El Niño, Interplay Between the Atmosphere
and the Oceans
Interactions between the atmosphere and the oceans can lead
to climate variability modes. El Niño is a perfect example.
Every two to ten years, an abnormal situation occurs in the
Pacific, manifested by the appearance of unusually warm
waters along the coast of Peru and major disruptions in the
tropical rainfall pattern.
In ‘normal’ periods, the water temperature is around 28–
29 °C in the western tropical Pacific, while it does not
exceed 20–25 °C in the east. This strong asymmetry in
temperature between the east and west maintains the atmospheric circulation which, in turn, maintains the temperature
gradient. The warmer waters provide the heat and humidity
necessary for the development of strong convective activity
over the western Pacific, which develops the air ascendance
associated with the Hadley circulation, while the air descends over the cold waters of the eastern Pacific. This
asymmetry between the eastern and western Pacific is
associated with a circulation called Walker. In turn, the trade
winds at the surface, which blow from east to west, maintain
the east-west temperature gradient.
In the eastern Pacific, they cause a surface current
deflected by the rotation of the Earth to the right in the
northern hemisphere and to the left in the southern hemisphere, which drives out the surface water on both sides of
the equator, and causes an upwelling of cold water to
compensate. The trade winds also propel surface waters
towards the west, where the accumulation of water inhibits
the upwelling process; heated by the Sun, these waters reach
the highest ocean temperatures, thereby favoring intense
convective activity. This situation is called La Niña when the
differences between east-west are particularly strong.
During an El Niño event, the circulation of both the ocean
and the atmosphere change simultaneously following a series of mutual actions and reactions in which it is impossible
to distinguish which of the ocean or the atmosphere triggers
the phenomenon. In particular, the water of the central
Pacific is warmed to 28–29 °C, which has the effect of
moving the high convective activity towards the east. A decrease in the strength of the trade winds in the western
Pacific follows, and possibly even a reversal of their direction. With weaker trade winds, the surface current weakens
and the warm waters of the western Pacific flow back
towards the east, a backlash that warms the central Pacific
and interrupts the upwelling of cold waters at the coasts of
South America. But El Niño starts a series of waves in the
ocean that eventually pans out and restores the so-called
‘normal’ situation.
Because of their well-recorded and varied consequences,
the existence of variations in the intensity and frequency of
El Niño events in the past are known. In the Andes, the
arrival of the warm waters on the Pacific coast brought
heavy, sometimes catastrophic, rainfall that caused floods or
even huge landslides that geologists are able to date. Prehistoric sites also bear traces of these events, and archaeologists have established that certain Andean civilizations
developed during periods when El Niño events were rare or
weak and regressed with the return of torrential rains.
However, the most precise way to reconstruct the sequence
of these events is by analyzing the geochemistry of the
corals abundant in the waters of the equatorial Pacific Ocean
and located at the heart of the phenomenon. For example,
Tarawa Atoll located near the International Date Line (180°
meridian) usually has a dry, almost desertic climate. When
an El Niño event occurs, the warm waters reach it, atmospheric convection becomes intense locally and heavy rains
fall on the entire atoll and the coral reef that surrounds it.
Corals, animals with a calcareous skeleton with recognizable
annual bands, record these rainy passages. Analysis of them
has allowed the number of El Niños in the last century to be
counted and to show that their frequency has changed over
the last hundred years. Conversely, near Australia, which is
usually in a region of warmer water, El Niño events are
characterized by decreased rains and cooling, which were
reliably recorded by the coral reefs of New Guinea or Fiji.
These recordings show that El Niño events have also existed
during periods of glacial climate, and confirm that their
frequency and intensity have varied in the past, the twentieth
century being a period during which they were particularly
strong (Fig. 1.9).
The Terrestrial and Marine Biosphere
The biosphere, defined as all living organisms, also intervenes in the operation of the climate system. Some theories
even propose that, throughout geological time, the biosphere
has contributed to the regulation of climate in order to create
conditions compatible with life.
The Geographical Distribution of the Biosphere
On land, the biosphere is mainly made up of vegetation
which is distributed according to the critical climate characteristics which are sunshine, temperature and precipitation.
Rainforests can only develop if temperature and humidity
conditions are favorable for the twelve months of the year.
They are replaced by a dry forest or savannah if the soil
water content decreases over several months. The savannah
itself becomes increasingly sparse as aridity increases
eventually becoming a desert. North of the tropics, seasonal
16
S. Joussaume and J.-C. Duplessy
and the Oceans
Interactions between the atmosphere and the oceans can lead
to climate variability modes. El Niño is a perfect example.
