Scales of External Forcing
117
our habitual use of the word normal for an Atlantic circulation that is no more normal
than any other, but just happens to be our own.
I have discussed the THC only as an example of the responsiveness of the ocean
to changes in the external forcing agents that together determine the quantity and
distribution of solar radiation received by Earth. These are, of course, the factors required
by the Milankevitch theory of climate variability: the obliquity of Earth’s axis and the
eccentricity and precession of our orbit around the sun. To these must (possibly) be added
variability in solar output, having periodicity at 2400, 200, 80–90 years, and on down to
the 22- and 11-year cycles. Noncyclic, episodic changes in volcanic activity also modify
the radiative balance of Earth through changes in atmospheric clarity. Episodes of low
activity (1100–1250 A.D.) and of strong vulcanism (1550–1700 A.D.) have been linked to
the Mediaeval Warm Period of the 11th to 14th centuries and to the Little Ice Age of the
17th and 18th centuries, respectively. The recovery of species distributions from this last
cold event was very slow: the progressive northward advance of “warm,” Atlantic water
benthic organisms (sponges, decapods, and echinoderms) along the continental slopes of
the Barents Sea continued on into the middle of the 20th century (Blacker, 1965).
Even single major eruptions, like that of Mount Pinatubo in 1991, may produce measurable atmospheric cooling. Vulcanism and solar output may also interact: one notes the
coincidence of the Maunder and the Spörer minima of sunspot activity during the recent
cold centuries. The failure of the trade winds of the tropical Pacific, to be discussed later, had
a return interval that varied on a millennial time scale during the Holocene, becoming more
frequent until about 1200 years ago, and then declining in frequency toward the present.
Because the integrated response of the atmosphere, cryosphere, and ocean to changes in
the radiative balance of Earth includes complex feedback mechanisms and nonlinearities,
it is not surprising that a steady state is not achieved. Rather, what takes our attention
is a series of major changes of state, at various time scales, but these occur against a
white-noise background of continual and apparently random change. It is this variability,
or weather, around the present-day climatic mean state that renders the analysis and
prediction of change so very complex and unsatisfactory. Yet the consequences of neither
the white-noise background nor the larger, discrete events (for which explanation is
relatively simple) can be ignored by marine ecologists: for just one example, consider the
lack of predictability in annual recruitment success to marine fish stocks. This is a serious
problem, both for ecosystem analysis and for the stability of marine fisheries, and it is
forced importantly by unpredictable between-year differences in ocean conditions.
The recurrent major changes that take our attention are generated through interaction
between ocean and atmospheric circulation. Because it is the latter that appears to lead
the dance, we are accustomed to using an index derived from the global atmospheric
pressure systems as an index of change in the ocean. As a matter of practical convenience,
of course, we have a much better capability for monitoring, daily or weekly, the state of
the atmosphere than that of the ocean. The low-frequency (or long time-scale) variability
of the circulation of the atmosphere exhibits recurrent and persistent patterns that may
persist for several years, or even several decades, and may be of ocean-basin or planetary
scale. These patterns involve shifts in atmospheric wave and jet-stream locations, of
centers of high and low atmospheric pressure, and are associated with anomalously low
or high sea surface temperatures. A dozen or so major teleconnections of this kind are
recognized by meteorologists: the North Atlantic Oscillation (NAO, winter months), East
Atlantic Pattern (winter), East Atlantic Jet (summer), North Pacific Pattern (spring),
Southern Oscillation (all months), and so on.
For each, we have recourse to an index, or comparison of atmospheric pressure at two
distant points, which provides a simple description of the general state of the wind pattern
over a major ocean basin. However, we should note that such indices may mislead us
in two ways. First, their use suggests greater independence of each indexed phenomenon
117
our habitual use of the word normal for an Atlantic circulation that is no more normal
than any other, but just happens to be our own.
I have discussed the THC only as an example of the responsiveness of the ocean
to changes in the external forcing agents that together determine the quantity and
distribution of solar radiation received by Earth. These are, of course, the factors required
by the Milankevitch theory of climate variability: the obliquity of Earth’s axis and the
eccentricity and precession of our orbit around the sun. To these must (possibly) be added
variability in solar output, having periodicity at 2400, 200, 80–90 years, and on down to
the 22- and 11-year cycles. Noncyclic, episodic changes in volcanic activity also modify
the radiative balance of Earth through changes in atmospheric clarity. Episodes of low
activity (1100–1250 A.D.) and of strong vulcanism (1550–1700 A.D.) have been linked to
the Mediaeval Warm Period of the 11th to 14th centuries and to the Little Ice Age of the
17th and 18th centuries, respectively. The recovery of species distributions from this last
cold event was very slow: the progressive northward advance of “warm,” Atlantic water
benthic organisms (sponges, decapods, and echinoderms) along the continental slopes of
the Barents Sea continued on into the middle of the 20th century (Blacker, 1965).
Even single major eruptions, like that of Mount Pinatubo in 1991, may produce measurable atmospheric cooling. Vulcanism and solar output may also interact: one notes the
coincidence of the Maunder and the Spörer minima of sunspot activity during the recent
cold centuries. The failure of the trade winds of the tropical Pacific, to be discussed later, had
a return interval that varied on a millennial time scale during the Holocene, becoming more
frequent until about 1200 years ago, and then declining in frequency toward the present.
Because the integrated response of the atmosphere, cryosphere, and ocean to changes in
the radiative balance of Earth includes complex feedback mechanisms and nonlinearities,
it is not surprising that a steady state is not achieved. Rather, what takes our attention
is a series of major changes of state, at various time scales, but these occur against a
white-noise background of continual and apparently random change. It is this variability,
or weather, around the present-day climatic mean state that renders the analysis and
prediction of change so very complex and unsatisfactory. Yet the consequences of neither
the white-noise background nor the larger, discrete events (for which explanation is
relatively simple) can be ignored by marine ecologists: for just one example, consider the
lack of predictability in annual recruitment success to marine fish stocks. This is a serious
problem, both for ecosystem analysis and for the stability of marine fisheries, and it is
forced importantly by unpredictable between-year differences in ocean conditions.
The recurrent major changes that take our attention are generated through interaction
between ocean and atmospheric circulation. Because it is the latter that appears to lead
the dance, we are accustomed to using an index derived from the global atmospheric
pressure systems as an index of change in the ocean. As a matter of practical convenience,
of course, we have a much better capability for monitoring, daily or weekly, the state of
the atmosphere than that of the ocean. The low-frequency (or long time-scale) variability
of the circulation of the atmosphere exhibits recurrent and persistent patterns that may
persist for several years, or even several decades, and may be of ocean-basin or planetary
scale. These patterns involve shifts in atmospheric wave and jet-stream locations, of
centers of high and low atmospheric pressure, and are associated with anomalously low
or high sea surface temperatures. A dozen or so major teleconnections of this kind are
recognized by meteorologists: the North Atlantic Oscillation (NAO, winter months), East
Atlantic Pattern (winter), East Atlantic Jet (summer), North Pacific Pattern (spring),
Southern Oscillation (all months), and so on.
For each, we have recourse to an index, or comparison of atmospheric pressure at two
distant points, which provides a simple description of the general state of the wind pattern
over a major ocean basin. However, we should note that such indices may mislead us
in two ways. First, their use suggests greater independence of each indexed phenomenon