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Arctic Warm Event – Exceptional Air Mass Transport and Dynamical Drivers of Extreme Wintertime Warm Event in the Arctic

Maximum temperature in the Arctic reached record high values at the turn of the years 2015/2016, which exceeded the melting point, leading to a strong reduction of the extent of the Arctic ice in the middle of the cold season. In this paper Binder et al. show by the use of a Lagrangian method that a combination of airstreams that differ greatly from each other contributed to this event:

i)                   Warm low-level air from the subtropics,

ii)                Cold low level air that was originally from the Arctic heated by surface fluxes, and

iii)              Air heated by adiabatic compression that was descending strongly.

The transport towards the pole of these air streams occurred along an intense low level jet between a series of cyclones and an anticyclone that was quasi stationary. A continuous warm conveyor belt ascent into the upper part of the anticyclone facilitated this transport that was enabled by the complex 3-D configuration. The combined role of multiple transport processes and transient dynamics on a synoptic scale, for establishing an extreme Arctic warm event was emphasised by this study.

In the Arctic the increase in surface temperatures has been much stronger compared with the global average in recent decades, the process being referred to as Arctic amplification (Serreze & Barry, 2011; Serreze & Francis, 2006; Stocker et al., 2013). Arctic cover of sea ice and concentrations of ice on land have consequently diminished significantly since the beginning of the modern satellite period in 1979 (Mernild et al., 2011; Simmonds, 2015; Stroeve et al., 2007). Possible reasons have been suggested for this enhanced warming in the Arctic which include positive albedo feedback that is associated with snow and ice that are melting (Arrhenius, 1896; Screen & Simmonds, 2010), different radiative feedbacks at low and high latitudes (Pithan & Mauritsen, 2014), changes in cloud cover and atmospheric water vapour in the Arctic (Graversen & Wang, 2009; Winton, 2006), changes in atmospheric circulation patterns (Willett, 1950), and the increased transport of heat and moisture towards the pole (Graversen et al., 2008; Rinke et al., 2017; Woods & Caballero, 2016).

Several short-term episodes with particularly high temperatures and enhanced melting have been superimposed on the long-term melting in the Arctic, such as the record decline in summer of Arctic sea ice in summer 2007 (Graversen et al., 2011), over Greenland an extreme heat and melt event in July 2012 (Nghiem et al., 2012), and in the Siberian Sea a period of rapid reduction of sea ice in August 2014 (Tjernström et al., 2015). During all these episodes the advection towards the pole of warm, moist air occurred (Bonne et al., 2015; Graversen et al., 2011; Neff et al., 2014; Sedlar & Devasthale, 2012; Tjernström et al., 2015) and radiative effects that are associated with the formation of low level liquid clouds and fog (Bennartz et al., 2013; Graversen et al., 2011; Tjernström et al., 2015), contributed essentially to high temperatures and strong melting of ice in the Arctic. The primary role of advection for the Arctic warm extremes contrasts with the processes that lead to warm extremes in the midlatitudes. In the midlatitudes warm extremes are generally a result of strong adiabatic warming in the colocated blocking anticyclones (Pfahl & Wernli, 2012), as well as diabatic heating from the enhanced insolation and surface sensible heat fluxes (Bieli et al,, 2015).

Extreme warm events in winter can also impact substantially sea ice conditions in the Arctic, as can the above mentioned Arctic summer heat episodes. Such a major winter warm event occurred in the Arctic in late December 2015 and early January 2016 (Boisvert et al., 2016; Cullather et al., 2016; Kim et al., 2017; Moore, 2016), which contributed to 2015/2016 being the warmest winter in the Arctic in the observational record (Cullather et al., 2016). The extreme event was the result of very unusual large-scale flow configuration in the early winter of 2015/2016, associated with overall anomalously warm conditions in Europe (NOAA, 2016) and other regional extremes, such as flooding in the UK (Marsh et al., 2016). In this study Binder et al. focused on the Arctic. Buoys at the North Pole measured maximum surface temperatures of -0.8^oC on 30 December (Moore, 2016), and values of 8.7^oC were observed at Svalbard airport station, the warmest temperatures that had ever been recorded at that station between November and April (The Norwegian Meteorological Institute, 2016). The maximum 2m temperature (T2m) to the north of 82^oN reached values greater than 0^oC during 3 short episodes between 29 December 2015 and 4 January 2016, which is almost 30 K above the climatological mean for winter in this region, According to operational analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF). They occurred in the Eurasian Arctic sector in the region around Svalbard and over the Kara Sea, reaching the highest values for winter since 1979. The sea ice thinned by more than 30 cm in the Barents Sea and the Kara Sea as a result of the warming event, and contributed to the record low sea ice extent observed in the Northern Hemisphere in January and February 2016 (National Snow and Ice Data Centre, 2016).

