atmosphere

Article Satellite-Derived Spatio-Temporal Distribution and Parameters of North Atlantic Polar Lows for 2015–2017

Pavel Golubkin *, Julia Smirnova and Leonid Bobylev

Nansen International Environmental and Remote Sensing Centre, 199034 St. Petersburg, Russia; [email protected] (J.S.); [email protected] (L.B.) * Correspondence: [email protected]

Abstract: A list of North Atlantic polar lows was compiled for 2015–2017. A total of 131 polar lows were found by analyzing the Moderate Resolution Imaging Spectroradiometer (MODIS) infrared imagery and auxiliary information. The study region was additionally divided by the 20◦ W meridian to assess possible differences in the polar lows occurring in the western and eastern parts of the region. The highest polar low activity was found over the and the northern . A large number of polar lows over this region were dual or multiple. When considering such systems as a single event, more polar lows were found in 2015 over the and southern Davis Strait, which is the region with the second highest number of polar lows. High interannual variability of polar low frequency was noted, which was more pronounced in the western part of the region. During the analyzed period, the largest number of polar lows occurred in January for the western part of the region and in February for the eastern part. The main polar low parameters were similar within the region, with the mean values slightly higher in the western part of the region, but  all extreme high values were observed in the eastern part. 

Citation: Golubkin, P.; Smirnova, J.; Keywords: polar lows; polar mesoscale ; North Atlantic; Bobylev, L. Satellite-Derived Spatio-Temporal Distribution and Parameters of North Atlantic Polar Lows for 2015–2017. Atmosphere 2021, 1. Introduction 12, 224. https://doi.org/10.3390/ Polar lows are intense maritime mesoscale cyclones occurring over the high latitude atmos12020224 ice-free areas [1]. High near-surface wind speeds allow distinguishing them from other polar mesoscale cyclones with 15 m/s often used as a threshold (e.g., [2,3]). Polar lows are Academic Editor: Mario characterized by short lifetimes (generally, less than 48 h) and small sizes (generally, less Marcello Miglietta than 600 km), and are often not captured by synoptic charts or atmospheric re-analyses [4–6]. Received: 21 December 2020 Near-surface wind speeds within polar lows may exceed 30 m/s and significant wave Accepted: 30 January 2021 Published: 6 February 2021 height may exceed 10 m [7–9]. Polar lows are, thus, certainly dangerous maritime events.

Publisher’s Note: MDPI stays neutral Over the years, many studies [10–13] revealed different mechanisms responsible for with regard to jurisdictional claims in polar low development, including the baroclinic instability, conditional instability of the published maps and institutional affil- second kind (CISK), and wind-induced surface heat exchange (WISHE). Later, it became iations. apparent that a variety of polar lows exists, referred to as the “polar low spectrum,” which has purely baroclinic systems at one end and purely convective at the other [1]. Consequently, there are different definitions of polar lows and criteria for their identification in different studies. Here, we adopt the widely used definition from Rasmussen and Turner [1], which reads: “A polar low is a small, but fairly intense, maritime that Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. forms poleward of the main baroclinic zone (the or other major baroclinic zone). This article is an open access article The horizontal scale of the polar low is approximately between 200 and 1000 km and distributed under the terms and surface winds near or above force.” conditions of the Creative Commons In the Northern Hemisphere, the main polar low genesis regions are the North Pacific Attribution (CC BY) license (https:// Ocean [14] including the and the Bering Sea [15], the Sea of [16], creativecommons.org/licenses/by/ and the North Atlantic, where the highest polar low activity is found [17]. Part of this 4.0/). region, i.e., the Nordic and Barents seas, is most extensively studied. First, climatologies

Atmosphere 2021, 12, 224. https://doi.org/10.3390/atmos12020224 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, x FOR PEER REVIEW 2 of 17

Atmosphere 2021, 12, 224 and the North Atlantic, where the highest polar low activity is found [17]. Part of2 ofthis 15 region, i.e., the Nordic and Barents seas, is most extensively studied. First, climatologies for the region used synoptic charts and only a few satellite images [18,19] and found about for6.5 polar the region lows per used year. synoptic Increased charts availability and only of a satellite few satellite data allowed images more [18,19 recent] and studies found aboutto reveal 6.5 polarconsiderably lows per more year. polar Increased lows. availabilityHowever, the of satellite estimates data vary allowed greatly more (i.e., recent from studiesabout 12 to to reveal 45 cases considerably per year) more mostly polar due lows. to the However, different the polar estimates low identification vary greatly (i.e.,ap- fromproaches about [2,3,20,21]. 12 to 45 cases per year) mostly due to the different polar low identification approachesIn contrast, [2,3, 20less,21 studies]. focused on the regions within the North Atlantic west of 20° W. DedicatedIn contrast, studies less [22,23] studies used focused polar-orbiting on the regions and geostationary within the North satellite Atlantic imagery west and of 20 re-◦ W.ported Dedicated a relatively studies high [22 ,number23] used of polar-orbiting polar lows and and mesoscale geostationary vortices satellite over imagery the eastern and reportedCanadian a waters relatively with high the number highest of activity polar lows over and the mesoscaleLabrador vorticesSea and oversouthern the eastern Davis CanadianStrait. In Kolstad waters with[24], thea frequent highest activityoccurrence over of the favorable Labrador conditions Sea and southern for polar Davis low devel- Strait. Inopment Kolstad was [24 documented,], a frequent occurrencebut rather few of favorable polar lows conditions (19 in 10 for ) polar low were development found west wasof 20° documented, W by analyzing but rather the Advanced few polar lowsVery (19High in 10Resolution seasons) Radiometer were found west(AVHRR) of 20◦ quick-W by analyzinglook imagery. the AdvancedA slightly Veryhigher High number Resolution of polar Radiometer lows (10 (AVHRR)in 2 years) quicklook was found imagery. in Blech- A slightlyschmidt higher[2] for numberthe Irminger of polar Sea lowsby using (10 in AVHRR 2 years) quicklook was found imagery. in Blechschmidt This contrasts [2] for with the Irmingerthe model-based Sea by using and reanalysis-based AVHRR quicklook objective imagery. climatologies, This contrasts which with the found model-based polar low and ac- reanalysis-basedtivity over the Irminger objective Sea climatologies, to be the highest which [25] found or second polar lowhighest activity [17] overwithin the the Irminger North SeaAtlantic. to be the highest [25] or second highest [17] within the North Atlantic. In this this study, study, we we identify identify polar polar lows lows over over the theNorth North Atlantic Atlantic from from 2015 2015to 2017 to using 2017 usinga uniform a uniform satellite-based satellite-based method method to estimate to estimate and intercompare and intercompare their their frequency frequency and andpa- parametersrameters within within the the region, region, and and to produce to produce a po alar polar low lowlist, which list, which might might be useful be useful in other in otherpolar polarlow-related low-related studies. studies.

