at many spatial scales. Here we provide only a short overview of the main interactions between mountain uplift and
climate dynamics to understand how the topographic changes of the Cenozoic have changed climate.
First, depending on its velocity, its angle with the
mountain range and the size of the obstacle, air arriving
towards a mountain range can undergo an orographic ascent
that leads to adiabatic cooling until water condensate and
precipitate on the windward slope, leading to a rainfall
gradient between both sides of the orographic barrier. At the
continental scale, this “rainshadow” produces strong
heterogeneities in rainfall patterns and associated biomes,
such as the ones observed today in Patagonia, for example.
The varying temperature lapse rates, i.e. temperature
decrease with elevation, that depends on air masses characteristics, also produce thermal heterogeneity in elevated
regions. High mountains can also alter the surface radiative
balance through snow-induced changes in albedo, creating
thermal and pressure gradients that alter air circulation.
Lastly, mountain ranges have long been shown to create
different kinds of waves that propagate on the lee side and
vertically, ultimately disturbing winds and the spatial distribution of precipitation (Warner 2004).
The impact of mountains on climate was considered as
early as the middle of the nineteenth century, but it was not
until the 1950s that studies quantified this relief-climate
relationship. The meteorologists, Charney and Eliassen
(1949), followed by Bolin (1950), assessed the impact of
mountain ranges on the mid-latitude westerlies, while Flohn
(1950) suggested that the Tibetan plateau was a source of
sensible heat significant enough to explain the establishment
of the Asian monsoon. Between the 1970s and 1990s, many
studies benefited from the advances in atmospheric general
circulation models to quantify the impact of mountain ranges
on the distribution of arid zones globally and the establishment of the Asian monsoon. Broccoli and Manabe (1992)
suggested that the aridity of Central Asia and the Great
Plains of North America was partly due to the generation of
stationary waves downwind of adjacent mountain ranges.
One specific geological event has captured the attention of
scientists: the uplift of the Tibetan plateau and the Himalayan
range in the context of the India-Asia collision, which occurred
at the beginning of the Eocene (Molnar et al. 2010). The first
assessment studies on the impact of the uplift of the Tibetan
Plateau using climate models were based on sensitivity
experiments with varying elevations of the plateau without
differentiating between the Himalayan and Tibetan uplifts and
without taking other paleogeographic changes into account
(e.g. Kutzbach et al. 1993; Ruddiman et al. 1997). These studies
suggested that the rise of the Tibetan plateau played a crucial
role for atmospheric circulation in general, and for the Asian
monsoon in particular. However, the physical mechanisms
involved, which use the temperature gradients induced by a
large continental area at high altitude, are still in question. Other
studies have shown (i) that the thermal contrast between a flat
continent and the Indian Ocean was enough to initiate a lower
intensity monsoon than the modern monsoon pattern and
(ii) that a prerequisite for this was the removal of the Paratethys
Sea which enhanced the heating of Central Asia, and therefore
the thermal contrast between the Indian Ocean and continental
Asia (Ramstein et al. 1997; Fluteau et al. 1999). More recently,
the separation of air masses on either side of a mountain range
has been mentioned as a major factor in the initiation of convection and associated precipitation (see Boos 2015 for a
review) and many modeling studies using various boundary
conditions have been undertaken to try to understand the exact
role of altitude (e.g., latitudinal position (Zhang et al. 2015,
2018), geographic extension (Chen et al. 2014) and of vegetation cover of the plateau (Hu and Boos 2017) in the establishment and intensification of the Asian monsoon.
Furthermore, the implications of the Himalayan-Tibetan
relief for the climate are not limited to the atmosphere. Rind
et al. (1997) used a general circulation model coupled with
an ocean-atmosphere model (AOGCM) to show that these
uplifts caused a rise in sea surface temperatures (*2 °C) in
the North Atlantic (Norwegian Sea), an increase in heat
transport at high latitudes (a crucial parameter for understanding climate change at high latitudes and possibly the
freezing-over of Greenland) as well as a reduction of about
10% in deep water production in the Norwegian Sea (linked
to the decrease in density of the water mass, due to the
warming of the North Atlantic Ocean). More recently, Su
et al. (2018) suggested that plateau uplift may have contributed to the establishment of the AMOC by changing the
intensity and latitude of the zonal winds, thereby altering
sea-ice formation and deep-water formation.
The impact of the reliefs on ocean-atmosphere dynamics
are not limited to the Late Cenozoic orogenesis, because the
Earth’s history is dotted with uplifts creating mountain ranges
with different locations, extensions, heights, and orientations.
By comparing climate simulations for a world with its current
topography and a “flat earth” world, Maffre et al. (2018)
showed that the orographic barriers of the Andes and the
Rockies constrain the transport of freshwater between the
Pacific and the Atlantic, and thus contribute to the high salinity
of the latter, favoring the formation of deep water. The Andean
uplift is also believed to be responsible for the establishment of
convective precipitation in tropical South America (Poulsen
et al. 2010) and the strengthening of the Humboldt Current
(Sepulchre et al. 2009). The establishment of reliefs in south
and east Africa during the Mio-Pliocene probably led to the
aridification of East Africa (Sepulchre et al. 2006), to the
strengthening of the coastal upwellings of the Benguela current (Jung et al. 2014) and a change in position of the ITCZ in
the Atlantic (Potter et al. 2017).
