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Australia: The Land Where Time Began |
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Arctic Melting of Sea Ice in Summer – Role of Polar Anticyclones and
Middle Latitude Cyclones
Since the late 1970s annual minima in the extent and volume of sea ice
in the Arctic have been decreasing rapidly, with substantial interannual
variability. Anticyclonic circulation anomalies that extend from the
surface to the upper troposphere have characterised summers that have
had a particularly strong reduction of the extent of the
Arctic sea ice. In
this study Wernli et al. investigated the origin of these seasonal
circulation anomalies by identifying individual Arctic anticyclones,
that have a lifetime of typically 10 days, and analysing the transport
of air masses into these systems. It was revealed by this study that
these Arctic anticyclones that were induced episodically were relevant
in the generation of seasonal circulation models. During the transient
episodes with Arctic anticyclones and the seasonal reduction of sea ice
volume correlates with the area averaged frequencies of Arctic
anticyclones polewards of 70oN (correlation coefficient of
0.57). It was shown by a trajectory analysis that these anticyclones
result from the extratropical air masses that have low potential
vorticity being injected into the upper troposphere in the Arctic by
extratropical cyclones. It was emphasised by the results of this study
that the fundamental role of extratropical cyclones and diabatic
processes that are associated with them in the establishment of Arctic
anticyclones and, in turn, seasonal circulation anomalies, which are of
key importance in the understanding of the variability of the Arctic sea
ice melting in summer.
The strong reduction of the extent of Arctic sea ice in the late summer
minimum during the last decades is considered to be one of the most
prominent signals of anthropogenic climate warming that is due to the
increased concentration of greenhouse gas (Simmonds, 2015; Kay, Holland
& Jahn, 2011). This reduction has been found to be nonlinear over time
and it seems the decrease has been stronger after 1996 (Ogi & Rigor,
2013). Also the interannual variability is high (Wettstein & Deser,
2014; Serreze et al., 2016), with the record low of sea ice coverage
being reached in the late summers of 2007 and 2012, and it is
surprisingly predictable, even on relatively short (intraseasonal)
timescales (Stroeve et al., 2014). The main driver of so-called Arctic
amplification is the long term declining trend of Arctic sea ice, i.e.,
the climate warming in the Arctic compared with that at lower latitudes
(Screen & Simmonds, 2010). Arctic amplification leads to, in turn, a
reduced meridional temperature gradient, with consequences for the
dynamics of middle latitude weather and extreme events, that is
potentially Important, though at the present is not well understood
(Cohen et al, 2014; Barnes & Screen, 2016). However, this study
addresses a potential influence in the opposite direction, i.e., the
role played by extratropical dynamics for the high interannual
variability of Arctic sea ice in summer.
There have been a number of different processes that have been suggested
to influence sea ice melting in the Arctic in summer. Polewards oceanic
heat fluxes are generally agreed to have a role, though atmospheric
processes are proposed to be the most important (Döscher, Vihma &
Maksimovich, 2014), including, in particular, low level Arctic clouds
(Kay et al, 2008; Kay & L’Ecuyer, 2013), transporting moisture into the
Arctic (Graversen et al., 2010), Arctic cyclones (Screen, Simmonds &
Kay, 2011; Zhang, Lindsay & Schweiger &Steele, 2013) and seasonal
anticyclonic circulation anomalies (Ogi & Wallace, 2012). Shortwave
radiation, which effectively cools the surface, is reflected by Arctic
clouds, and increase downwards of longwave radiation, that warms the
surface, and there is a net cooling effect in summer (Kay & L’Ecuyer,
2013). A reduction of cloudiness in the summer of 2007 resulted in a
positive shortwave radiation anomaly and warming at low level in the
part of the Arctic Ocean that had the strongest loss of sea ice (Kay &
L’Ecuyer, 2008; Kay & L’Ecuyer, 2013). Also, it is expected that
increasing specific humidity in the Arctic is expected to have
contributed to the loss of sea ice by way of its impact on longwave
radiation (Gong, Feldstein & Lee, 2017; Lee et al, 2017). The transport
of moisture into the Arctic is related strongly to subtropical cyclones
(Jacobson & Vihma, 2010; Doyle et al, 2015). According to Wernli et al.
the direct impact of Arctic cyclones in summer is controversial.
