29
and convection processes. Hence, the presence of sea ice
strongly modulates interactions between ocean and atmosphere, namely heat, mass, and momentum transfers.
In addition to all physical aspects, sea ice acts as a key
component also for the Arctic ecosystem, it also determines
marine transportation and offshore activities, and is of crucial societal importance. A detailed description of these
aspects is beyond the scope of the present review, but we
refer to the Arctic Climate Impact Assessment – Scientific
Report (ACIA 2004) for a more thorough perspective.
Arctic – Subarctic Atmosphere
Atmospheric Circulation: Why Does It Matter?
The polar regions are the world’s heat sink: at low latitudes
the amount of incoming solar radiation (shortwave) exceeds
the emitted infrared radiation (longwave), whereas there is
an annual energy deficit at the poles, where more heat is
emitted than absorbed. The surplus of energy is then transported from the equatorial region towards the poles in the
atmosphere and ocean. In the atmosphere, this manifests as
global circulation cells, which, due to turbulent interactions,
transfer energy to smaller processes of regional and local
importance forcing climate and weather patterns. The latter
play a very important role in the coupling with ocean and sea
ice, which on the other hand also force changes on the atmospheric circulation. Therefore, global climate and weather
are highly dependent on these interactions between the components of the earth system (Taylor 2009).
Though temperatures have been increasing in polar and
equatorial regions, it has been amplified at high latitudes,
especially over the Arctic (Serreze and Barry 2011). This
amplification is attributed to several feedback mechanisms
(Taylor et al. 2013) and, even though the ice-albedo feedback
is often cited as primary contributor, some studies suggest
that other interactions, like the warming of the lower atmosphere might play a bigger role (Pithan and Mauritsen 2014).
Serreze and Barry (2011) provide a thorough synthesis of
research on Arctic amplification.
The fact that the temperature increase over the Arctic has
been happening at a faster rate than the global average,
decreases the overall meridional temperature gradient over
the globe, which in turn may affect the atmospheric circulation pattern locally as well as remotely (Barnes and Screen
2015). The scientific community has been broadly concerned
with possible changes over mid-latitude weather such as,
e.g., the occurrence of extreme weather events and the weakening and shifting of the westerly winds (Overland 2016).
These winds are strongly coupled to the track and intensity
of storm systems travelling at mid-latitudes, hence it is
expected that changes in the position and strength of the jet
stream leads to noticeable changes in the northern hemispheric daily weather (e.g., Barnes and Screen 2015; Serreze
and Barry 2011).
The particular role and responses of the atmosphere in a
warming climate are beyond the scope of this work. Thus,
for more comprehensive understanding we refer here to several studies which review and investigate responses of largescale atmospheric circulation to changes in sea ice cover
over the Arctic (Budikova 2009; Bader et al. 2011; Vihma
2014; Semmler et al. 2016). Nevertheless, an overview on
the background characteristics of the Arctic atmospheric system are given next.
Major Modes of Atmospheric Circulation
in the Arctic
As explained above, atmospheric circulation and weather are
linked to gradients. The system has an intrinsic seasonal
variability upon which these gradients oscillate. To characterize the major atmospheric modes over the Arctic, a brief
illustration on its climatology is given in terms of sea level
pressure.
The prevailing atmospheric circulation over the Arctic is
anticyclonic, which results from an average high-pressure
system that spawns winds over the region. Although prevalent, the circulation regime may shift to cyclonic on the time
scales of 5–7 years (Proshutinsky et al. 2009). Shifts from
one regime to another are forced by changes in the location
and intensity of the pressure systems described below. This
oscillatory mode is part of the Arctic system’s natural variability and may help to explain the significant, basin-scale
changes of the Arctic atmosphere-ice-ocean system
(Polyakov and Johnson 2000; Proshutinsky et al. 2009,
2015).
The two semi-permanent centers of low pressure, the oceanic Aleutian and Icelandic Lows, and the continental
Siberian High, which extends into the Arctic as the Beaufort
High, are observed as pronounced features during winter. In
summer, the gradients of the polar and subpolar regions are
relatively weak, and sea level pressure distribution is dominated by the subtropical, the Azores and the Pacific Highs
(McBean et al. 2005). To describe the main states of the
atmospheric circulation, indices were created. Based on a
surface variable and obtained through statistical analysis,
these are used to characterize complex climate processes and
explain past variability.
The major mode of variability in the Arctic is the Arctic
Oscillation (AO), and is characterized by the relation between
the surface pressure anomaly in the Arctic and in mid- latitudes
(Thompson and Wallace 1998). When the AO is in its positive
phase, surface pressure in the polar region is low. This mode
manifests as the strengthening of the zonal westerly winds
The Physical System of the Arctic Ocean and Subarctic Seas in a Changing Climate
and convection processes. Hence, the presence of sea ice
strongly modulates interactions between ocean and atmosphere, namely heat, mass, and momentum transfers.
