(wind convergence) at 200 hPa (Fig. 3.18c) and by “D” (wind divergence) at
850 hPa (Fig. 3.18d).
Wei et al. (2018) showed that dynamical and thermodynamical interactions
between the summertime MBL clouds, Azores High circulation, and local SSTs in
the subtropical North Atlantic constitute an important aspect of the summer
interannual climate variability.
The intensity of subtropical highs over the NH oceans during summer has
changed in recent years. However, whether such changes are related to climate
warming remains unclear.
The hydrological cycle is linked to the general circulation of the atmosphere by
water vapor transport. The symmetric component of the general circulation explains
around 40% of the total spatial variance of annual mean net precipitation (precipitation minus evaporation, P–E): precipitation in tropics occurs in the intertropical
convergence zone, leading to vertical motion and moisture transport and forming the
ascending branch of the Hadley cell. In the subtropics, strong subsidence associated
with the descending branch of the Hadley cell leads to a minimum in precipitation
and a region of net evaporation. Poleward this dry zone, transient eddies transport
water vapor into midlatitudes, where precipitation associated to storm track eddies is
high, leading to positive net precipitation. Part of the spatial variance rest of annual
mean P–E is due to asymmetric component of the circulation, taking the form of
extratropical stationary Rossby waves, dry subtropical lows, monsoons, storm
tracks, and Walker circulations (e.g., Wills and Schneider 2015).
Under global warming, model-based studies have diagnosed a weakening and
poleward expansion of the Hadley circulation, and poleward expansion of the
subtropical dry zone (e.g., Lu et al. 2007); an intensification of summertime highs
(Li et al. 2012); and changes in the structure of stationary waves (e.g., Wills and
Schneider 2018). These changes in the general circulation could cause an increasing
anticyclonic circulation over the Mediterranean area in winter at the end-of-century,
as diagnosed in various studies (Giorgi and Lionello 2008; Hoerling et al. 2012). The
atmospheric mechanisms involved and its relevance for the climate change response
have not been clarified yet. Changes in the North Atlantic SST and storm track (e.g.,
Woollings et al. 2012) might be important. Increased high pressure over the Mediterranean in winter is related to increase mass divergence that could have large
regional impact on the hydrological cycle tending to cause a stronger negative P–E.
Such changes have already had implications on the increasing aridification of the
Mediterranean region (Seager et al. 2014).
The Arctic is warming more than twice as quickly as the global surface average.
This phenomenon is known as “Arctic amplification.” It stems predominantly from
the rapid loss of sea ice cover in the region that is one of the most striking
manifestations of climate change (Kay et al. 2011; Stroeve et al. 2012). Some studies
have shown that the Arctic sea ice decline can significantly affect the large-scale
atmospheric dynamics at mid-to-high latitudes of the Northern Hemisphere by
altering storm tracks, jet stream (position and strength), and planetary waves
(Deser et al. 2010; Screen et al. 2013; Peings and Magnusdottir 2014a). Two effects
are identified by Francis and Vavrus (2012), which each contributes to a slower
56
J. M. Sánchez-Laulhé et al.
850 hPa (Fig. 3.18d).
Wei et al. (2018) showed that dynamical and thermodynamical interactions
between the summertime MBL clouds, Azores High circulation, and local SSTs in
the subtropical North Atlantic constitute an important aspect of the summer
interannual climate variability.
The intensity of subtropical highs over the NH oceans during summer has
changed in recent years. However, whether such changes are related to climate
warming remains unclear.
The hydrological cycle is linked to the general circulation of the atmosphere by
water vapor transport. The symmetric component of the general circulation explains
around 40% of the total spatial variance of annual mean net precipitation (precipitation minus evaporation, P–E): precipitation in tropics occurs in the intertropical
convergence zone, leading to vertical motion and moisture transport and forming the
ascending branch of the Hadley cell. In the subtropics, strong subsidence associated
with the descending branch of the Hadley cell leads to a minimum in precipitation
and a region of net evaporation. Poleward this dry zone, transient eddies transport
water vapor into midlatitudes, where precipitation associated to storm track eddies is
high, leading to positive net precipitation. Part of the spatial variance rest of annual
mean P–E is due to asymmetric component of the circulation, taking the form of
extratropical stationary Rossby waves, dry subtropical lows, monsoons, storm
tracks, and Walker circulations (e.g., Wills and Schneider 2015).
Under global warming, model-based studies have diagnosed a weakening and
poleward expansion of the Hadley circulation, and poleward expansion of the
subtropical dry zone (e.g., Lu et al. 2007); an intensification of summertime highs
(Li et al. 2012); and changes in the structure of stationary waves (e.g., Wills and
Schneider 2018). These changes in the general circulation could cause an increasing
anticyclonic circulation over the Mediterranean area in winter at the end-of-century,
as diagnosed in various studies (Giorgi and Lionello 2008; Hoerling et al. 2012). The
atmospheric mechanisms involved and its relevance for the climate change response
have not been clarified yet. Changes in the North Atlantic SST and storm track (e.g.,
Woollings et al. 2012) might be important. Increased high pressure over the Mediterranean in winter is related to increase mass divergence that could have large
regional impact on the hydrological cycle tending to cause a stronger negative P–E.
Such changes have already had implications on the increasing aridification of the
Mediterranean region (Seager et al. 2014).
The Arctic is warming more than twice as quickly as the global surface average.
This phenomenon is known as “Arctic amplification.” It stems predominantly from
the rapid loss of sea ice cover in the region that is one of the most striking
manifestations of climate change (Kay et al. 2011; Stroeve et al. 2012). Some studies
have shown that the Arctic sea ice decline can significantly affect the large-scale
atmospheric dynamics at mid-to-high latitudes of the Northern Hemisphere by
altering storm tracks, jet stream (position and strength), and planetary waves
(Deser et al. 2010; Screen et al. 2013; Peings and Magnusdottir 2014a). Two effects
are identified by Francis and Vavrus (2012), which each contributes to a slower
56
J. M. Sánchez-Laulhé et al.