Changes Of Arctic Cyclone Activities Under The Global Warming

Based on the 6-hourly mean sea level pressure datasets from the ERA-Interim,a data set of Arctic cyclones?ACs?over 1979–2018 was obtained by employing an automatic cyclone tracking algorithm.Then according to genesis position,the ACs were divided into different categories,whose seasonal and inter-annual variations were investigated respectively.The AC frequency,intensity,velocity,lifespan,moving distance,typical path,etc.have all been analysed.It is worth mentioning that the meridional variation of AC tracks has also been explored by using a new index?_np,the latitude where a AC reaches northernmost point.Finally,the causes of two typical anomalies were further discussed,including the more and more summertime cyclogenesis events near the Arctic coast and the northward shift of wintertime AC tracks in the Arctic Atlantic sector in the last 40 years.The main conclusions are as follows:?1?Generally,the ACs are more and stronger in winter,but less and weaker in summer.However,compared to the wintertime ACs,the lifespan and moving distance of summertime ACs are both slightly longer.In fact,the seasonal variations of ACs from different origins are not consistent,especially in the frequency of cyclogenesis events.In the East Greenland Sea and Barents Sea of the Arctic Atlantic sector,there are more cyclogenesis events in winter than summer,while less in the north of central Siberia and Kolyma.Therefore,from winter to summer,the location of the largest Arctic cyclogenesis density would gradually move from the Arctic Atlantic sector to the north of the Eurasian continent.Under the background of global warming,the frequency of cyclogenesis events has significantly increased in summer,while decreased in winter.The spatial distribution of the linear trends of Arctic cyclogenesis density shows that,in the past 40 years,there are more and more cyclogenesis events in summer near the Arctic coast.Although the intensity and lifespan of ACs have large inter-annual variability,no significant linear trend is detected for the period 1979–2018 overall.In addition,by analysing the long-term tendency of?_np,we found that the wintertime cyclones originated from the Arctic Atlantic sector are significantly shifting northward in the past 40 year,whose tracks mainly turn northward in the Barents sea since 2000.?2?There is a band of strong meridional temperature gradient near the Arctic coastlines in summer,which is called Arctic Frontal Zone?AFZ?.It was found that the intensity of AFZ and the latitude of Arctic Frontal Line?AFL?,which is derived by connecting the maximal values of meridional temperature gradients in AFZ,could both influence the frequency of Arctic cyclogenesis events,but the affected areas of two factors are different.With the strengthen of AFZ,there would be more cyclogenesis events in the central Arctic Ocean.However,in the past 40 years,the strength of AFZ just increased slightly.Therefore,it has little contribution to the overall rise of summertime cyclogenesis events.As for AFL,with its northward shifting,the cyclogenesis events near the Arctic coast,especially the northern coast belonging to Eurasia and North America,would significantly increase.For the period 1979-2018,AFL shows a significant northward shifting,especially the section near the Kara Sea and Laptev Sea.Accompanying with its northward shifting,the baroclinic instability also increased near the Arctic coast,leading to more and more cyclogenesis events.It shows that the northward shifting of AFL plays a significant role in the increase of the summertime cyclogenesis events for the last 40 years.?3?Through the linear decomposition of?_np,it is found that increases in latitudinal distance between genesis position and the northernmost latitude????as well as the poleward shift of AC genesis latitude??_G?could be both responsible for the northward shift of wintertime AC tracks in the Arctic Atlantic sector in the last 40 years.The former is largely related to the rapid decrease of sea ice in the Barents Sea in winter.Since 2000,a large amount of ice melting in the Barents Sea in winter has made the ice edge of the region retreat northward,leading to the strong meridional temperature gradient zone moving northward.In this context,the baroclinic instability is obviously weakened on the south of Barents Sea,but strengthened on the north of Barents Sea,which drives cyclones to turn northward near the Barents Sea,increasing the cyclone meridional velocity and??.The poleward shift of?_G is likely linked to the winter North Arctic Oscillation?NAO?.When the winter NAO phase is negative,with the Icelandic low weakening and retreating southward,the cyclogenesis events decrease significantly in the south of East Greenland.Meanwhile,due to the weakening of southern wind,the temperature in the north of the Arctic Atlantic sector could turn to lower,and the sea ice from the East Greenland Sea to Svalbard could increase,leading to a strong temperature gradient in the northwest of the East Greenland.That would be beneficial to cyclogenesis.Less southern cyclogenesis and more northern cyclogenesis could lead to the overall cyclogenesis moves northward,contributing to the increase of the?_G.