The onset of the extreme event was explained by media reports (Gosden, 2015; Samenow, 2015) and previous studies (Boisvert et al., 2016; Cullather et al., 2016; Kim et al., 2017) to be advection of heat and moisture towards the pole by the storm “Frank,” which passed over Iceland on 30 December 2015 as one of the strongest storms in the North Atlantic on record (Kim et al., 2017). The subsequent formation of a blocking anticyclone over Scandinavia and northwestern Russia, as well as strong long-wave radiation that was associated with the surface warm anomaly contributed, according to Kim et al. (2017), to sustaining the high temperatures in the Arctic. In contrast to this, the warming was attributed (Moore, 2016) to a perturbed Polar Vortex and an Arctic cyclone that advected warm, moist air from the Nordic Seas which are nearby. It is illustrated by these diverging interpretations that the processes which led to this exceptionally extreme event are yet to be explained fully.

For the establishment of anomalous seasonal flow patterns individual weather patterns can be important, and thereby they account for interannual variability of flow (Davies, 2015; Wernli & Papritz, 2017). Understanding processes that resulted in high surface temperatures in the Arctic at the turn of the years 2015/2016 is therefore important because it helps in the gaining of insight into factors contributing to interannual variability of temperatures in the Arctic and concentrations of ice, which are superimposed onto the long-term Arctic warming and decline of the ice, as well as because of the extreme character of the event. This study was aimed at clarifying the origin of the air masses that led to the extreme event and to uncover the meteorological processes that were responsible for the transport towards the pole of the air masses. Also, to place the results in a climatological context, the identified regions, transport processes and synoptic features are compared with reanalysis data from the previous 36 winters.

Unusual Weather Evolution

Binder et al. explored the meteorological setting that enabled the progression of the air masses into the Arctic after investigating the history of the air that contributed to the warm event in the Arctic, in which 3 types of airstreams, each of which had a fundamentally different origin, temperature evolution, and experienced physical processes. Several cyclones developed close to an upper level trough over the central North Atlantic during the days leading up to the warm event. Warm and moist airstreams denoted as warm conveyor belts (WCBs; Browning, 1971; Wernli & Davies, 1997) ascended into the upper troposphere from the surface warm sector (see Madonna et al., 2014 for details of the identification of WCBs). Intense lateral heating, cloud formation and precipitation are always associated with the ascent of such a strong WCB (Browning, 1990). Potential vorticity (PV) modifications are led to by the cloud diabatic processes (Hoskins et al., 1985), whereby early in the phase of the ascent of WCB, PV is produced at low levels and destroyed in the WCB outflow at upper levels (Binder et al., 2016; Madonna et al., 2014, Wernli & Davies, 1997). Therefore, as a net effect the WCB transport introduces air of low PV into the tropopause region, which is illustrated by the intersection positions of WCB with the 310 K isentropic surface. This upward transport of low-PV air in the WCB is attributed to the diabatic amplification of the ridge (see also Grams et al. 2011; Pfahl et al., 2015), as is evident from the location of the WCB intersections at the polewards edge of the upper level ridge between Iceland and Scandinavia.

A poleward upper-level jet developed (blue contours in Fig. 3b, Binder et al., 2017) along the western side of the ridge, and at the surface, and a pronounced poleward low-level jet (black arrows in Fig. 3b, Binder et al., 2017) developed between the Icelandic lows “L₁” and “L₂” and the high pressure system “H” over Europe (Figs. 3a and 3b, Binder et al., 2017). Cyclone “Frank” was located near Newfoundland and still very weak at this time. The initiation of the extreme Arctic warm event was linked to a series of other strong cyclones, as well as being associated with WCBs that were located over the central North Atlantic before the genesis of “Frank”, while this exceptional storm contributed to the maintenance of the unusual synoptic situation (see cyclone marked “L₃” in Figs. 3e, 3f, and S8, discussed below, Binder et al., 2017). A cyclone clustering is the term used for such a series of cyclones (Mailier e al., 2006; Pinto et al., 2014).

