.
2.5 Regime shifts and unprecedented events
A discussion of conceptual models of temporal climate variability would not
be complete without mention of special temporal signatures, which are not
likely to be emphasized in studies based on the conventional tools used in
time series analysis. For example, the concept of an abrupt 'regime shift'
has direct relevance to the interdecadal time scale, as discussed below,
and it figures prominently in the influential work of Hansen and Sutera
(1986) who have argued that transitions ofthe Northern Hemisphere winter
circulation between high and low wave amplitude regimes account for a
significant fraction of the low-frequency variability.
The existence of regime shifts could be manifested in a number of different ways. If the individual regimes can be characterized in terms of
distinctive ranges of values of some index, defined as a continuous function
of time, the probability density function (PDF) of that index should be
bi-or multi-modal. Another manifestation could be the existence of persistent flow regimes, marked by rapid onset and termination, as discussed by
Horel (1985).
An oft cited example of a 'regime shift' on the interdecadal time scale
is the warming of the equatorial Pacific, accompanied by a drop in the
'Southern Oscillation Index' (SOl, the Darwin minus Tahiti sea-level pressure difference) that took place in 1976. These changes corr~sponded to
the onset of a warm episode of the ENSO cycle, but the SOl has remained
depressed ever since, apart from brief positive excursions in 1984-85 and
1988-89, as documented in Fig. 22 of chapter 2. Trenberth (1990) has
suggested that the same regime shift was accompanied by a change in the
planetary-scale flow pattern over the extratropical North Pacific, marked
by a pronounced drop in sea-level pressure over this region. We will discuss
these and other changes in the climate system that took place around the
same time in §2.7.
variability within a particular segment of a climate record will happen to
assume the form of a linear trend within a particular period of record
like a century, as is sometimes presumed in studies of global change. For
example, in Figs. 16 and 17 of chapter 2 there is little indication of a
linear trend in Northern Hemisphere surface air temperature during the
period 1945-90, yet there is a distinct 'U shaped' signature of interdecadal
variability during this period.
2.5 Regime shifts and unprecedented events
A discussion of conceptual models of temporal climate variability would not
be complete without mention of special temporal signatures, which are not
likely to be emphasized in studies based on the conventional tools used in
time series analysis. For example, the concept of an abrupt 'regime shift'
has direct relevance to the interdecadal time scale, as discussed below,
and it figures prominently in the influential work of Hansen and Sutera
(1986) who have argued that transitions ofthe Northern Hemisphere winter
circulation between high and low wave amplitude regimes account for a
significant fraction of the low-frequency variability.
The existence of regime shifts could be manifested in a number of different ways. If the individual regimes can be characterized in terms of
distinctive ranges of values of some index, defined as a continuous function
of time, the probability density function (PDF) of that index should be
bi-or multi-modal. Another manifestation could be the existence of persistent flow regimes, marked by rapid onset and termination, as discussed by
Horel (1985).
An oft cited example of a 'regime shift' on the interdecadal time scale
is the warming of the equatorial Pacific, accompanied by a drop in the
'Southern Oscillation Index' (SOl, the Darwin minus Tahiti sea-level pressure difference) that took place in 1976. These changes corr~sponded to
the onset of a warm episode of the ENSO cycle, but the SOl has remained
depressed ever since, apart from brief positive excursions in 1984-85 and
1988-89, as documented in Fig. 22 of chapter 2. Trenberth (1990) has
suggested that the same regime shift was accompanied by a change in the
planetary-scale flow pattern over the extratropical North Pacific, marked
by a pronounced drop in sea-level pressure over this region. We will discuss
these and other changes in the climate system that took place around the
same time in §2.7.