predominate in the equatorial regions, while high pressures
belts predominate in subtropical regions. A convergence of
winds towards the equator, the trade winds, is thus observed
in the tropics at sea level. The trade winds are dry at the start
of their journey, since they are powered by the descending
branch of the Hadley cell. Like the Harmattan over Africa,
they maintain desert conditions on tropical continents. Over
the ocean, there is high evaporation of surface waters heated
by solar radiation and the trade winds pick up this vapor and
carry it towards the low latitudes.
At the equator, with its ascending branch of warm moist
air, low-altitude convergence is manifested by strong convective activity and heavy rainfall. These allow the development of a lush, tropical rainforest on land, while over the
great ocean basins, convective activity is focused along a
narrow longitudinal strip of one hundred kilometers wide:
the intertropical convergence zone. It is in this area that
storms and rain are concentrated: the ‘doldrums’, dreaded by
sailing vessels in the past, and now by solo sailors.
Beyond 30° latitude, the flow of air, deflected eastward
by the Coriolis force, reaches such speeds that it becomes
unstable and breaks into eddies and meanders. Large
meanders in this western circulation appear as vast oscillations, usually between three and six of them, which encircle
the Earth. Depressions and anticyclones succeed each other
in the middle latitudes between 30 and 60° north and south,
creating very variable weather conditions. This is the cause
of the ‘temperate’ climate prevailing in Western Europe. By
mixing the hot subtropical air and cold polar air, these
vortices take over the transfer of the excess energy from the
tropics to the poles from the Hadley circulation. However,
this circulation is affected by the contrast between land and
oceans, and by the presence of mountains, both favoring the
anchoring of global planetary waves whose intensity and
position change over time. These waves impact on the
geographical distribution of climate and cause, for example,
a warmer climate on the west coasts than on the east coasts
of the continents of the northern hemisphere. The contrast
between the climate of Canada and that of France is a
striking example.
There have been very significant changes in the intensity
and location of the winds in the past, particularly during
glacial-interglacial oscillations. They are evidenced by the
presence, more or less marked, of pollen or desert dust
transported to the ocean, sometimes very far from the coast,
where they contribute to marine sedimentation. In the marine
environment, when the wind blows parallel to the coast, it
causes upwelling of deep cold water. Variations in their
intensity, reflecting that of the wind, result in variations in
the temperature of surface water that paleo-oceanographers
have managed to reconstruct (see Chap. 21).
For recent periods, historical records provide information,
sometimes subtle, on the variability of the winds and storms.
For example, during the Little Ice Age, variations in the
position of the winds were detected in the Pacific Ocean by
analyzing the travelling time of galleons transporting goods
between Manila (Philippines) and Acapulco (Mexico). The
General Archive of the Indies held in Seville relates that the
journey could take between less than three months and more
than four months. The routes were always the same:
departing from Manila, the galleons went east, allowing
themselves to be carried by the stable westerly winds. For
the return, they headed west, catching the northeast trade
winds, and the duration of the journey was determined, in
the end, by the location of the opposing winds (from the
southwest) that they encountered as they approached Manila.
Historians were thus able to show the existence of a period
of about forty years in the middle of the seventeenth century
when the headwinds were very common due to a northward
shift of the large depressions. Changes in the strength and
direction of winds are therefore an important manifestation
of past climate changes.
Water Vapor, Clouds and Rainfall
Water in the atmosphere, in the form of vapor in the air, or as
a liquid or ice in clouds, plays an important role in climate
dynamics. Firstly, the amount of water vapor contained in
the air is a function of increasing temperature as defined by
the Clausius-Clapeyron relationship that links the saturation
level of water vapor in air to temperature. When air temperature increases, its water vapor content increases. Furthermore, water vapor is the most important greenhouse gas
in the atmosphere. Any increase in its concentration in the
air in turn induces further warming of the atmosphere,
engaging a positive feedback mechanism which amplifies
the original disturbance.
When the water vapor content exceeds the threshold of
saturation, water vapor condenses, causing cloud formation.
Clouds have a particularly complex role, because of two
opposite effects. On the one hand, they reflect part of the
solar radiation, which has the effect of cooling the surface of
the Earth. On the other hand, they have a greenhouse effect
that causes it to heat up. These two effects are not completely
balanced. On average, with current climate conditions, the
reflectivity effect is more important that the greenhouse
effect, and so, overall, clouds cool the Earth. But in the case
of climate change, will clouds play a moderating or amplifying role? Low-level clouds and clouds in the upper layers
of the troposphere are very different. Low-level clouds are
usually thicker, and reflect solar radiation, but being near the
surface, they have little impact on the greenhouse effect.