Indications were found that fewer cyclones throughout summer favoured a
stronger retreat of sea ice (Screen, Simmonds & Keay, 2011), on the one
hand, whereas, on the other hand, reduction of sea ice can be
accelerated by intense cyclones, by the induction of motion in the sea
ice (Belchansky, Douglas & Platanov, 2004), or through mechanical
breakup and melt resulting from upwards transport of heat (Zhang et al.,
2013), in late summer in particular.
Arctic cyclones
The importance of anticyclonic circulation models for the variability of
the minimum extent of sea ice (Ogi & Wallace, 2012; Ding et al., 2017)
has been emphasised by several studies in recent years. It was found
that in the summer months with an anomalously anticyclonic mean
circulation at 925 hPa a decrease in the extent of sea ice was
particularly large. It was revealed by a recent study (Ding et al.,
2017) that the structure of these anticyclonic flow anomalies that
extended from the surface to the upper troposphere, that was vertically
coherent, and quantified by the use of a general circulation model that
was coupled to a simple ocean-sea-ice model, that in late summer up to
60% of the decline of the extent of sea ice since 1979 can be attributed
to trends in the summer mean atmospheric circulation. These seasonal
circulation anomalies can, in principle, affect sea ice in 2
complementary ways, at least. They can, on one hand, lead to enhanced
export of sea ice out of the Arctic (kinematic effect), via the Fram
Strait, in particular, when the centre of the anticyclonic flow anomaly
is located over northern Greenland (Wettstein & Deser, 2014; Kwok, 2009;
Tsukernik et al., 2010; Smedsrud et al., 2017), and subsequent melt
outside the Arctic Ocean. The anticyclonic anomalies are, on the other
hand, associated with downwelling motion through most of the
troposphere, which leads to adiabatic warming and a shift towards lower
clouds, which eventually leads to increased longwave downwards radiation
(Ding et al, 2017) (thermodynamic effect). An increase in water vapour
in the warm lower atmosphere amplifies this effect. It has been
speculated that teleconnection with the Asian summer monsoon and the
tropics are responsible for this internal variability (Ding et al, 2017;
Grunsceich & Wang, 2016), as the seasonal upper-level anticyclonic
circulation anomalies are manifestations of internal atmospheric
variability (Ding et al, 2017; Davies, 2015). It is shown by the
following that light is shed on the origin of these seasonal circulation
anomalies. It has been shown that they have resulted from a few episodic
events, which have typical lifetime of 10 days, of Artic anticyclones
per summer season, and that these events are related intimately to the
injection of extratropical air masses to the Arctic that are associated
with cyclones of the middle latitudes.
Circulation anomalies and frequency maps of Arctic anticyclones and
blocks for the summer of 2007 have been shown by Wernli & Papritz. This
summer is characterised by a low tropospheric anticyclonic flow anomaly
in the Beaufort Sea and central Arctic and cyclonic anomalies that are
weaker in the eastern Arctic. The seasonal Arctic anticyclonic anomaly
has a structure that is barotropic (Ding et al, 2017) i.e., it is
revealed by Fig. 1b in this paper by
Wernli & Papritz that almost the same anomaly pattern at 300 hPa
as is seen at 925 hPa. The configuration of this flow anomaly is similar
to the Arctic dipole (Watanabe et al, 2006; Wang et al., 2009), though
shifted towards the west, and it is consistent with a transport drift
that is amplified and larger loss of ice via the Fram Strait (Kwok,
2009). Wernli & Papritz noted as an aside that the seasonal mean Arctic
flow at 300 hPa is cyclonic in all summers, in spite of these pronounced
flow anomalies. It is important that the barotropic anticyclonic flow
anomaly in the Beaufort Sea in the summer of 2007 coincides with a
region where there were Arctic anticyclones that peaked at a frequency
of more than 35%. It is shown by this that the formation of about 3
synoptic scale anticyclones, each of which had a lifetime of about 10
days, accounts for the seasonal circulation anomaly. These anticyclones
are characterised by negative potential vorticity anomalies that are
very strong such that they are classified as blocks. There are other
summers, e.g. in the summers of 2011 and 2012, where the same agreement
is found between the location of barotropic seasonal mean anticyclone
circulation anomalies and frequency maxima of Arctic anticyclones and
blocks.
