Abstract Using 39 years of ECMWF renalysis data, an established energetics framework and simple composite analysis this study has shown that South African cut-off low (COL) pressure systems are preceded by downstream development of a baroclinic wave. Downstream development is characterised by the evolution of two energy centres, one located upstream in the midlatitudes and another downstream in the subtropics. The upstream eddy kinetic energy, which is associated with a midlatitude jet streak, develops and reaches its maximum before the formation of the closed COL cyclonic circulation. The downstream eddy kinetic energy centre maximises at the point where the closed circulation forms. The upstream eddy kinetic energy centre grows from baroclinic conversion from eddy available potential energy to eddy kinetic energy, whilst the downstream centre grows by receiving energy by means of ageostrophic geopotential fluxes that transport eddy kinetic energy in a north-eastward direction from the upstream centre. These ageostrophic geopotential fluxes are induced, increased in magnitude and directed by processes associated with Rossby wave breaking (RWB) on the midlatitude dynamical tropopause. and so the downstream energy transfer connects South African COLs to midlatitude processes. The study has further shown that the baroclinic kinetic energy configuration previously associated with wet seasons over South Africa is consistent with times when COLs forms over the country.

中文翻译：

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南非截止低压系统的下游开发

摘要 本研究使用 39 年的 ECMWF 再分析数据、已建立的能量学框架和简单的综合分析表明，南非截止低压 (COL) 压力系统之前有斜压波的下游发展。下游发展的特点是两个能源中心的演化，一个位于中纬度上游，另一个位于亚热带下游。与中纬度急流相关的上游涡动能在闭合 COL 气旋环流形成之前发展并达到最大值。下游涡动能中心在形成闭合循环的点处最大化。上游涡动能中心从涡可用势能向涡动能的斜压转换增长，而下游中心通过地转地势通量接收能量而增长，这些地势通量从上游中心向东北方向输送涡流动能。这些地转地势通量是由与中纬度动力对流层顶上的罗斯比波破碎 (RWB) 相关的过程引起的，在幅度上增加和引导。因此下游的能量转移将南非的 COL 连接到中纬度过程。该研究进一步表明，以前与南非雨季相关的斜压动能配置与该国形成 COL 的时间一致。这些地转地势通量是由与中纬度动力对流层顶上的罗斯比波破碎 (RWB) 相关的过程引起的，在幅度上增加和引导。因此下游的能量转移将南非的 COL 连接到中纬度过程。该研究进一步表明，以前与南非雨季相关的斜压动能配置与该国形成 COL 的时间一致。这些地转地势通量是由与中纬度动力对流层顶上的罗斯比波破碎 (RWB) 相关的过程引起的，在幅度上增加和引导。因此下游的能量转移将南非的 COL 连接到中纬度过程。该研究进一步表明，以前与南非雨季相关的斜压动能配置与该国形成 COL 的时间一致。