2. Data and Methods ◦ The study region includes thethe NorthNorth AtlanticAtlantic OceanOcean northnorth ofof 5050° N and adjacent parts of the , i.e., the BaffinBaffin BayBay andand thethe GreenlandGreenland andand BarentsBarents Seas,Seas, which,which, forfor brevity, are collectively referred to here as the North Atlantic (Figure1). brevity, are collectively referred to here as the North Atlantic (Figure 1).

FigureFigure 1.1. TheThe studystudy regionregion andand thethe mainmain areasareas withinwithin it.it. The blue shaded area denotes the median cold (October- April) sea ice extent for 2015–2017. April) sea ice extent for 2015–2017.

Mesoscale vortices in this region were first identified using the Moderate Resolu- tion Imaging Spectroradiometer (MODIS) thermal infrared imagery (channel 31, 10.78– 11.28 µm). Level 1B calibrated radiances with 1-km spatial resolution (MOD021/MYD021)

Atmosphere 2021, 12, x FOR PEER REVIEW 3 of 17

Atmosphere 2021, 12, 224 3 of 15 Mesoscale vortices in this region were first identified using the Moderate Resolution Imaging Spectroradiometer (MODIS) thermal infrared imagery (channel 31, 10.78–11.28 µm). Level 1B calibrated radiances with 1-km spatial resolution (MOD021/MYD021) along along with the corresponding geolocation products (MOD03/MYD03) were obtained from with the corresponding geolocation products (MOD03/MYD03) were obtained from the the NASA’s Level-1 and Atmosphere Archive & Distribution System (LAADS) Distributed NASA’s Level-1 and Atmosphere Archive & Distribution System (LAADS) Distributed Active Archive Center (DAAC) (https://ladsweb.modaps.eosdis.nasa.gov/ (accessed on Active Archive Center (DAAC) (https://ladsweb.modaps.eosdis.nasa.gov/). The MODIS 28 January 2021)). The MODIS instruments onboard the Terra and Aqua polar orbiting instruments onboard the Terra and Aqua polar orbiting satellites provide almost uniform satellites provide almost uniform coverage of the northern areas of the study region (i.e., coverage of the northern areas of the study region (i.e., Barents Sea, northern Norwegian Barents Sea, northern Norwegian and Seas) as consecutive satellite swaths for and Greenland Seas) as consecutive satellite swaths for a certain area are separated by no a certain area are separated by no more than about 1 h 40 min with only one temporal more than about 1 h 40 min with only one temporal gap between the swaths per day, gap between the swaths per day, which does not exceed 4 h. However, for more south- which does not exceed 4 h. However, for more southern latitudes (i.e., <65° N), there are ern latitudes (i.e., <65◦ N), there are two 6–8 h gaps between the available data per day. two 6–8 h gaps between the available data per day. Hence, the AVHRR channel 4 quick- Hence, the AVHRR channel 4 quicklook infrared imagery from the MetOp-B and NOAA-19 satelliteslook infrared provided imagery by thefrom Dundee the MetOp-B Satellite and Receiving NOAA-19 Station satellites at the provided University by the of Dundee Dundee Satellite Receiving Station at the University of Dundee (presently, Dundee Satellite Station (presently, Dundee Satellite Station Ltd. Dundee, UK) was additionally used to better assess theLtd. development Dundee, UK) and was displacement additionally used of a to system better in assess question the development when they were and uncertain.displace- ment of a system in question when they were uncertain. However, this did not consider- However, this did not considerably reduce the gaps (generally, by about only 1 h) as the groundably reduce tracks the of gaps the MetOp-B(generally, and by about NOAA-19 only 1 satellites h) as the were ground close tracks in space of the and MetOp-B time to thoseand NOAA-19 of the Terra satellites and Aqua were satellites, close in respectively.space and time AVHRR to those imagery of the Terra was thus and notAqua used sat- in theellites, construction respectively. of polar AVHRR low imagery tracks or was estimation thus not of used their in parameters. the construction of polar low tracksMaximum or estimation near-surface of their parameters. wind speeds within the identified mesoscale vortices were estimatedMaximum using near-surface the data from wind the Advancedspeeds within Scatterometer the identified (ASCAT) mesoscale instrument vortices onboard were theestimated MetOp-A using and the MetOp-B data from satellites. the Advanced Level 2 Scatterometer wind vectors with(ASCAT) 12.5-km instrument spatial resolutiononboard werethe MetOp-A obtained and from MetOp-B the NASA’s satellites. Physical Level Oceanography 2 wind vectors DAAC with 12.5-km (https://podaac.jpl.nasa. spatial resolution gov/were (accessedobtained onfrom 28 January the NASA’s 2021)). ExceedancePhysical ofOceanography the 15 m/s thresholdDAAC at(https://po- least once duringdaac.jpl.nasa.gov/). lifetime was requiredExceedance for aof system the 15 tom/s be th furtherreshold considered at least once as a during polar low. lifetime Examples was ofrequired the identified for a system polar lowsto be overfurther the consider Labradored and as a Barents polar low. Seas Exam are shownples of in the Figure identified2. The accuracypolar lows of over the ASCATthe Labrador wind and speed Barents data, whichSeas are is shown about 1in m/s Figure [26 ],2. isThe only accuracy of concern of the in borderlineASCAT wind cases, speed where data, it couldwhich impact is about the 1 m/s decision [26], whetheris only of to concern include in or borderline dismiss the cases, case. Towhere mitigate it could the possibilityimpact the ofdecision the incorrect whether inclusion to include of cases,or dismiss the wind the case. speed To threshold mitigate the was requiredpossibility to of be the exceeded incorrect in inclusion at least four of cases, adjacent the gridwind cells. speed There threshold is, however, was required a possibility to be ofexceeded wind speed in at underestimationleast four adjacent leading grid cells. to the There incorrect is, however, omission a possibility of cases. of wind speed underestimation leading to the incorrect omission of cases.

