19.1 Introduction
327
Ross Couper-Johnston [3] describes the process above as a self-perpetuating
loop:
Whatever the trigger, a self-perpetuating loop between the ocean and atmosphere is initiated. In the case of an anomalous burst of westerly winds, the winds prompt a switch in the
local current, which moves the warm water pool a little to the east. This causes a further
relaxation of the trade winds that allows even more warm water to shift east again. The
angle of the thermocline flattens, and less cold water is upwelled in the east. This decreases
the difference in sea temperatures between east and west and similarly reduces the pressure
gradient. Like a sumo wrestler caught off balance and pushed to the edge of the doyo, the
interplay between atmosphere and the ocean can force the warm pool all the way across the
equator to the boundary of the eastern Pacific.
At the peak of an El Niño, the broad picture in the Pacific is very different from the
norm. The thermocline has flattened out considerably. So the deep, cold water normally
close to the surface off South America is up to 30 metres deeper than usual. The sea-levels
on both sides of the ocean are comparable. And the pressure difference between east and
west has disappeared and, at times, even reversed. This is the flipside of the atmospheric
seesaw that Gilbert Walker first noticed when he coined it the Southern Oscillation. Because
the pressure difference drives the winds, the trade winds disappear and are replaced by
westerlies that can blow nearly all the way to the Americas (pp. 39–40).
Ross Couper-Johnston goes on to describe the processes that lead to the end of
the oscillation. These have relevance for prehistoric voyaging in tropical Polynesia,
for it was the restoration of normal trade winds and westward-flowing currents that
ensured a fast return voyage for tropical Polynesian voyagers who had sailed east
taking advantage of reversed currents and winds:
The changes at the ocean surface are reinforced by two other very important and related
processes evolving tens of metres below. The first is the excitation of a series of massive ‘gravity’ or Kelvin waves, which travel eastwards close to the equator several tens
of metres below the surface. Their effect is to send ‘packets’ of warm water towards the
South American coast, further depressing the thermocline in the east. The second, which is
sparked in response to the Kelvin waves, is the generation of a series of another type of subsurface wave, known as a Rossby wave. These have the opposite effect of Kelvin waves,
heading westward and pulling the thermocline closer to the surface. . .The signal to stop the
physical processes that maintain the El Niño phase begins a new feedback loop, this time in
the opposite direction. Easterly trade winds strengthen and warmer water is pushed further
west, once more enhancing the pressure differential from east to west which drives stronger
trade winds (pp. 40–41).
Because of the risk of hypothermia, oceanic migration in outriggers lying low
enough in the water to allow for paddling is inherently more complicated and perilous than land migration and has to be effected quickly. After the Exodus, the
Israelites (accompanied, no doubt, by herd animals) are said to have wandered for
40 years in the desert in their migration to the Promised Land. Three or four weeks
is probably the very most a migration canoe can afford to be on its ocean road. And
the impact of weather conditions at sea is critical. The reason El Niño events hold
an important place in climate-modulated exploration and migration in the southern
Pacific is that for many months they control both winds and currents in such a way
as to optimally support west–east voyages within tropical Polynesia. Such support
327
Ross Couper-Johnston [3] describes the process above as a self-perpetuating
loop:
Whatever the trigger, a self-perpetuating loop between the ocean and atmosphere is initiated. In the case of an anomalous burst of westerly winds, the winds prompt a switch in the
local current, which moves the warm water pool a little to the east. This causes a further
relaxation of the trade winds that allows even more warm water to shift east again. The
angle of the thermocline flattens, and less cold water is upwelled in the east. This decreases
the difference in sea temperatures between east and west and similarly reduces the pressure
gradient. Like a sumo wrestler caught off balance and pushed to the edge of the doyo, the
interplay between atmosphere and the ocean can force the warm pool all the way across the
equator to the boundary of the eastern Pacific.
At the peak of an El Niño, the broad picture in the Pacific is very different from the
norm. The thermocline has flattened out considerably. So the deep, cold water normally
close to the surface off South America is up to 30 metres deeper than usual. The sea-levels
on both sides of the ocean are comparable. And the pressure difference between east and
west has disappeared and, at times, even reversed. This is the flipside of the atmospheric
seesaw that Gilbert Walker first noticed when he coined it the Southern Oscillation. Because
the pressure difference drives the winds, the trade winds disappear and are replaced by
westerlies that can blow nearly all the way to the Americas (pp. 39–40).
Ross Couper-Johnston goes on to describe the processes that lead to the end of
the oscillation. These have relevance for prehistoric voyaging in tropical Polynesia,
for it was the restoration of normal trade winds and westward-flowing currents that
ensured a fast return voyage for tropical Polynesian voyagers who had sailed east
taking advantage of reversed currents and winds:
The changes at the ocean surface are reinforced by two other very important and related
processes evolving tens of metres below. The first is the excitation of a series of massive ‘gravity’ or Kelvin waves, which travel eastwards close to the equator several tens
of metres below the surface. Their effect is to send ‘packets’ of warm water towards the
South American coast, further depressing the thermocline in the east. The second, which is
sparked in response to the Kelvin waves, is the generation of a series of another type of subsurface wave, known as a Rossby wave. These have the opposite effect of Kelvin waves,
heading westward and pulling the thermocline closer to the surface. . .The signal to stop the
physical processes that maintain the El Niño phase begins a new feedback loop, this time in
the opposite direction. Easterly trade winds strengthen and warmer water is pushed further
west, once more enhancing the pressure differential from east to west which drives stronger
trade winds (pp. 40–41).
Because of the risk of hypothermia, oceanic migration in outriggers lying low
enough in the water to allow for paddling is inherently more complicated and perilous than land migration and has to be effected quickly. After the Exodus, the
Israelites (accompanied, no doubt, by herd animals) are said to have wandered for
40 years in the desert in their migration to the Promised Land. Three or four weeks
is probably the very most a migration canoe can afford to be on its ocean road. And
the impact of weather conditions at sea is critical. The reason El Niño events hold
an important place in climate-modulated exploration and migration in the southern
Pacific is that for many months they control both winds and currents in such a way
as to optimally support west–east voyages within tropical Polynesia. Such support