Impact of melting sea ice
According to Wernli & Papritz as there is striking qualitative agreement
between the frequency of Arctic anticyclones and seasonal flow
anomalies, and the importance of these anomalies for the melting of
summer sea ice (Ogi & Wallace, 2012; Ding et al, 2017) provides the
motivation for considering the time series of the frequency of summer
Arctic anticyclones and the concomitant decrease in the volume
(Schweiger et al., 2011) of sea ice instead of its extent is because the
volume is better constrained by thermodynamics and is affected less by
preconditioning. After 2007 the increased frequency of anticyclones is
in line with the general tendency towards a more anticyclonic summer
circulation in the Arctic (Ogi & Wallace, 2012; Ding et al, 2017). The
loss of sea ice in summer is systematically above the long-term mean
over the same period, with 2014 being the only exception, when there was
also a minimum in the frequency of Arctic cyclones. However, there is
also a high correlation before 2000 when the volume of sea ice in early
summer was less depleted and, therefore, less vulnerable to mechanical
breakup. The 2time series have consequently a relatively high Pearson
correlation of 0.57 over the entire period. The frequency of Arctic
anticyclones is indicated by this to influence significantly the
variability of the sea ice independently of the total amount of sea ice
that is present in early summer. An inverse analysis that quantified the
probability density of 300 hPa geopotential height anomalies for days in
which there was a loss of sea ice that was particularly strong, it is
revealed by a broadening towards more anticyclonic flow conditions
compared with days when there are low and normal volume loss of sea ice,
In order to understand better the underlying physical processes, Wernli
& Papritz quantify the increasing melting of sea ice during the 126
strongest anticyclone events. A suitable statistical approach is
required to identify and quantify a systematic enhancement of daily loss
of sea ice by Arctic anticyclones because of the high case-to-case
variability of Arctic anticyclones and daily loss of sea ice volume
tendencies. When a statistical bootstrap technique is applied in order
to estimate sea ice volume tendency anomalies averaged over many
anticyclone events and compared with climatological conditions, a robust
signal does, however, emerge. Average volume of sea ice that is lost
during anticyclonic events in the Arctic is, in fact, intensified by
about 6 km3/day at the 10% significance level. Within Arctic
anticyclones subsidence induces significant levels of adiabatic warming
to the degree that the temperature at 850 hPa above sea ice within
Arctic anticyclones is enhanced by almost 6 K in the mean. It was
confirmed by a trajectory analysis that air masses in Arctic anticyclone
at 850 hPa are actually mainly heated within the Arctic by adiabatic
descent over the preceding 2 days. Reduced cloud water content results
from the warming and subsidence in the anticyclones, and therefore
enhancement of net shortwave radiation at the surface. A similar
mechanism was found to be relevant in the case of the melting of the
Greenland ice sheet (Hofer, 2017). Also, and consistent with the results
of an earlier study (Ding et al, 2017), downwelling longwave radiation
also increases significantly during Arctic anticyclone events. However,
this increase is limited to the region outside Arctic anticyclones,
where there is a smaller reduction in the cloud content of water than is
the case inside but total column water vapour is enhanced significantly.
It was shown by a case study that the uneven changes of total column
vapour inside and outside Arctic anticyclones result from the import of
moist air masses into the Arctic along the periphery of Arctic
anticyclones, where the meridional transport is at its strongest.
Consequently, surface longwave radiation anomalies are strongest at the
edge and outside of Arctic anticyclones where subsidence reaches its
strongest, though surface shortwave radiation anomalies at the edge and
outside Arctic anticyclones, where subsidence is strongest.
Arctic anticyclones – formation
The formation mechanism of Arctic anticyclones was addressed in this
study by using large ensembles of backwards trajectories from all
anticyclones at 300 hPa. Air masses that are involved in Arctic cyclones
are revealed by their trajectories to be injected into the Arctic upper
troposphere from mid-latitudes. These injections have been found to
occur associated to a large extent with extratropical cyclones and about
half of the air masses experience substantial ascent and latent heating
up to more than 20 K while being transported pole wards, though the
remaining half moves into the Arctic almost along isentropic surfaces
while experiencing a moderate degree of radiative cooling of a few K.