In addition to all physical aspects, sea ice acts as a key
component also for the Arctic ecosystem, it also determines
marine transportation and offshore activities, and is of crucial societal importance. A detailed description of these
aspects is beyond the scope of the present review, but we
refer to the Arctic Climate Impact Assessment – Scientific
Report (ACIA 2004) for a more thorough perspective.
Arctic – Subarctic Atmosphere
Atmospheric Circulation: Why Does It Matter?
The polar regions are the world’s heat sink: at low latitudes
the amount of incoming solar radiation (shortwave) exceeds
the emitted infrared radiation (longwave), whereas there is
an annual energy deficit at the poles, where more heat is
emitted than absorbed. The surplus of energy is then transported from the equatorial region towards the poles in the
atmosphere and ocean. In the atmosphere, this manifests as
global circulation cells, which, due to turbulent interactions,
transfer energy to smaller processes of regional and local
importance forcing climate and weather patterns. The latter
play a very important role in the coupling with ocean and sea
ice, which on the other hand also force changes on the atmospheric circulation. Therefore, global climate and weather
are highly dependent on these interactions between the components of the earth system (Taylor 2009).
Though temperatures have been increasing in polar and
equatorial regions, it has been amplified at high latitudes,
especially over the Arctic (Serreze and Barry 2011). This
amplification is attributed to several feedback mechanisms
(Taylor et al. 2013) and, even though the ice-albedo feedback
is often cited as primary contributor, some studies suggest
that other interactions, like the warming of the lower atmosphere might play a bigger role (Pithan and Mauritsen 2014).
Serreze and Barry (2011) provide a thorough synthesis of
research on Arctic amplification.
The fact that the temperature increase over the Arctic has
been happening at a faster rate than the global average,
decreases the overall meridional temperature gradient over
the globe, which in turn may affect the atmospheric circulation pattern locally as well as remotely (Barnes and Screen
2015). The scientific community has been broadly concerned
with possible changes over mid-latitude weather such as,
e.g., the occurrence of extreme weather events and the weakening and shifting of the westerly winds (Overland 2016).
These winds are strongly coupled to the track and intensity
of storm systems travelling at mid-latitudes, hence it is
expected that changes in the position and strength of the jet
stream leads to noticeable changes in the northern hemispheric daily weather (e.g., Barnes and Screen 2015; Serreze
and Barry 2011).
The particular role and responses of the atmosphere in a
warming climate are beyond the scope of this work. Thus,
for more comprehensive understanding we refer here to several studies which review and investigate responses of largescale atmospheric circulation to changes in sea ice cover
over the Arctic (Budikova 2009; Bader et al. 2011; Vihma
2014; Semmler et al. 2016). Nevertheless, an overview on
the background characteristics of the Arctic atmospheric system are given next.
Major Modes of Atmospheric Circulation
in the Arctic
As explained above, atmospheric circulation and weather are
linked to gradients. The system has an intrinsic seasonal
variability upon which these gradients oscillate. To characterize the major atmospheric modes over the Arctic, a brief
illustration on its climatology is given in terms of sea level
pressure.
The prevailing atmospheric circulation over the Arctic is
anticyclonic, which results from an average high-pressure
system that spawns winds over the region. Although prevalent, the circulation regime may shift to cyclonic on the time
scales of 5–7 years (Proshutinsky et al. 2009). Shifts from
one regime to another are forced by changes in the location
and intensity of the pressure systems described below. This
oscillatory mode is part of the Arctic system’s natural variability and may help to explain the significant, basin-scale
changes of the Arctic atmosphere-ice-ocean system
(Polyakov and Johnson 2000; Proshutinsky et al. 2009,
2015).
The two semi-permanent centers of low pressure, the oceanic Aleutian and Icelandic Lows, and the continental
Siberian High, which extends into the Arctic as the Beaufort
High, are observed as pronounced features during winter. In
summer, the gradients of the polar and subpolar regions are
relatively weak, and sea level pressure distribution is dominated by the subtropical, the Azores and the Pacific Highs
(McBean et al. 2005). To describe the main states of the
atmospheric circulation, indices were created. Based on a
surface variable and obtained through statistical analysis,
these are used to characterize complex climate processes and
explain past variability.
The major mode of variability in the Arctic is the Arctic
Oscillation (AO), and is characterized by the relation between
the surface pressure anomaly in the Arctic and in mid- latitudes
(Thompson and Wallace 1998). When the AO is in its positive
phase, surface pressure in the polar region is low. This mode
manifests as the strengthening of the zonal westerly winds
The Physical System of the Arctic Ocean and Subarctic Seas in a Changing Climate