Characteristics of cyclone climatology and variability in the Southern Ocean

A new climatology of cyclones in the Southern Ocean is generated by applying an automated cyclone detection and tracking algorithm （developed by Hodges at the Reading University） for an improved and relatively high- resolution European Centre for Medium-Range Weather Forecasts atmospheric reanalysis during 1979-2013. A validation shows that identified cyclone tracks are in good agreement with a available analyzed cyclone product. The climatological characteristics of the Southern Ocean cyclones are then analyzed, including track, number, density, intensity, deepening rate and explosive events. An analysis shows that the number of cyclones in the Southern Ocean has increased for 1979-2013, but only statistically significant in summer. Coincident with the circumpolar trough, a single high-density band of cyclones is observed in 55^-67~S, and cyclone density has generally increased in north of this band for 1979-2013, except summer. The intensity of up to 70% cyclones in the Southern Ocean is less than 980 hPa, and only a few cyclones with pressure less than 920 hPa are detected for 1979-2013. Further analysis shows that a high frequency of explosive cyclones is located in the band of 45^-55~S, and the Atlantic Ocean sector has much higher frequent occurrence of the explosive cyclones than that in the Pacific Ocean sector. Additionally, the relationship between cyclone activities in the Southern Ocean and the Southern Annular Mode is discussed.

Seasonal and inter-annual variations of Arctic cyclones and their linkage with Arctic sea ice and atmospheric teleconnections

The seasonal and inter-annual variations of Arctic cyclone are investigated. An automatic cyclone tracking algorithm developed by University of Reading was applied on the basis of European Center for Medium-range Weather Forecasts（ECMWF） ERA-interim mean sea level pressure field with 6 h interval for 34 a period. The maximum number of the Arctic cyclones is counted in winter, and the minimum is in spring not in summer.About 50% of Arctic cyclones in summer generated from south of 70°N, moving into the Arctic. The number of Arctic cyclones has large inter-annual and seasonal variabilities, but no significant linear trend is detected for the period 1979–2012. The spatial distribution and linear trends of the Arctic cyclones track density show that the cyclone activity extent is the widest in summer with significant increasing trend in CRU（central Russia）subregion, and the largest track density is in winter with decreasing trend in the same subregion. The linear regressions between the cyclone track density and large-scale indices for the same period and pre-period sea ice area indices show that Arctic cyclone activities are closely linked to large-scale atmospheric circulations, such as Arctic Oscillation（AO）, North Atlantic Oscillation（NAO） and Pacific-North American Pattern（PNA）. Moreover,the pre-period sea ice area is significantly associated with the cyclone activities in some regions.

中国近海温带气旋的时空变化特征

基于1979－2012年共34年的 ECMWF 逐日4次平均海平面气压的再分析资料，采用英国雷丁大学气旋客观追踪算法，对出现在我国近海的温带气旋（气旋生命史1 d 以上，移动距离大于500 km）的时空分布特征进行统计分析。结论包括以下几点：（1）1979－2012年进入中国近海的温带气旋平均每年45个，气旋数量呈现春夏多而秋冬少的特点。20世纪90年代初至今，气旋数量呈增加趋势，其中北部海区气旋数量增加达到显著水平，东部海区气旋数量表现为不显著减少，故认为影响中国近海的气旋路径有北移的趋势。（2）进入我国近海的温带气旋主要有4个生成源地，按比例由高到低分别是江淮气旋（38．9％），东海气旋（25．2％），黄河气旋（24．3％）以及蒙古气旋（11．6％）。气旋入海后，当大气海洋条件适合时，可以爆发性增长，气旋爆发性增长的主要区域在朝鲜半岛及以东洋面以及日本以东洋面，在我国近海气旋爆发的比例较小。（3）气旋生命史主要为1～7 d，但生命史为1～4 d 的气旋比例最大，平均占气旋总数的52％，其中夏季长生命史气旋（大于10 d）的比例最大，为8％，冬季最少，接近3％。冬季气旋最强，气压分布区间大；夏季弱气旋多，中心气压分布集中。

进入极区温带气旋的时空变化特征分析

基于气旋自动追踪方案,利用ERA-Interim逐日4次(每6 h一次)平均海平面气压再分析资料,对1979—2012年共34年生成于65°N以南,北上进入极圈的气旋的时空特征进行统计分析。结果表明:(1)该类气旋的数量与强度具有明显的季节变化,呈现春夏多强度弱,秋冬少但强度强的特点;(2)气旋年总数呈现显著减少趋势;(3)进入极区的气旋其主要源地在北大西洋一侧,北上跨入极区的关键区域位于格陵兰岛以东的丹麦海峡以及冰岛以东的海域(60°—30°W),气旋在该区域一年四季都很活跃;(4)该类气旋主要在50°—65°N生成,也有在中纬度生成,长途跋涉进入极区;(5)该类气旋在陆地上生成的比例比海上高,尤其是在夏季,冬季海上气旋源地相对变得活跃,生成比例接近陆地的生成比例,几乎各占一半。