FigureFigure 2. 2.Examples Examples of of polarpolar lowslows identifiedidentified in MODIS infrared imagery imagery with with ASCAT ASCAT near-surface near-surface wind wind speeds speeds overlayed. overlayed. (a()a 1) 1 March March 2017. 2017. ( b(b)) 11 11 March March 2017. 2017.

The methodology described above could, however, lead to the inclusion of numerous small-scale extratropical cyclones, which may appear similar to polar lows on infrared im- Atmosphere 2021, 12, 224 4 of 15

agery [1,20]. To better distinguish polar lows from other systems, it is, therefore, necessary to assess the development mechanisms of the analyzed cyclones. For this purpose, we used surface synoptic charts produced by the Met Office and Deutscher Wetterdienst along with the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth generation reanalysis ERA5 [27] data, which included mean sea level pressure, air temperature at 2 m and at 850 hPa, and geopotential at 500 and 1000 hPa. Joint analysis of these data with the available infrared imagery allowed excluding frontal systems following the classical Norwegian or Shapiro-Keyser cyclone models and retaining polar lows, which generally develop during cold air outbreaks along low-level baroclinic zones and are triggered by the positive vorticity advection supplied by upper-level cold lows or troughs. The tracks of all identified polar lows were constructed by recording the locations of polar low centers on successive MODIS swaths. Polar low center locations were set to the -free or, if not present, to the circulation center determined by the rotation of the system on successive infrared images when possible, or subjectively identified considering previous track points, the overall movement of the system, and smoothness of the resulting track. The number of track points per polar low in the resulting dataset spans from 2 (only one case) to 59 (only one case as well) with the median number of track points being 8. Main polar low parameters were calculated as follows. The polar low diameter was approximated by averaging the lengths of two roughly orthogonal lines crossing the main cloud structure (excluding the cloud tail when present). The reported values are maximum diameters reached during the cyclone mature stage. Lifetimes were calculated as the difference between the last and the first MODIS observation of a given polar low. Distance travelled is the sum of distances between consecutive track points. Translation speed is the total distance travelled divided by the lifetime. Maximum wind speed refers to the maximum value of near-surface wind speed reached within a polar low lifetime. To ensure that only the values within a given polar low are considered and not those belonging to, e.g., large-scale synoptic fields or adjacent or polar lows in the case of multiple systems, and to eliminate the possible effect of retrieval errors, the maximum wind speed was determined manually by relating the ASCAT wind speed fields rounded down and plotted as integers with a 1-m/s step and MODIS imagery and then selecting the lowest value within a group of at least four adjacent grid cells with maximum wind speeds within the polar low area. Dominant propagation direction was approximated first by binning the track segments of a given polar low into 30◦ bins and then selecting the bin with the longest total duration of track segments within it. Dual and multiple polar lows refer to the cases when two or more polar lows formed within the same synoptic environment in proximity to each other and were simultaneously active during any time period. Total polar low numbers are given both when considering such systems as a single event and when counting each system individually.

3. Results and Discussion 3.1. Temporal Distribution Overall, 131 polar lows were found over the North Atlantic for 2015–2017. The largest number of polar lows (61) was found in 2015, 45 polar lows were identified in 2017, and only 25 cases occurred in 2016 (Table1). Hereinafter, the study region is additionally divided by the 20◦ W meridian to assess possible differences in the polar lows occurring in the western (mostly corresponding to the Davis Strait and Labrador and Irminger Seas) and eastern (mostly corresponding to the more studied Nordic and Barents seas) parts of the region. Accordingly, polar lows, which formed west (east) of 20◦ W, are referred to as western (eastern) polar lows. Atmosphere 2021, 12, 224 5 of 15

Table 1. Temporal distribution of identified polar lows. Numbers in brackets are obtained when considering dual and multiple polar lows as a single event.

