326
19 Correlation of Significant Voyaging Activity with Rare Extreme Climate Events
he claims too that most, and perhaps all, “of the later waves of migration which
brought first one and then another of these linguistic groups to dominance in this
or that part of Europe and southwest Asia seem from their dates to be related to
climate change” [1]. Both macro and micro events, the main thrust of migration and
successive expansions, he sees as climate-modulated.
The maritime expansions involved in the later west–east exploration and colonization of the southern Pacific were likewise climate-modulated. Two climatic
factors stand out as relevant: global warmth for protection against hypothermia
and strong El Niño events capable of reversing the direction of contrary winds and
currents. The major impact of El Niños in tropical Polynesia during the Medieval
Climatic Optimum, however, was as much in precipitating as in supporting migrations. As we have seen, in tropical Polynesia after c. AD 900, and possibly before
where populations were high in relation to resources, El Niños brought in their wake
drought, famine and conflict over land and resources, triggering migrations for the
starving and defeated. Documenting the pattern of strong to severe El Niño events
helps to determine the pulse of migrations in tropical Polynesia and to suggest an
associated chronology.
Since Lamb’s classic was published in 1972–1977, ENSO (El Niño Southern
Oscillation) events have become a major focus for research,largely because of interest generated by significant El Niños in 1972/1973, 1982/1983 and 1997/1998. This
has led to an enriched understanding of the interactions of ocean and atmosphere
and their controlling effects on weather patterns during an El Niño, not just in the
Pacific itself but in the continents round its rim. Droughts in Australia, forest fires
in Indonesia, a doubling of the frequency of cyclones in tropical regions are but a
few of the climatic consequences of a strong ENSO event.
A brief summary of the oceanic and atmospheric forces driving an El Niño might
help to explain its dynamic relevance to oceanic migration. Below we quote from
two clear descriptions of aspects directly affecting sailing conditions in the Pacific.
The first is given by William Burroughs [2]:
The tropical Pacific can be regarded as a thin layer roughly 100m thick, of warm light water
sitting on top of a much deeper layer of colder denser water. The interface between these
two layers is known as the ‘thermocline.’ High SST’s correlate with a deep thermocline and
vice versa. As an ENSO warm event develops, the easterly trade winds that normally drive
the currents in the equatorial Pacific become exceptionally weak. The sea level in the western Pacific falls and the depth of the thermocline is reduced. . . . Intense eastwards currents
between the equator and 10 degrees N carry warm waters away from the western Pacific.
Along the western coast of the Americas there is an increase in sea level that propagates
polewards in both hemispheres. This motion, which may be associated with cyclone pairs
in the atmosphere north and south of the equator. . .reinforces the early flow, propagates
an eastward motion or wave. This is called a “Kelvin wave” (named after Lord Kelvin in
recognition of his fundamental work in wave dynamics).
The changes in the ocean contain two important pieces of information. First, the
observed movement in the ocean is a consequence of the alteration of the winds that normally drive the currents away from South America. This standard pattern produces lower
sea levels in the east than in the west. It also means that cold water is drawn in from higher
latitudes and also from greater depths. As the wind weakens so does the current. Sea levels
rise and warm water spreads back to cover the cold water.
19 Correlation of Significant Voyaging Activity with Rare Extreme Climate Events
he claims too that most, and perhaps all, “of the later waves of migration which
brought first one and then another of these linguistic groups to dominance in this
or that part of Europe and southwest Asia seem from their dates to be related to
climate change” [1]. Both macro and micro events, the main thrust of migration and
successive expansions, he sees as climate-modulated.
The maritime expansions involved in the later west–east exploration and colonization of the southern Pacific were likewise climate-modulated. Two climatic
factors stand out as relevant: global warmth for protection against hypothermia
and strong El Niño events capable of reversing the direction of contrary winds and
currents. The major impact of El Niños in tropical Polynesia during the Medieval
Climatic Optimum, however, was as much in precipitating as in supporting migrations. As we have seen, in tropical Polynesia after c. AD 900, and possibly before
where populations were high in relation to resources, El Niños brought in their wake
drought, famine and conflict over land and resources, triggering migrations for the
starving and defeated. Documenting the pattern of strong to severe El Niño events
helps to determine the pulse of migrations in tropical Polynesia and to suggest an
associated chronology.
Since Lamb’s classic was published in 1972–1977, ENSO (El Niño Southern
Oscillation) events have become a major focus for research,largely because of interest generated by significant El Niños in 1972/1973, 1982/1983 and 1997/1998. This
has led to an enriched understanding of the interactions of ocean and atmosphere
and their controlling effects on weather patterns during an El Niño, not just in the
Pacific itself but in the continents round its rim. Droughts in Australia, forest fires
in Indonesia, a doubling of the frequency of cyclones in tropical regions are but a
few of the climatic consequences of a strong ENSO event.
A brief summary of the oceanic and atmospheric forces driving an El Niño might
help to explain its dynamic relevance to oceanic migration. Below we quote from
two clear descriptions of aspects directly affecting sailing conditions in the Pacific.
The first is given by William Burroughs [2]:
The tropical Pacific can be regarded as a thin layer roughly 100m thick, of warm light water
sitting on top of a much deeper layer of colder denser water. The interface between these
two layers is known as the ‘thermocline.’ High SST’s correlate with a deep thermocline and
vice versa. As an ENSO warm event develops, the easterly trade winds that normally drive
the currents in the equatorial Pacific become exceptionally weak. The sea level in the western Pacific falls and the depth of the thermocline is reduced. . . . Intense eastwards currents
between the equator and 10 degrees N carry warm waters away from the western Pacific.
Along the western coast of the Americas there is an increase in sea level that propagates
polewards in both hemispheres. This motion, which may be associated with cyclone pairs
in the atmosphere north and south of the equator. . .reinforces the early flow, propagates
an eastward motion or wave. This is called a “Kelvin wave” (named after Lord Kelvin in
recognition of his fundamental work in wave dynamics).
The changes in the ocean contain two important pieces of information. First, the
observed movement in the ocean is a consequence of the alteration of the winds that normally drive the currents away from South America. This standard pattern produces lower
sea levels in the east than in the west. It also means that cold water is drawn in from higher
latitudes and also from greater depths. As the wind weakens so does the current. Sea levels
rise and warm water spreads back to cover the cold water.