High southern latitude cyclone behaviour during the FROST SOPs and its longer term variability

Using the ECMWF analyses for the three FROST periods, a data set has been extracted to show the anomalous mean sea level pressure over these periods. In addition a comprehensive analysis of all cyclones in the sub Antarctic region during the special observing periods is part of the set.From the abstracts of some of the referenced papers:The data collected during the three special observing periods (SOPs) of the Antarctic First Regional Observing Study of the Troposphere project provide an excellent base upon which to study the behaviour of cyclonic systems in winter, spring, and summer in the Southern Hemisphere. This paper provides a report on the behaviour of these cyclonic systems during the three SOPs as revealed in the twice-daily ECMWF operational analyses.The study has been undertaken with an objective cyclone tracking algorithm applied to the digital analyses. The results revealed cyclone behaviour generally in accord with long-term climatologies developed with this scheme. In the SOPs the authors observed many systems to be generated in the western part of the ocean basins and then to move east and, to a lesser extent, south. In the three periods they found a concentration of tracks just to the north of the Antarctic continent, being particularly noticeable in the Indian Ocean. At the same time, they found significant differences in cyclone behaviour between the climatology and the SOPs in specific regions. The monthly mean sea level pressure (MSLP) anomalies during the SOPs were quite large (and exceeded 10 hPa in places), particularly in the Pacific and in the region to the south of Australia. It appears that the anomalous cyclone structure during the SOPs could be related to the anomalies of the MSLP. The authors suggest that the three SOPs cannot be regarded as typical of their time of year, but it could be argued that no specific period could be so regarded.The results obtained with these high quality analyses during the SOPs have confirmed the Antarctic coast as a region of high cyclone density and of very active cyclogenesis. The identification of these high levels of coastal cyclogenesis appears to differ from early studies that suggested the greatest (winter) cyclogenetic activity to be much farther north in the 40-50S region, The results presented here, however, concur with recent studies undertaken with high-resolution satellite data and four-dimensional data analyses, and the theoretical consequences of the baroclinic structure of the Antarctic coastal region.The Antarctic First Regional Observing Study of the Troposphere (FROST) project had three one-month Special Observing Periods (SOPs) during which the commitment was made to ensure that all additional data collected were passed on via the Global Telecommunication System (GTS) to operational centres for use in the construction of the analyses. These analyses can be regarded as the best available for these times of year, given the special effort to include additional data south of 50S during these periods.The availability of these high-quality analyses has stimulated us to refine the Melbourne University numerical cyclone tracking algorithm, with additional synoptic guidance gained from a manual analysis of southern hemisphere cyclones in the winter SOP (July 1994). Using the refined scheme we have compiled and compared statistics of cyclone tracks obtained objectively from the Australian GASP (Global Assimilation and Prediction) system analyses and manually from semi-independent analyses. Our results show that the cyclones found by the numerical and manual approaches bear considerable similarity to each other, even for complex systems for which such unanimity might not have been expected. In general, the automatic algorithm tended to 'find' more systems than did the manual analyst, with these extra systems being predominantly those identified as weak and/or open. The results emphasise the difference in perception of what constitutes a low.The overall behaviour of cyclones revealed by the objective scheme in July 1994 was consistent with that identified in various climatologies in that many systems were generated in the western part of the ocean basins and moved to the east and, to a lesser extent, to the south. A concentration of tracks was found just to the north of the Antarctic continent. On the other hand, this specific month was anomalous in a number of respects; this was reflected in the nature and distribution of cyclone activity. The consistency of the findings with those of an experienced, practicing synoptician means that the state-of-the-art numerical algorithm can be applied to numerical analyses and model output with confidence.It is argued that mathematical and numerical models can be of immense value to the climatologist and palaeoclimatologist as these tools can provide the 'glue' and framework which can tie together various pieces of climatic information. The power of these models lies in the fact that they are based on the basic physics governing the complex processes which determine climate and its variability and changes.The discussion presents some specific examples of where the modelling philosophy is able to contribute significantly to the task of interpreting palaeoclimatic information, ensuring the internal consistency of proxy data, and gaining new perspectives on the climate matrix.