北极地区1979—2015年西北航道7-10月份气旋的特征分析

本文采用气旋自动识别与追踪算法,基于欧洲中期天气预报中心ERA-Interim平均海平面气压场,建立1979-2015年共37年7-10月北极西北航道东西两个区域的气旋数据。数据包括气旋中心经纬度以及中心最低气压值。基于这套数据,分析了北极7-10月西北航道气旋数量特征、空间密度分布、强度特征、加深以及爆发性气旋的活动情况。西北航道东西两段的气旋个数存在显著的差异,东段气旋个数是西段区域的2～2.5倍,并且两段个数变化趋势不一致,西段的气旋个数趋势呈不显著的减少,东段的个数呈不显著上升趋势。西北航道气旋强度偏弱,中心最低气压达980 hPa的气旋仅占不到总数的5%。最低中心气压集中分布在990～1 000 hPa之间。1979-2015年以来,东段的气旋强度趋势增强,西段气旋在2002年以前也是显著的增强,2002年以后强度减弱。气旋的生命史集中在7 d以内,东段1 d以内的气旋个数明显偏多。西段气旋高密度区域主要分布在74°N以北的波弗特海北部,东段主要分布在巴芬湾的东北侧和巴芬岛的东南侧,近几年来其主要密度分布区域东西两段存在南移以及略微变化。西北航道内爆发性气旋的增长位置集中在70°N的沿岸附近,尤其是加拿大北部以及格陵兰西海岸附近。大西洋震荡指数与东段气旋的个数有较好的正相关。

江淮气旋气候学特征的统计研究

从江淮气旋的定义出发,采用欧洲中期天气预报中心的ERA-Interim再分析资料,运用气旋的客观判定与追踪算法追踪江淮气旋,分析了1979-2010年江淮气旋的气候特征。结果表明:江淮气旋发生的频数有显著的年际变化,但随时间变化的长期趋势并不明显。由于春季冷空气活动频繁,且易与副高西南侧气流汇合形成气旋,因此春季是江淮气旋最活跃的季节,其中5月份江淮气旋发生次数最多;而在冬季,东亚地区受大陆冷高压控制,形势稳定,不易形成气旋,故秋冬季江淮气旋出现较少。受地形和下垫面等因素影响,江淮气旋生成的源地主要位于洞庭湖地区、鄱阳湖地区及大别山区东北侧。43.9%的江淮气旋中心平均降压率为0^-1 h Pa/6h,大多数江淮气旋中心最大降压率为0^-2 h Pa/6h(占66.4%),较难形成暴发性气旋。江淮气旋生命史较短,主要为1~2天。

A Climatology of Extratropical Cyclones over East Asia During 1958-2001

A climatology of extratropical cyclones （ECs） over East Asia （20~ 75~N, 60^-160~E） is analyzed by applying an improved objective detection and tracking algorithm to the 4-time daily sea level pressure fields from the European Centre for Medium-range Weather Forecasts （ECMWF） reanalysis data. A total of 12914 EC processes for the period of 1958-2001 are identified, with an EC database integrated and EC activities reanalyzed using the objective algorithm. The results reveal that there are three major cyclogenesis regions： West Siberian Plain, Mongolia （to the south of Lake Baikal）, and the coastal region of East China; whereas significant cyclolysis regions are observed in Siberia north of 60~N, Northeast China, and Okhotsk Se^Northwest Pacific. It is found that the EC lifetime is largely 1 7 days while winter ECs have the shortest lifespan. The ECs are the weakest in summer among the four seasons. Strong ECs often appear in West Siberia, Northeast China, and Okhotsk Sea-Northwest Pacific. Statistical analysis based on k-means clustering has identified 6 dominating trajectories in the area south of 55~N and east of 80~E, among which 4 tracks have important impacts on weather/climate in China. ECs occurring in spring （summer） tend to travel the longest （shortest）. They move the fastest in winter, and the slowest in summer. In winter, cyclones move fast in Northeast China, some areas of the Yangtze-Huaihe River region, and the south of Japan, with speed greater than 15 m s-1. Explosively-deepening cyclones are found to occur frequently along the east coast of China, Japan, and Northwest Pacific, but very few storms occur over the inland area. Bombs prefer to occur in winter, spring, and autumn. Their annual number and intensity in 1990 and 1992 in East Asia （EA） are smaller and weaker than their counterparts in North America.