The upper level trough over the western North Atlantic and the downstream ridge amplified strongly over the subsequent days (Figs. 3b and 3c, Binder et al., 2017). Ahead of the Icelandic cyclones WCB air masses continued to ascend, and the low-PV air in their outflow enhanced the upper level ridge. This eventually led to a strong, persistent blocking anticyclone (see Croci-Maspoli et al., 2007; Pfahl & Wernli, 2012; Schwierz et al., 2004; for details of the blocking identification) that extended far into the Atlantic from Central Europe (green contours in Figs. 3c and 3e, Binder et al., 2017; Pfahl et al., 2015).

Along with the formation of one of the most intense west-to-east surface pressure gradients and the strongest polewards low level and upper level meridional jets between Iceland and Northern Europe in the entire reanalysis period (Table S1 in supporting information and Figs. 3d and 3f, Binder et al., 2017) went this dipolar pattern in the upper troposphere. Warm air was transported northwards by the intense low level jet, which resulted in anomalously far polewards extension of the warm plume. This plume resulted in Arctic surface air temperatures above the melting point, beginning at 12 UTC 29 December 2015. Over the following days this synoptic situation remained similar, with the Arctic block being maintained by the low-PV air in the outflow from the WCB, and continuing polewards across the persistent west-east SLP gradient. The region closest to the pole returned to conditions that were slightly colder when there were interruptions by 2 short episodes, 2 additional warm plumes with T2m ≥ 0^oC reached the Arctic on 31 December 2015 and 3 January 2016 in the warm sector of the 2 Arctic mesocyclones.

The rapid horizontal transport of the warm air by a polewards low level jet that was exceptionally strong, and because transport occurred mainly over the ocean that was relatively warm, explain how the subtropical air in category S still reached the Arctic with temperature above 0^oC. The cold sector of one of the surface cyclones that contributed to the low pressure over Iceland produced the cold air outbreak that was responsible for the equatorwards advection and strong heating of the Arctic air masses in category A. These parcels of air were again transported into the Arctic along the strong polewards jet in the warm sector of a subsequent Icelandic cyclone. Also, the unusual synoptic situation can explain the temperature evolution of the parcels of air in category M: Strong subsidence in this region resulted from the intense high pressure system that formed over North Europe below the upper level blocking anticyclone. Therefore, the air parcels of category M that moved from west to east along the upper level wave guide descended over western Norway adiabatically, enlarging further the warm pool extending poleward associated with the approach of airstreams S and A.

Conclusions

In this study Binder et al. (2017) investigated the dynamical and physical mechanisms leading to high temperatures in the Arctic at the turn pf the years 2015/2016. The extreme event resulted from a combination of several very unusual processes:

1)    Rapid meridional transport of warm subtropical air (airstream S,

2)    Intense heating of polar air, that was originally cold, by sea air heat fluxes (airstream A), and

3)    Strong adiabatic warming of upper tropospheric air that was originally cold (airstream M).

The polewards extension of an intense upper level blocking anticyclone, which was supported by continuous ascent of the WCB in association with a series of Icelandic cyclones, facilitated the transport of these warm air masses S,A and M, that were fundamentally different from each other, to the North Pole. An unusually strong low level jet that formed between the Icelandic cyclones and a Scandinavian surface anticyclone, which was quasi-stationary, along which the polewards warm advection occurred. It was shown by the quantitative Lagrangian analysis of Binder et al. that it was the complex superposition of diverse dynamical processes of synoptic scale which made the events so extreme, that to attribute the Arctic warm event to a single process would therefore be an unjustified simplification.  As well as these short-term processes, Binder et al. suggest it is likely that the pronounced long-term warming trend in polar regions also played a role for the extreme amplitude of the event. According to Binder et al. attribution studies are needed to quantify the contribution of climate change and Arctic Amplification to such extreme events, and their potentially increased occurrence in a future climate.

Sources & Further reading

1. Binder, H., et al. (2017). "Exceptional Air Mass Transport and Dynamical Drivers of an Extreme Wintertime Arctic Warm Event." Geophysical Research Letters 44(23): 12,028-012,036.