| The Arctic Ocean is a critical area for global water, heat and salinity exchange and it plays an important role in many key research topics such as global climate and ecosystem change and development of Arctic sea channels. Over the past few decades, drastic changes had taken place in the Arctic Ocean. Through data processing and analyzing, this paper studies changes in Arctic sea ice, focuses on its winter and summer motion features in various Arctic regions. By comparing features of sea ice motion with extent, the correlation between them can be found. Furthermore, factors that have impact on sea ice changes are investigated. Considering sea ice as an isolation layer between ocean and atmosphere, it will necessarily be influenced by both of them because of direct contact. Therefore the primary issue in this dissertation is atmospheric cyclones’ effects on sea ice. Major features of the Arctic cyclones such as numbers, spatial distribution and velocity are statistically analyzed by automatic recognition and tracking method. The correlation between the numbers of strong cyclones and ice extent is discussed as well, and effects on sea ice motion pattern, velocity and coverage during the occurrence of cyclone is investigated through one case.First of all, we utilized monthly mean sea ice motion data from NSIDC to analyze kinematic characteristics of the Arctic sea ice in the period from 1979 to 2012 in the whole Arctic as well as in Beaufort-Chukchi sea and Fram Strait. Results show that the mean sea ice velocity had an increasing trend in the past 34 years, the trend was more significant in winter than in summer. Specifically the increasing rate of winter and summer mean sea ice movement velocity in the North Pole, Beaufort-Chukchi Sea and Fram Strait was 2.1%/year and 1.7%/year, 2.0%/year and 1.6%/year, 4.9/year and 2.2/year respectively. By monthly average sea ice extent data from NSIDC,we analyzed the relationship between sea ice extent and ice motion speed, with the coefficient of association is-0.77 for 1979-2012, which is obviously a negative correlation.Based on monthly averaged wind field reanalysis data provided by NCEP/NCAR, wind changes in winter and summer in 1979-2012 were investigated. The long-term changing trend of Arctic winter and summer wind field was in accordance with that of sea ice mean movement velocity and growth rate in winter is greater than that in summer, also. the correlation coefficient was 0.50 and 0.48 between wind and sea ice velocity in winter and summer, respectively. Combined with daily information of sea level pressure and wind field as well as sea ice concentration provided by ECMWF, sea ice motion features including convergence and divergence under the impact of Arctic surface wind forcing in 2007, 2012 were studied. Wind and air pressure field play a vital role in sea ice motion, divergence and redistribution. In the summer of 2007, high air pressure dominated in Beaufort Sea and south wind prevailed in Bering Strait and Chukchi Sea as well as their northern area during day 234 th to 273 rd. The strong Beaufort Gyre and Transpolar Drift Stream made a part of sea ice converge at north Greenland and Canadian Archipelago, another part transport out of Fram Strait across central Arctic. In 2012, Bering Strait and Chukchi Sea located at the boundary of low and high air pressure systems, where northerly wind prevailed, which led to an advection of sea ice from eastern Arctic to the west. Then the perennial ice at the Canada Basin diverged due to offshore motion, thinner ice thus was created and moved to east Siberia Sea together with ice in Kara Sea and Laptev Sea, entered Chukchi Sea finally and melt under effects of lower-latitude warm inflow.In addition, we counted numbers, life circle, moving distance and velocity, spatial distribution of Arctic cyclones from 1979 to 2012, using automatic recognition and tracking method and several ECMWF reanalyzed data such as daily 4-time step(6 hours) wind, sea level pressure, sea surface temperature and 500 h Pa air temperature. Effects of cyclones on sea ice was analyzed through case study. It turned out that the quantity of Arctic cyclones exhibited significant temporal and spatial variations. both winter and summer showed a fluctuated feature. The number of cyclones reached its largest value 13.69 in October and 15.43 in September. The cyclone life circle distributions were basically the same in both winter and summer, within 5 days mostly. The moving distance of the relatively static or zonal motion sort of cyclones was within 60 km, accounted for 80% at least. The other three types of cyclones showed relatively flat variation tendency. From 1979 to 2012, the mean velocity for Arctic cyclones was 4.73m·s-1 and the highest portion were between 3 to 6m·s-1. The major area for cyclone generation and dissipation located at Baffin Bay, reaching Davis Strait. Besides, the Greenland Sea north of Iceland and centered here to the Norwegian Sea, south of Svalbard, Barents Sea, Kara Sea were also extreme areas for cyclone generation. As the channel that connects the Arctic Ocean and lower-latitude ocean, Chukchi Sea was where extreme cyclones extinction took place.At last, based on the thinning and area reducing of summer sea ice and increasing open water, which caused Arctic cyclones’ profound influences on sea ice, the relationship between the number of summer Arctic cyclones and ice extent was discussed. We found that there was an obvious negative correlation between the number of strong cyclone occurrence and sea ice extent from 1996 to 2012. And through case study, ice movement velocity would surge while ice cover would suddenly decrease under the impact of cyclones, which provides evidence for research on Arctic cyclones’ effects on sea ice. |
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