本数据集基于欧洲中期天气预报中心（ECMWF）针对三场FROST时段的分析场提取所得，用以呈现上述时段内的平均海平面气压异常特征。此外，数据集还包含南极亚极地区域内所有气旋的综合分析，分析时段覆盖前述特殊观测期。

据部分参考文献摘要记载：南极对流层首次区域观测研究（FROST）项目的三个特殊观测期（SOPs）所采集的数据，为研究南半球冬、春、夏三季的气旋系统活动特征提供了优质基础。相关研究基于每日两次更新的ECMWF业务分析场，对三个SOP期内的气旋系统活动特征进行了报道。

本研究采用客观气旋追踪算法对数字化分析场进行处理。结果显示，气旋活动特征总体符合基于该算法构建的长期气候学统计结果。研究团队在SOP期内观测到，多数气旋系统生成于海盆西部，随后向东移动，另有少部分系统向南移动。三个观测期内，气旋移动轨迹均集中于南极大陆北侧，其中以印度洋区域最为显著。与此同时，在部分特定区域，SOP期内的气旋活动特征与长期气候学统计结果存在显著差异。SOP期内的月平均海平面气压（Mean Sea Level Pressure, 下称MSLP）异常幅度较大，部分区域甚至超过10百帕，尤其在太平洋及澳大利亚南部区域更为明显。研究表明，SOP期内异常的气旋结构或与MSLP异常存在关联。作者指出，三个SOP期不能被视为对应季节的典型时段，但也有观点认为，不存在可被视作典型的特定观测时段。

基于SOP期内高质量分析场得到的结果证实，南极海岸是气旋密度高、气旋生成活动极为活跃的区域。此前有早期研究认为，南半球冬季气旋生成活动的峰值区域位于南纬40°-50°的更北地带，而本研究观测到的沿岸高强度气旋生成活动与该结论存在差异。不过，本次研究结果与近期基于高分辨率卫星数据及四维数据分析得到的研究结论一致，同时也符合南极沿岸区域斜压结构的理论推演结果。

南极对流层首次区域观测研究（FROST）项目共设置三个为期一个月的特殊观测期（SOPs），项目期间承诺将所有新增采集的数据通过全球电信系统（Global Telecommunication System, 下称GTS）传输至业务中心，用于分析场的构建。鉴于项目期间特别增设了南纬50°以南区域的额外数据采集工作，上述分析场可被视为对应时段内质量最优的观测分析结果。

上述高质量分析场的可用性推动我们对墨尔本大学数值气旋追踪算法进行优化，优化过程中还借助了1994年7月冬季SOP期内南半球气旋的人工分析结果，以获取额外的天气学指导。基于优化后的算法，我们对两类气旋轨迹统计结果进行了整理与对比：一类是基于澳大利亚全球同化与预报（Global Assimilation and Prediction, 下称GASP）系统分析场得到的客观轨迹，另一类是基于半独立人工分析得到的轨迹。结果显示，数值方法与人工方法识别出的气旋系统具有较高的相似性，即便对于那些原本难以达成一致识别结果的复杂气旋系统亦是如此。总体而言，自动算法比人工分析识别出更多的气旋系统，其中额外识别出的系统大多为弱气旋或开放型气旋。该结果凸显了不同研究对“低压系统”定义的认知差异。

1994年7月SOP期内，客观算法揭示的气旋活动总体特征与各类气候学统计结果一致：多数气旋生成于海盆西部，随后向东移动，另有少部分向南移动。气旋轨迹仍集中于南极大陆北侧。但另一方面，该特定月份在多个方面均表现出异常，这一点从气旋活动的特征与分布中即可体现。本研究结果与资深天气分析专家的观测结果一致，这意味着当前最先进的数值算法可放心应用于数值分析场及模式输出结果的气旋识别工作。

有观点认为，数学与数值模型对气候学家及古气候学家具有极高的应用价值，因为这类工具可提供“粘合剂”与框架，将各类气候信息整合为统一整体。这类模型的优势在于，其构建基于支配气候及其变化与变异的复杂过程的基本物理规律。

本次讨论将通过若干具体案例，展示建模思路如何为古气候信息解译、保障代理数据的内部一致性，以及深化对气候系统整体的认知提供重要助力。