Polar Jan Feb Mar Apr Oct Nov Dec Total Lows 2015 15 (12) 16 (11) 6 (5) 1 1 7 (4) 15 (8) 61 (42) 2016 5 (3) 0 5 (2) 4 (3) 0 4 (2) 7 (5) 25 (15) All 2017 8 (6) 15 (9) 8 (5) 3 0 6 (5) 5 (4) 45 (32) Total 28 (21) 31 (20) 19 (12) 8 (7) 1 17 (11) 27 (17) 131 (89) 2015 9 6 (5) 3 1 0 4 (3) 4 (3) 27 (24) 2016 3 (1) 0 0 0 0 0 2 5 (3) ≤20◦ W 2017 1 2 1 2 0 1 3 (2) 10 (9) Total 13 (11) 8 (7) 4 3 0 5 (4) 9 (7) 42 (36) 2015 6 (3) 10 (6) 3 (2) 0 1 3 (1) 11 (5) 34 (18) 2016 2 0 5 (2) 4 (3) 0 4 (2) 5 (3) 20 (12) >20◦ W 2017 7 (5) 13 (7) 7 (4) 1 0 5 (4) 2 35 (23) Total 15 (10) 23 (13) 15 (8) 5 (4) 1 12 (7) 18 (10) 89 (53)

A total of 89 polar lows were found over the eastern region including 34 that occurred in 2015, 20 that occurred in 2016, and 35 that occurred in 2017. The interannual variability is even higher in the western region, where 27 polar lows formed in 2015, 5 polar lows formed in 2016, and 10 formed in 2017 (42 in total). There is a considerably higher number of dual and multiple polar lows in the eastern region. If such cases are counted as a single event, the difference in the number of polar lows between the regions is reduced, i.e., there are 36 events over the western and 53 events over the eastern region. Moreover, more polar low events were found over the western region (24) than over the eastern (18) in 2015. The mean annual number of eastern polar lows obtained here (about 30) lies within the range found in the literature. In particular, about 45 polar lows per year are found on average in Smirnova et al. [3]. Some of this difference may possibly be attributed to the different data used and to the large interannual variability of polar low frequency, which certainly affects the robustness of our estimates due to the rather short period analyzed. Another source of this difference is that no additional information was used in Smirnova et al. [3] to analyze the development mechanisms of selected systems, which might have led to the selection of smaller extratropical cyclones or other vortices that would not be considered polar lows in the present study. In contrast, lower mean annual numbers are found in Noer et al. [20] and Rojo et al. [21]. We find that their updated dataset [28] contains 21 cases on average for the same years as analyzed here (2015–2017). Some of this difference is most likely because our list contains more minor cases as in the present study we did not consider the magnitude of wind speed exceedance within a cyclone relatively to the ambient wind speed field as a selection criterion and could possibly include cases with less magnitude than would be required in these studies. Although identification was performed for all months, no polar lows were found from May to September, and only one was found in October (Figure3). On average, for the western region, the largest number of polar lows is observed in January. For the eastern region, more polar lows were found in February. Monthly polar low frequencies are also highly variable, e.g., no polar lows were observed in February 2016. Longer-term satellite- derived polar low climatologies for the Nordic and Barents seas find maximum polar low frequency in March [3,21]. However, if we compare our results to the updated dataset by Rojo et al. [28], which includes more recent years, we find a similar monthly distribution for 2015–2017 to the one obtained here, with a clear February maximum. Atmosphere 2021, 12, 224 6 of 15 Atmosphere 2021, 12, x FOR PEER REVIEW 7 of 17

Figure 3. MonthlyFigure distribution3. Monthly distribution of polar lows. of Totalpolar barlows. lengths Total correspondbar lengths tocorrespond all observed to all polar observed lows. Darkerpolar colored bases representlows. the Darker numbers colored obtained bar bases when represent considering the dualnumb anders multipleobtained polarwhen lows cons asidering a single dual event. and multi- ple polar lows as a single event. 3.2. Spatial Distribution Although identification was performed for all months, no polar lows were found Tracks of all identified polar lows are plotted in Figure4. The highest polar low activity from May to September, and only one was found in October (Figure 3). On average, for is observed in the region encompassing the Barents Sea and northern Norwegian Sea (north the western region, the largest number of polar lows is observed in January. For the east- of 70◦ N), where more than half of identified polar lows (68) originated. This is in line with ern region, more polar lows were found in February. Monthly polar low frequencies are other studies, which reported peak polar low numbers within this region [3,20,21]. At the also highly variable, e.g., no polar lows were observed in February 2016. Longer-term sat- same time, these studies also found a high number of polar lows over the Norwegian Sea ellite-derived polar low climatologies for the Nordic and Barents seas find maximum po- south of 70◦ N, where, rather surprisingly, very few polar lows (8) are observed here. This lar low frequency in March [3,21]. However, if we compare our results to the updated may be due to the high variability of polar low frequency and formation locations, which dataset by Rojo et al. [28], which includes more recent years, we find a similar monthly depend on the large-scale circulation patterns (e.g., [21,29]). distribution for 2015–2017 to the one obtained here, with a clear February maximum. Another region with high polar low activity is found over the Labrador Sea and southern Davis Strait where 28 cyclones formed. Similar to the Nordic and Barents seas, 3.2. Spatial Distribution this region is prone to frequent cold air outbreaks and is located close to one of the low- Tracks ofpressure all identified centers polar within lows the are circumpolar plotted in vortexFigure providing4. The highest the upper-level polar low forcingac- necessary tivity is observedfor polarin the low region development encompas [sing1]. Using the Barents the indices Sea and representing northern staticNorwegian stability and upper- Sea (north of 70°level N), forcing, where more Kolstad than [24 half] found of identified that the polar favorable lows (68) conditions originated. for polarThis is low in development line with otherare studies, observed which in reported the Labrador peak polar Sea evenlow numbers more often within than this in region the Nordic [3,20,21]. and Barents seas, At the same time,although these thestudies lower also sea found surface a temperatureshigh number in of the polar Labrador lows over Sea maythe Norwe- be a factor decreasing gian Sea souththe of probability70° N, where, of polarrather low surpri formation.singly, very If we few consider polar lows the dual (8) are and observed multiple polar lows as here. This maya be single due event,to the whichhigh variability better characterizes of polar low the frequency frequency and of theformation environmental loca- conditions tions, which dependthat led on to the polar large-scale low formation circulation regardless patterns of (e.g., one [21,29]). or several cyclones developed within Another regionthis environment, with high polar the low difference activity between is found theover regions the Labrador with the Sea highest and south- polar low activity ern Davis Straitdecreases, where 28 i.e., cyclones 24 polar formed. low events Similar occurred to the Nordic over the and Labrador Barents Sea seas, and this southern Davis region is proneStrait to frequent and 36 overcold theair Barentsoutbreak Seas and and is northern located Norwegianclose to one Sea. of the Furthermore, low-pres- in 2015, more sure centers withinpolar lowthe circumpolar events occurred vortex over providing the Labrador the upper-level Sea and southern forcing Davis necessary Strait (15) than over for polar low developmentthe Barents Sea [1]. and Using northern the indi Norwegiances representing Sea (11). static Analyzing stability the and influence upper- of large-scale level forcing, Kolstadatmospheric [24] found circulation that the on favorable polar low conditions formation, for the polar positive low phasedevelopment of the North Atlantic are observed inOscillation the Labrador (NAO) Sea was even found more to often set favorable than in the conditions Nordic forand polar Barents low seas, development over

