Arctic Melting of Sea Ice in Summer – Role of Polar Anticyclones and Middle Latitude Cyclones

Australia: The Land Where Time Began

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 70^oN (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 30-50^oN 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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