Every two to ten years, an abnormal situation occurs in the
Pacific, manifested by the appearance of unusually warm
waters along the coast of Peru and major disruptions in the
tropical rainfall pattern.
In ‘normal’ periods, the water temperature is around 28–
29 °C in the western tropical Pacific, while it does not
exceed 20–25 °C in the east. This strong asymmetry in
temperature between the east and west maintains the atmospheric circulation which, in turn, maintains the temperature
gradient. The warmer waters provide the heat and humidity
necessary for the development of strong convective activity
over the western Pacific, which develops the air ascendance
associated with the Hadley circulation, while the air descends over the cold waters of the eastern Pacific. This
asymmetry between the eastern and western Pacific is
associated with a circulation called Walker. In turn, the trade
winds at the surface, which blow from east to west, maintain
the east-west temperature gradient.
In the eastern Pacific, they cause a surface current
deflected by the rotation of the Earth to the right in the
northern hemisphere and to the left in the southern hemisphere, which drives out the surface water on both sides of
the equator, and causes an upwelling of cold water to
compensate. The trade winds also propel surface waters
towards the west, where the accumulation of water inhibits
the upwelling process; heated by the Sun, these waters reach
the highest ocean temperatures, thereby favoring intense
convective activity. This situation is called La Niña when the
differences between east-west are particularly strong.
During an El Niño event, the circulation of both the ocean
and the atmosphere change simultaneously following a series of mutual actions and reactions in which it is impossible
to distinguish which of the ocean or the atmosphere triggers
the phenomenon. In particular, the water of the central
Pacific is warmed to 28–29 °C, which has the effect of
moving the high convective activity towards the east. A decrease in the strength of the trade winds in the western
Pacific follows, and possibly even a reversal of their direction. With weaker trade winds, the surface current weakens
and the warm waters of the western Pacific flow back
towards the east, a backlash that warms the central Pacific
and interrupts the upwelling of cold waters at the coasts of
South America. But El Niño starts a series of waves in the
ocean that eventually pans out and restores the so-called
‘normal’ situation.
Because of their well-recorded and varied consequences,
the existence of variations in the intensity and frequency of
El Niño events in the past are known. In the Andes, the
arrival of the warm waters on the Pacific coast brought
heavy, sometimes catastrophic, rainfall that caused floods or
even huge landslides that geologists are able to date. Prehistoric sites also bear traces of these events, and archaeologists have established that certain Andean civilizations
developed during periods when El Niño events were rare or
weak and regressed with the return of torrential rains.
However, the most precise way to reconstruct the sequence
of these events is by analyzing the geochemistry of the
corals abundant in the waters of the equatorial Pacific Ocean
and located at the heart of the phenomenon. For example,
Tarawa Atoll located near the International Date Line (180°
meridian) usually has a dry, almost desertic climate. When
an El Niño event occurs, the warm waters reach it, atmospheric convection becomes intense locally and heavy rains
fall on the entire atoll and the coral reef that surrounds it.
Corals, animals with a calcareous skeleton with recognizable
annual bands, record these rainy passages. Analysis of them
has allowed the number of El Niños in the last century to be
counted and to show that their frequency has changed over
the last hundred years. Conversely, near Australia, which is
usually in a region of warmer water, El Niño events are
characterized by decreased rains and cooling, which were
reliably recorded by the coral reefs of New Guinea or Fiji.
These recordings show that El Niño events have also existed
during periods of glacial climate, and confirm that their
frequency and intensity have varied in the past, the twentieth
century being a period during which they were particularly
strong (Fig. 1.9).
The Terrestrial and Marine Biosphere
The biosphere, defined as all living organisms, also intervenes in the operation of the climate system. Some theories
even propose that, throughout geological time, the biosphere
has contributed to the regulation of climate in order to create
conditions compatible with life.
The Geographical Distribution of the Biosphere
On land, the biosphere is mainly made up of vegetation
which is distributed according to the critical climate characteristics which are sunshine, temperature and precipitation.
Rainforests can only develop if temperature and humidity
conditions are favorable for the twelve months of the year.
They are replaced by a dry forest or savannah if the soil
water content decreases over several months. The savannah
itself becomes increasingly sparse as aridity increases
eventually becoming a desert. North of the tropics, seasonal
16
S. Joussaume and J.-C. Duplessy