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the western North Atlantic promoting colder temperatures, lower 500 hPa geopotential heights and northerly (for the Davis Strait), westerly (for the Labrador Sea), and northerly to northeasterly (for the Irminger Sea) flows inducing cold air outbreaks, and unfavorable conditions over the Norwegian and Barents Sea, where southerly to southwesterly flows are promoted, which prevents cold air outbreaks [29,30]. Indeed, our results likely reflect this since using the values provided by the NOAA Prediction Center [31], we find that the average monthly mean NAO index for the colder months (January-April and October- December) of 2015 is 1.39, i.e., a strongly positive NAO phase, while less pronounced, albeit also positive values are found for 2016 and 2017 (0.51 and 0.72, respectively), which Atmosphere 2021, 12, x FOR PEER REVIEWpresumably implies that other weather regimes were prevalent for more extended periods9 of 17

during these years.

Figure 4. Tracks ofof identifiedidentified polar polar lows. lows. Circles Circles represent represent locations locations of theof the first first observation observation of a givenof a given polar polar low color-coded low color- codedaccording according to the yearsto the theyears cyclones the cyclones occurred. occurred. Lines representLines repr medianesent median sea ice sea extent ice extent during during colder colder months months (January-April, (January- April, October-December) of the analyzed years. October-December) of the analyzed years.

NoHigh polar polar lows low were activity found over over the the Irminger Baffin SeaBay, found which in is the mostly model-based ice-covered or reanalysis- through- outbased the objective cold season. climatologies During the (e.g., analyzed [17,25]) period, is not present freeze-up, in our on results.average, Only started nine in polar late Octoberlows were and identified a small ice-free over this area region near includingGreenland seven could that be observed occurred until in 2015 mid-December. and two that occurred in 2017. This difference might also be attributed to the more positive NAO phase 3.3.in 2015. Polar We Low note, Parameters however, a large number of small extratropical cyclones and mesoscale vorticesThe withindistribution this region. of polar Some lows ofby the diameter latter hadis shown wind in speeds Figure exceeding 5a. Most thepolar 15 lows m/s (60%)threshold, have but,maximum nevertheless, diameters were at notmature included stage since,that are as in far the as 200 we– could400 km ascertain, range. Diam- their etersdevelopment of only six differed polar lows from exceeded that expected 600 km for, polarall in lows,the eastern e.g., there region, was and no one upper-level of them exceededforcing or 700 wind km. speeds Smaller exceeding polar lows 15 m/swith wereless than only 200-km on periphery diameters and wereaccount considered for 15% of to cases.belong Such to synoptic small systems flow. are more frequent in the eastern region, where 20% fall into this category.No polar Overall, lows western were found polar over lows the are Baffin slight Bay,ly larger which than is mostly eastern ice-covered ones with throughoutmore cases inthe the cold 300 season.–600-km During range. the The analyzed mean diameter period, is freeze-up, 351 km for on western average, and started 321 km in late for Octobereastern polarand a lows. small This ice-free difference area near might Greenland be due could to the be considerably observed until higher mid-December. number of multiple systems in the eastern region, which can be rather small (e.g., several minor, smaller polar lows may accompany a major, larger polar low), but the mean diameter of eastern polar lows only grows by 10 km when neglecting multiple systems. Figure 5b shows the distribution of polar lows by lifetime. Most polar lows (56%) existed for less than 24 h and 26% are in the 12–18 h range. The maximum recorded polar low lifetime is 102 h. Western polar lows generally existed slightly longer than eastern ones with the mean lifetimes of 20.6 h and 19.6 h, respectively, although all the longest- lived systems (more than 42 h) were found in the eastern region. Since almost all western polar lows originated south of 65° N latitude, the local minima at 18–24 h and 30–36 h lifetimes are most likely due to the decreased temporal resolution of MODIS imagery at lower latitudes.

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3.3. Polar Low Parameters The distribution of polar lows by diameter is shown in Figure5a. Most polar lows (60%) have maximum diameters at mature stage that are in the 200–400 km range. Diam- eters of only six polar lows exceeded 600 km, all in the eastern region, and one of them exceeded 700 km. Smaller polar lows with less than 200-km diameters account for 15% of cases. Such small systems are more frequent in the eastern region, where 20% fall into this category. Overall, western polar lows are slightly larger than eastern ones with more cases in the 300–600-km range. The mean diameter is 351 km for western and 321 km for eastern polar lows. This difference might be due to the considerably higher number of multiple systems in the eastern region, which can be rather small (e.g., several minor, smaller polar Atmosphere 2021, 12, x FOR PEER REVIEW 10 of 17 lows may accompany a major, larger polar low), but the mean diameter of eastern polar lows only grows by 10 km when neglecting multiple systems.