Conversely, high altitude clouds like cirrus, are much thinner
and are very cold. They let solar radiation pass through, but
they contribute strongly to an increase in the greenhouse
10
S. Joussaume and J.-C. Duplessy
belts predominate in subtropical regions. A convergence of
winds towards the equator, the trade winds, is thus observed
in the tropics at sea level. The trade winds are dry at the start
of their journey, since they are powered by the descending
branch of the Hadley cell. Like the Harmattan over Africa,
they maintain desert conditions on tropical continents. Over
the ocean, there is high evaporation of surface waters heated
by solar radiation and the trade winds pick up this vapor and
carry it towards the low latitudes.
At the equator, with its ascending branch of warm moist
air, low-altitude convergence is manifested by strong convective activity and heavy rainfall. These allow the development of a lush, tropical rainforest on land, while over the
great ocean basins, convective activity is focused along a
narrow longitudinal strip of one hundred kilometers wide:
the intertropical convergence zone. It is in this area that
storms and rain are concentrated: the ‘doldrums’, dreaded by
sailing vessels in the past, and now by solo sailors.
Beyond 30° latitude, the flow of air, deflected eastward
by the Coriolis force, reaches such speeds that it becomes
unstable and breaks into eddies and meanders. Large
meanders in this western circulation appear as vast oscillations, usually between three and six of them, which encircle
the Earth. Depressions and anticyclones succeed each other
in the middle latitudes between 30 and 60° north and south,
creating very variable weather conditions. This is the cause
of the ‘temperate’ climate prevailing in Western Europe. By
mixing the hot subtropical air and cold polar air, these
vortices take over the transfer of the excess energy from the
tropics to the poles from the Hadley circulation. However,
this circulation is affected by the contrast between land and
oceans, and by the presence of mountains, both favoring the
anchoring of global planetary waves whose intensity and
position change over time. These waves impact on the
geographical distribution of climate and cause, for example,
a warmer climate on the west coasts than on the east coasts
of the continents of the northern hemisphere. The contrast
between the climate of Canada and that of France is a
striking example.
There have been very significant changes in the intensity
and location of the winds in the past, particularly during
glacial-interglacial oscillations. They are evidenced by the
presence, more or less marked, of pollen or desert dust
transported to the ocean, sometimes very far from the coast,
where they contribute to marine sedimentation. In the marine
environment, when the wind blows parallel to the coast, it
causes upwelling of deep cold water. Variations in their
intensity, reflecting that of the wind, result in variations in
the temperature of surface water that paleo-oceanographers
have managed to reconstruct (see Chap. 21).
For recent periods, historical records provide information,
sometimes subtle, on the variability of the winds and storms.
For example, during the Little Ice Age, variations in the
position of the winds were detected in the Pacific Ocean by
analyzing the travelling time of galleons transporting goods
between Manila (Philippines) and Acapulco (Mexico). The
General Archive of the Indies held in Seville relates that the
journey could take between less than three months and more
than four months. The routes were always the same:
departing from Manila, the galleons went east, allowing
themselves to be carried by the stable westerly winds. For
the return, they headed west, catching the northeast trade
winds, and the duration of the journey was determined, in
the end, by the location of the opposing winds (from the
southwest) that they encountered as they approached Manila.
Historians were thus able to show the existence of a period
of about forty years in the middle of the seventeenth century
when the headwinds were very common due to a northward
shift of the large depressions. Changes in the strength and
direction of winds are therefore an important manifestation
of past climate changes.
Water Vapor, Clouds and Rainfall
Water in the atmosphere, in the form of vapor in the air, or as
a liquid or ice in clouds, plays an important role in climate
dynamics. Firstly, the amount of water vapor contained in
the air is a function of increasing temperature as defined by
the Clausius-Clapeyron relationship that links the saturation
level of water vapor in air to temperature. When air temperature increases, its water vapor content increases. Furthermore, water vapor is the most important greenhouse gas
in the atmosphere. Any increase in its concentration in the
air in turn induces further warming of the atmosphere,
engaging a positive feedback mechanism which amplifies
the original disturbance.
When the water vapor content exceeds the threshold of
saturation, water vapor condenses, causing cloud formation.
Clouds have a particularly complex role, because of two
opposite effects. On the one hand, they reflect part of the
solar radiation, which has the effect of cooling the surface of
the Earth. On the other hand, they have a greenhouse effect
that causes it to heat up. These two effects are not completely
balanced. On average, with current climate conditions, the
reflectivity effect is more important that the greenhouse
effect, and so, overall, clouds cool the Earth. But in the case
of climate change, will clouds play a moderating or amplifying role? Low-level clouds and clouds in the upper layers
of the troposphere are very different. Low-level clouds are
usually thicker, and reflect solar radiation, but being near the
surface, they have little impact on the greenhouse effect.
Conversely, high altitude clouds like cirrus, are much thinner
and are very cold. They let solar radiation pass through, but
they contribute strongly to an increase in the greenhouse
10
S. Joussaume and J.-C. Duplessy