22 Climate Evolution on the Geological Timescale and the Role …
263
climate dynamics to understand how the topographic changes of the Cenozoic have changed climate.
First, depending on its velocity, its angle with the
mountain range and the size of the obstacle, air arriving
towards a mountain range can undergo an orographic ascent
that leads to adiabatic cooling until water condensate and
precipitate on the windward slope, leading to a rainfall
gradient between both sides of the orographic barrier. At the
continental scale, this “rainshadow” produces strong
heterogeneities in rainfall patterns and associated biomes,
such as the ones observed today in Patagonia, for example.
The varying temperature lapse rates, i.e. temperature
decrease with elevation, that depends on air masses characteristics, also produce thermal heterogeneity in elevated
regions. High mountains can also alter the surface radiative
balance through snow-induced changes in albedo, creating
thermal and pressure gradients that alter air circulation.
Lastly, mountain ranges have long been shown to create
different kinds of waves that propagate on the lee side and
vertically, ultimately disturbing winds and the spatial distribution of precipitation (Warner 2004).
The impact of mountains on climate was considered as
early as the middle of the nineteenth century, but it was not
until the 1950s that studies quantified this relief-climate
relationship. The meteorologists, Charney and Eliassen
(1949), followed by Bolin (1950), assessed the impact of
mountain ranges on the mid-latitude westerlies, while Flohn
(1950) suggested that the Tibetan plateau was a source of
sensible heat significant enough to explain the establishment
of the Asian monsoon. Between the 1970s and 1990s, many
studies benefited from the advances in atmospheric general
circulation models to quantify the impact of mountain ranges
on the distribution of arid zones globally and the establishment of the Asian monsoon. Broccoli and Manabe (1992)
suggested that the aridity of Central Asia and the Great
Plains of North America was partly due to the generation of
stationary waves downwind of adjacent mountain ranges.
One specific geological event has captured the attention of
scientists: the uplift of the Tibetan plateau and the Himalayan
range in the context of the India-Asia collision, which occurred
at the beginning of the Eocene (Molnar et al. 2010). The first
assessment studies on the impact of the uplift of the Tibetan
Plateau using climate models were based on sensitivity
experiments with varying elevations of the plateau without
differentiating between the Himalayan and Tibetan uplifts and
without taking other paleogeographic changes into account
(e.g. Kutzbach et al. 1993; Ruddiman et al. 1997). These studies
suggested that the rise of the Tibetan plateau played a crucial
role for atmospheric circulation in general, and for the Asian
monsoon in particular. However, the physical mechanisms
involved, which use the temperature gradients induced by a
large continental area at high altitude, are still in question. Other
studies have shown (i) that the thermal contrast between a flat
continent and the Indian Ocean was enough to initiate a lower
intensity monsoon than the modern monsoon pattern and
(ii) that a prerequisite for this was the removal of the Paratethys
Sea which enhanced the heating of Central Asia, and therefore
the thermal contrast between the Indian Ocean and continental
Asia (Ramstein et al. 1997; Fluteau et al. 1999). More recently,
the separation of air masses on either side of a mountain range
has been mentioned as a major factor in the initiation of convection and associated precipitation (see Boos 2015 for a
review) and many modeling studies using various boundary
conditions have been undertaken to try to understand the exact
role of altitude (e.g., latitudinal position (Zhang et al. 2015,
2018), geographic extension (Chen et al. 2014) and of vegetation cover of the plateau (Hu and Boos 2017) in the establishment and intensification of the Asian monsoon.
Furthermore, the implications of the Himalayan-Tibetan
relief for the climate are not limited to the atmosphere. Rind
et al. (1997) used a general circulation model coupled with
an ocean-atmosphere model (AOGCM) to show that these
uplifts caused a rise in sea surface temperatures (*2 °C) in
the North Atlantic (Norwegian Sea), an increase in heat
transport at high latitudes (a crucial parameter for understanding climate change at high latitudes and possibly the
freezing-over of Greenland) as well as a reduction of about
10% in deep water production in the Norwegian Sea (linked
to the decrease in density of the water mass, due to the
warming of the North Atlantic Ocean). More recently, Su
et al. (2018) suggested that plateau uplift may have contributed to the establishment of the AMOC by changing the
intensity and latitude of the zonal winds, thereby altering
sea-ice formation and deep-water formation.
The impact of the reliefs on ocean-atmosphere dynamics
are not limited to the Late Cenozoic orogenesis, because the
Earth’s history is dotted with uplifts creating mountain ranges
with different locations, extensions, heights, and orientations.
By comparing climate simulations for a world with its current
topography and a “flat earth” world, Maffre et al. (2018)
showed that the orographic barriers of the Andes and the
Rockies constrain the transport of freshwater between the
Pacific and the Atlantic, and thus contribute to the high salinity
of the latter, favoring the formation of deep water. The Andean
uplift is also believed to be responsible for the establishment of
convective precipitation in tropical South America (Poulsen
et al. 2010) and the strengthening of the Humboldt Current
(Sepulchre et al. 2009). The establishment of reliefs in south
and east Africa during the Mio-Pliocene probably led to the
aridification of East Africa (Sepulchre et al. 2006), to the
strengthening of the coastal upwellings of the Benguela current (Jung et al. 2014) and a change in position of the ITCZ in
the Atlantic (Potter et al. 2017).
22 Climate Evolution on the Geological Timescale and the Role …
263