FigureFigure 5. Distribution 5. Distribution of polar of polar lows lows by by (a) (a diameter,) diameter, ( (bb)) lifetime, lifetime, ((cc)) distance distance travelled, travelled, and and (d) translation(d) translation speed. speed.

DistanceFigure5 travelledb shows the is also distribution generally of larger polar lows for western by lifetime. polar Most lows polar (Figure lows 5c). (56%) Almost 30%existed of them for lesstravelled than 24 500 h and–750 26% km. are For in the eastern 12–18 hpolar range. lows, The maximum37% are in recorded the 250 polar–500 km low lifetime is 102 h. Western polar lows generally existed slightly longer than eastern ones range. The mean values are 625 and 569 km for western and eastern polar lows, respec- with the mean lifetimes of 20.6 h and 19.6 h, respectively, although all the longest-lived tively.systems There (more are thannine 42cases h) were in total found when in the pola easternr lows region. travelled Since almostmore than all western 1500 km, polar two of whichlows travelled originated more south than of 65 2000◦ N latitude, km (both the in local the minimaeastern at region). 18–24 h and 30–36 h lifetimes Polar low translation speeds are similar in both regions (Figure 5d). About 30% of cases are in the 6–9 m/s range. Seven polar lows exceeded the translation speed of 18 m/s, and the fastest moving one reached 21 m/s. The mean values are 8.8 m/s for the western region and 8.9 m/s for the eastern region. As mentioned above, the maximum wind speeds were determined by rounding down the ASCAT data and recording integers with a 1 m/s step. Since exact decimal val- ues were unknown, they were approximated by adding 0.5 m/s to each recorded wind speed value. The resulting distribution of polar lows by maximum wind speed is shown in Figure 6. As for most other parameters, generally larger values are found for the west- ern polar lows. About 40% of them have wind speeds in the 19–21 m/s range. More than half of the eastern polar lows are in the 17–21 m/s range. The mean wind speed is 21 m/s for the western polar lows and 20 m/s for the eastern polar lows. Only two polar lows were found to exceed 27 m/s wind speed, one of which exceeded 28 m/s wind speed. Figure 7 shows the distribution of polar lows by dominant propagation direction. More than half of the western polar lows propagate predominantly eastwards and about one-third propagate northeastward or southeastward. The eastern polar lows predomi- nantly propagate southward or southeastward.

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are most likely due to the decreased temporal resolution of MODIS imagery at lower latitudes. Distance travelled is also generally larger for western polar lows (Figure5c). Almost 30% of them travelled 500–750 km. For eastern polar lows, 37% are in the 250–500 km range. The mean values are 625 and 569 km for western and eastern polar lows, respectively. There are nine cases in total when polar lows travelled more than 1500 km, two of which travelled more than 2000 km (both in the eastern region). Polar low translation speeds are similar in both regions (Figure5d). About 30% of cases are in the 6–9 m/s range. Seven polar lows exceeded the translation speed of 18 m/s, and the fastest moving one reached 21 m/s. The mean values are 8.8 m/s for the western region and 8.9 m/s for the eastern region. As mentioned above, the maximum wind speeds were determined by rounding down the ASCAT data and recording integers with a 1 m/s step. Since exact decimal values were unknown, they were approximated by adding 0.5 m/s to each recorded wind speed value. The resulting distribution of polar lows by maximum wind speed is shown in Figure6. As for most other parameters, generally larger values are found for the western polar lows. About 40% of them have wind speeds in the 19–21 m/s range. More than half of the eastern polar lows are in the 17–21 m/s range. The mean wind speed is 21 m/s for the western Atmosphere 2021, 12, x FOR PEER REVIEW 11 of 17 polar lows and 20 m/s for the eastern polar lows. Only two polar lows were found to exceed 27 m/s wind speed, one of which exceeded 28 m/s wind speed.

FigureFigure 6. 6.Distribution Distribution of polar of polar lows by lows maximum by maximum near-surface near wind-surface speed wind reached speed during reached their lifetimes. during their lifetimes. Figure7 shows the distribution of polar lows by dominant propagation direction. More than half of the western polar lows propagate predominantly eastwards and about one- third propagate northeastward or southeastward. The eastern polar lows predominantly propagate southward or southeastward.

Figure 7. Distribution of polar lows by dominant propagation direction. The radial axes represent the proportion of polar lows (%).

3.4. Formation Conditions The large-scale synoptic environment during the formation of western and eastern polar lows may be assessed using Figure 8, which shows composites of 500-hPa geopo- tential height (Z500) anomalies and 925-hPa wind vectors. The composites were obtained from ERA5 data using the closest 1-h time step to the first observation of each polar low in our list. Anomaly of Z500 represents the difference between Z500 at a given time step and the mean Z500 for October–April 2015–2017. For the western polar lows, there is a strong negative anomaly centered above south- ern Greenland and covering the Labrador Sea, the Davis Strait, and the Irminger Sea (Fig- ure 8a). The largest anomaly magnitude exceeds -200 m, which confirms the presence of strong upper-forcing during formation of the identified polar lows. Lower-level winds indicate the presence of cold-air outbreaks as intense northeasterly and easterly flows ad- vect the colder air located over the continent and sea ice toward the warmer sea surface of the Davis Strait and Labrador Sea. During formation of the western polar lows, gener- ally unfavorable conditions for polar low development are found in the eastern region,

Atmosphere 2021, 12, x FOR PEER REVIEW 11 of 17

Atmosphere 2021, 12, 224 10 of 15 Figure 6. Distribution of polar lows by maximum near-surface wind speed reached during their lifetimes.

FigureFigure 7. Distribution7. Distribution of of polar polar lows lows by by dominant dominant propagationpropagation direct direction.ion. The The radial radial axes axes represent represent the the proportion proportion of polar of polar lows (%). lows (%). 3.4. Formation Conditions 3.4. Formation Conditions The large-scale synoptic environment during the formation of western and eastern polarThe lows large-scale may be assessed synoptic using environment Figure 8, during which the shows formation composites of western of 500-hPa and eastern geopo- po- lartential lows height may be (Z500) assessed anomalies using and Figure 925-hPa8, which wind shows vectors. composites The composites of 500-hPa were geopotential obtained heightfrom ERA5 (Z500) data anomalies using the and closest 925-hPa 1-h time wind step vectors. to the Thefirst compositesobservation wereof each obtained polar low from ERA5in our data list. usingAnomaly the closestof Z500 1-hrepresents time step the to difference the first observationbetween Z500 of at each a given polar time low step in our list.and Anomaly the mean ofZ500 Z500 for represents October–April the difference 2015–2017. between Z500 at a given time step and the meanFor Z500 the for western October–April polar lows, 2015–2017. there is a strong negative anomaly centered above south- ern ForGreenland the western and covering polar lows, the thereLabrador is a strongSea, the negative Davis Strait, anomaly and centeredthe Irminger above Sea southern (Fig- Greenlandure 8a). The and largest covering anomaly the Labrador magnitude Sea, exceeds the Davis -200 Strait, m, which and theconfirms Irminger the Seapresence(Figure of8 a) . Thestrong largest upper-forcing anomaly magnitudeduring formation exceeds of -200the identified m, which polar confirms lows. the Lower-level presence ofwinds strong upper-forcingindicate the presence during of formation cold-air outbreaks of the identified as intense polar northeasterly lows. Lower-level and easterly winds flows indicate ad- thevect presence the colder of cold-airair located outbreaks over the ascontinent intense and northeasterly sea ice toward and the easterly warmer flows sea advectsurface the colderof the airDavis located Strait over and the Labrador continent Sea. and During sea ice formation toward theof the warmer western sea polar surface lows, of thegener- Davis Straitally unfavorable and Labrador conditions Sea. During for formationpolar low ofdevelopment the western are polar found lows, in generallythe eastern unfavorable region, conditions for polar low development are found in the eastern region, with small, albeit negative, Z500 anomalies and weak southwesterly to southeasterly flow. Rather favorable conditions are found over the Greenland Sea closer to the Greenland coast, but sea ice in this region suppresses any possible developments. Similarly, during the formation of the eastern polar lows, a strong negative Z500 anomaly is observed over the area of the highest polar low activity in the eastern region (Figure8b). This anomaly is centered over the Barents Sea and has the largest magnitudes exceeding −180 m. Strong northerly to north-easterly flow leads to the outbreaks of cold Arctic air over the region. During this situation, unfavorable conditions for polar low formation are found in the western region, where a weak positive anomaly is present and only a minor cold air flow may be observed over the Davis Strait and northwestern Labrador Sea. Overall, slightly weaker negative Z500 anomaly and lower-level flow are found during formation of the eastern polar lows compared to the conditions during formation of the western polar lows. The distributions in Figure8 are in excellent agreement with those presented in Mallet et al. [29] for Noer et al. [20] and Kolstad [24] polar low lists. Another view of polar low formation conditions is presented in Figure9, which shows the composites of the main parameters within a 500-km radius centered around each polar low. Same as above, the closest in time to the first polar low observation ERA5 data were used for these composites. Atmosphere 2021, 12, 224 11 of 15 Atmosphere 2021, 12, x FOR PEER REVIEW 13 of 17

Figure 8.FigureComposite 8. Composite of 500-hPa of geopotential 500-hPa geopotential height anomaly height (m, anomaly shaded) (m, and shaded) 925-hPa and wind 925-hPa vectors wind (arrows) vec- during formationtors of ( a(arrows)) western during and (b) formation eastern polar of lows.(a) western and (b) eastern polar lows.

Atmosphere 2021, 12, 224 12 of 15 Atmosphere 2021, 12, x FOR PEER REVIEW 14 of 17

Figure 9.9. CompositesComposites withinwithin aa 500-km 500-km radius radius of of polar polar low low centers centers of of (a ),(a (),d )(d difference) difference between between the the sea sea surface surface and and 500-hPa 500- potentialhPa potential temperatures, temperatures, (b, e )(b temperature, e) temperature at 850 at hPa 850 andhPa (andc, f) ( geopotentialc, f) geopotential height height at 500 at hPa 500 for hPa (a –forc) western(a–c) western and ( dand–f) (d–f) eastern polar lows. eastern polar lows.