Prior to contributing to the formation of an Arctic anticyclone, some of
the parcels of ascending air reside even in the subtropical oceanic
boundary layer for about 10 days. With their region of origin, strong
ascent and latent heating, they contribute to so-called warm conveyor
belts, which are airstreams that are strongly diabatic, within
extratropical cyclones (Browning, 1990; Madonna et al., 2014) that have
been shown to be of central importance for the amplification of upper
level ridges and blocks in the mid-latitudes (Madonna et al., 2014;
Grams et al, 2011; Pfahl et al., 2015). Their contribution to the
formation of strongly negative potential vorticity anomalies in the
upper troposphere (Madonna et al., 2014; Grams et al., 2011), is the
main reason for the relevance of diabatic airstreams. It was revealed by
the findings of this study that such airstreams and the negative
potential vorticity anomalies that are associated with them also play an
essential role in formation of Arctic anticyclones. In Fig. 4c-h of this
paper the formation of an Arctic anticyclone and the role of latent
heating are portrayed exemplarily for an event that occurred in early
June 2007 (additional fields for this episode are shown in Supplementary
Figs. 10-14 of this paper. Many air parcels over the North Pacific
between 30-50oN on 2 June that will form the upper
tropospheric portion of an Arctic anticyclone a few days later are shown
in Fig. 4c in this paper. Many of them were found to be in the
mid-troposphere following the flow that is imposed by the trough-ridge
pattern. Some air parcels are located in the oceanic boundary layer to
the south of extratropical cyclones that are developing. Over the
following days, the air parcels in the lower troposphere undergo intense
latent heating as they rise within the cyclones’ warm conveyor belts of
the North Pacific, with most of them entering the Arctic between 5 and 9
June over North America as part of the prominent ridge downstream of the
cyclone in the Gulf of Alaska. A weaker plume of extratropical air
entered the Arctic over Siberia on 9 June and merged with the Arctic
anticyclone that was well established in the Beaufort Sea. The
barotropic nature of this exemplary Arctic cyclone was demonstrated by
the sea level pressure and upper level potential vorticity contours.
In conclusion, the importance of the transient, synoptic-scale polar
anticyclones for the melting of Arctic summer sea ice was revealed by
the analyses by Wernli & Papritz. Downwelling leads to adiabatic warming
and a reduction in cloudiness, as well as enhanced net surface
radiation, during these anticyclone events – within anticyclones that
are due to increased shortwave fluxes, and due to enhanced longwave
radiation in their periphery that is associated with an increase in
water vapour. Export and mechanical breakup of thin ice (Ogi & Wallace,
2012) may also be contributed to by winds that are anomalous and at low
level that are associated with these anticyclones. The importance of
seasonal circulation anomalies that have been documented previously, for
the interannual variability of melting (Ogi & Wallace, 2012; Ding et
al., 2017) of summer sea ice is, therefore, as quantified in this study,
was to a large extent caused by variations in the frequency of episodic
Arctic anticyclones.
According to Wernli & Papritz another important conclusion is that
injections of air masses with low potential vorticity into the upper
troposphere by lower latitude cyclones are the means by which these
anticyclones form. Intense heating in the cloud systems of the cyclones
is associated with about half of this transport, which corroborates
further the important role of diabatic processes for atmospheric
dynamics (Grams et al, 2011; Pfahl et al., 2015; Joos & Wernli, 2012).
In the Northern Hemisphere summer cyclone activity is high over the
oceans and the continents (Wernli & Schwierz, 2006) and, therefore, the
injection of air masses into the Arctic occur at all longitudes, with
maximum over Siberia, North America and Europe. This contributes to the
large variability that is observed in the location of the summer mean
anticyclonic circulation anomalies and their impact on the extent
(Serreze et al., 2016) of sea ice in late summer. The results of this
study are in line with an earlier study (Davies, 2015), which emphasises
the role of transient processes on a synoptic scale for the
establishment of seasonal flow anomalies that are, in this case, of
direct relevance for the interannual variability of the extent of
melting of sea ice in the Artic in the northern summer. Therefore, the
realistic representation of the formation of Arctic anticyclones in
global climate models is an essential prerequisite for studying the
interannual variability of Arctic sea ice in the future warmer climate.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |