## 3 Results and Discussion  
### Review of Synoptic and Sea Ice Conditions  
Anomalously low sea ice coverage observed in the Chukchi Sea during both autumn and early winter 2017 [43] definitely contributed to the intensification of the mesocyclonic activity in this region. The sea ice concentration map, obtained from the University of Bremen on 19 October 2017, exhibits a significant retreat of sea ice compared to September (Figure 1a, red dotted line) and October (Figure 1a, black dotted line) climatological sea ice edge. A notable increase in the ice-free area in October 2017 led to the favorable conditions for mesoscale cyclogenesis over the Chukchi and Beaufort Seas and adjacent open water areas from the north. The high SST (more than 4 \({}^{\circ}\)C) along the coast of Alaska (Figure 2) can also contribute to PL development. Favorable conditions for the mesocyclogenesis were defined by: low-level baroclinic instability in the marginal zone between the sea ice edges or cold coast land areas and relatively warm sea surface; convective instability expressed as the differences between the sea surface temperature (SST) and the air temperature at the 500 hPa level (T\({}_{500}\)) that should exceed the threshold. Here, we use 40 \({}^{\circ}\)K for \(\Delta\)T = SST \(-\) T\({}_{500}\) criterion [44]. Figure 1b illustrates the frequency of conditions with \(\Delta\)T \(\geq\) 40 \({}^{\circ}\)K in October 2017, which for most of the study domain exceeds 10%. The areas with a recorded historical maximum frequency of PL favorable conditions since 1979 (marked with crosses) cover a significant part of the Chukchi Sea and the ice-free waters of the Beaufort Sea. From 18 to 24 October, mesocyclones series were observed in this region. Four mesocyclones with wind speeds \(\geq\)15 m/s were classified as PLs. Their origin locations--derived from MODIS, VIIRS, and AVHRR satellite images--are shown in Figure 1b. One of them (marked as PL1) occurred over the Beaufort Sea on 18 October 2017 and existed for over 24 h. According to ASCAT and AMSR2 data, wind speed in the PL1 reached 18 m/s. The PL4 occurred on October 22 over the Chukchi Sea in the northeastern sector of the upper-level low centered over the Norton Bay. It was weaker and with a noticeably shorter lifetime--approximately 10 h--and moved south to the coast of Chukotka, where it dissipated. Wind speed in the PL4 was reaching 15-16 m/s by ASCAT data. The most intense of this series was a PL pair--containing the PL2 and PL3--which occurred on the border of the Chukchi and Beaufort seas on October 19. In the mature stage, the wind speed in the PL2 exceeded 25 m/s and in the most intense PL3 reached  
Figure 1: Sea ice concentration on 19 October 2017 **(a)**: September (red dotted line) and October (black dotted line) median sea ice edge for 1981–2010 obtained from the National Snow and Ice Data Center (NSIDC) Sea Ice Index, version 3; frequency of favorable conditions for mesoscale cyclogenesis in October 2017 (as a percentage of the month length) **(b)**. Hatching areas recorded the highest frequency since 1979. The red circles on **(b)** indicate the location of the PLs formation areas as detected on satellite images: 1—20:16 UTC, October 17, 2—1:59 UTC, October 19, 3—4:23 UTC, October 19, 4—5:02 UTC, October 22. Figure 2: PL1 (circles), PL2 (triangles), PL3 (asterisks) and PL4 (diamonds) tracks: obtained from satellite measurements (black); derived from ERA5 (red), MERRA-2 (yellow) and CFSv2 (blue) relative vorticity at 850 hPa **(a)** and SLP **(b)** fields. The sea surface temperature field (color) is for 19 October 2017.  
near hurricane values--over 30 m/s. Both interacting PLs moved under the upper-level vortex with the cold core along the northwest coast of Alaska to the Bristol Bay. The PL**3** was developed as the vortex of a multicenter mesocyclonic system like the \"merry-go-round\" [1] and was unusually strong for the PLs in the northern part of the PA region.