4. ConclusionsFigure9a,d show composites of the difference between the sea surface ( θSST) and 500 hPaIn this (θ500 study,) potential polar temperatures lows over the for North the western Atlantic and were eastern identified regions. in MODIS The differences infrared imagerybetween and the seatheir surface intensity or skin was temperature assessed using or potentialASCAT scatterometer temperatureand data. that The at develop- a higher mentlevel areof selected often used systems to characterize was examined atmospheric using synoptic static stability surface and charts identify and atmospheric marine cold reanalysisair outbreaks data (e.g., to better [17,32 exclude,33]). Stoll smaller et al. extr [17atropical] found thatcyclones the difference or more minor between vortices. the sea A totalsurface of 131 and polar 500-hPa lows potential were found temperatures for 2015–2017. performs The number better of than polar other lows similar over the criteria. east- ernIn Figure part of9a,d the, study higher region values (>20° indicate W) is less 89, stablewhich atmosphere, is more than and two the times highest higher values than arefor thefound western within part 100 km(<20° of W), polar where low centers 42 polar in bothlows regions. were found. The magnitude A substantial of the partθSST of− θthis500 difference is similardue to the between larger thenumber regions, of du exceptal or formultiple the area polar of thelows highest over the values, eastern ex- region.ceeding When−5 K, considering which is considerably them as a largersingle inev theent, western the difference region. is The markedly highest decreased, values are i.e.,approximately there are 53 locatedevents in at the the eastern northwestern region sectorand 36 forevents the westernin the western polar lowsregion. and at the northeasternMost of sectorthe identified for the eastern polar lows polar (68) lows, or likelyiginated reflecting over the the Barents orientations Sea and of the northern upper- Norwegianlevel troughs Sea present (north in of Z500 70° composite N). The second fields (Figurehighest9c,f). number Figure of9 b,epolar show lows composites (28) formed of over850-hPa the airLabrador temperature Sea and (T850). southern Strong Davis baroclinic Strait. environmentsCounting dual are and evident, multiple as systems the largest as atemperature single event gradients results in area decreased about 2 Kdifference over 100 as km well. with In colder particular, air located more polar in the low west events and occurrednorth-northwest over the for Labrador the western Sea and southern eastern polar Davis lows, Strait respectively. than over the Barents Sea and northern4. Conclusions Norwegian Sea in 2015. This is likely a consequence of a strongly positive NAO phase in the colder months of 2015, during which the number of polar lows is increased over Inthe this Labrador study, polarSea and lows decreased over the over North the Atlantic Norwegian were and identified Barents in Sea MODIS [29,30]. infrared imagery and their intensity was assessed using ASCAT scatterometer data. The develop- Overall, polar low frequency exhibits high interannual variability, which was noted ment of selected systems was examined using synoptic surface charts and atmospheric in many studies (e.g., [3,20,21]). In particular, in the present study, 61, 25, and 45 polar reanalysis data to better exclude smaller extratropical cyclones or more minor vortices. lows were found in 2015, 2016, and 2017, respectively. Some of this variability might be

Atmosphere 2021, 12, 224 13 of 15

A total of 131 polar lows were found for 2015–2017. The number of polar lows over the eastern part of the study region (>20◦ W) is 89, which is more than two times higher than for the western part (<20◦ W), where 42 polar lows were found. A substantial part of this difference is due to the larger number of dual or multiple polar lows over the eastern region. When considering them as a single event, the difference is markedly decreased, i.e., there are 53 events in the eastern region and 36 events in the western region. Most of the identified polar lows (68) originated over the Barents Sea and northern Norwegian Sea (north of 70◦ N). The second highest number of polar lows (28) formed over the Labrador Sea and southern Davis Strait. Counting dual and multiple systems as a single event results in a decreased difference as well. In particular, more polar low events occurred over the Labrador Sea and southern Davis Strait than over the Barents Sea and northern Norwegian Sea in 2015. This is likely a consequence of a strongly positive NAO phase in the colder months of 2015, during which the number of polar lows is increased over the Labrador Sea and decreased over the Norwegian and Barents Sea [29,30]. Overall, polar low frequency exhibits high interannual variability, which was noted in many studies (e.g., [3,20,21]). In particular, in the present study, 61, 25, and 45 polar lows were found in 2015, 2016, and 2017, respectively. Some of this variability might be due to the variation of the large-scale atmospheric circulation patterns (e.g., [21,29]). For instance, the strong dependence of the western polar low frequency on the NAO phase found in previous studies [29,30,34] with more events occurring during the positive NAO phase, have likely influenced the numbers we obtained for this region as indicated by our simplified estimates using the average values of the monthly mean NAO index for colder months. Overall, during the analyzed period, more polar lows were observed in February, while for the western region the maximum number was found in January. The monthly distribution is, however, highly variable, as found in previous studies (e.g., [3,21]) and also here (e.g., no polar lows were identified in February 2016). The distributions of polar lows by their main parameters derived here for the North Atlantic generally agree with those previously reported for polar lows over the Nordic and Barents seas (e.g., [3,21]). The following mean values for the main polar low parameters were obtained: diameter of 331 km, lifetime of 19.9 h, distance travelled of 587 km, translation speed of 8.9 m/s, and wind speed of 20.3 m/s. Comparing the mean values for the two subregions, except for the mean translation speeds, which are almost equal, higher mean values were found for the western polar lows, although the difference is rather small. In particular, the mean diameter and distance travelled are larger by about 10%, mean lifetime is longer by 1 h, and mean wind speed is higher by 1 m/s. At the same time, all extreme high values of these parameters are observed in the eastern region. The estimated dominant polar low propagation directions are mostly westward for the western polar lows and mostly southward and southwestward for the eastern polar lows. Given the high interannual variability in polar low frequency and formation locations and our quite limited study period, we certainly do not consider the derived polar low spatio-temporal distributions as robust in the sense that they would hold for other years. Rather, they are only intended to reflect the polar low activity during the period analyzed here. The polar low list compiled here (Table S1) might also be useful for other polar low-related studies and might be extended in the future.

Supplementary Materials: The following are available online at https://www.mdpi.com/2073-443 3/12/2/224/s1. Table S1: North Atlantic polar low tracks for 2015–2017. Author Contributions: Conceptualization, P.G. and L.B. Methodology, P.G. Validation, J.S. Formal analysis and investigation, P.G. and J.S. Data curation and writing—original draft preparation, P.G. Writing—review and editing, P.G., J.S., and L.B. Supervision, project administration, and funding acquisition, L.B. All authors have read and agreed to the published version of the manuscript. Atmosphere 2021, 12, 224 14 of 15

Funding: This study was funded by the Ministry of Science and Higher Education of the Russian Federation in the framework of the project “An integrated Arctic observation system,” Unique Project Identifier RFMEFI61618x0103. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available in Table S1. Acknowledgments: The authors thank two anonymous reviewers for their comments, which helped to improve the manuscript. The authors are grateful to NASA for providing MODIS and ASCAT data and to ECMWF for producing and providing ERA5 data. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

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