Cold surge episodes over southeastern Brazil – a potential vorticity perspective
Cold surge episodes over southeastern Brazil – a potential vorticity perspective
复制标题
巴西东南部的冷潮事件——潜在涡度的视角
DOI：
10.1002/joc.3618
复制
发表时间：
2013
期刊：
影响因子：
--
通讯作者：
J. Arnold
中科院分区：
文献类型：
--
作者：
M. Sprenger;O. Martius;J. Arnold
在巴西南部和东南部，最强烈的寒潮和相关的霜冻事件的特点是，南美洲上空有一个向热带纬度延伸的大振幅槽，其以西有一个在太平洋上空的脊。在本研究中，位涡（PV）流线用于检测导致冷涌流的流动条件。案例研究表明，几个PV异常与冷潮事件有关：(1)在冷潮事件之前和期间，南美洲上游的位涡度单位（2‐PVU）等值线逐渐扭曲，表明气流状况有利于罗斯比波破碎，因此气流强烈偏离纬向性；(2)冷潮初始阶段的特征是高PV气流向北膨胀至安第斯山脉以东，形成一条PV流带，其北端到达乌拉圭和巴西东南部；西侧的强PV梯度形成了一个气流结构，诱导并维持了亚南极空气向亚热带的输送；(3)南美洲板块上游的南太平洋东部出现明显的负PV异常，形成阻塞。对平流层PV流频率在几个等熵面上的27次冷涌流进行了综合分析。它揭示了南美洲和南大西洋西部的赤道破波是强烈冷潮动力学的一个重要潜在组成部分。这些迹象在320 K等熵水平附近和巴西亚热带地区气温降幅最大的前一天最为明显。
Most intense cold surges and associated frost events in southern and southeastern Brazil are characterized by a large amplitude trough over South America extending toward tropical latitudes and a ridge to the west of it over the Pacific Ocean. In this study, potential vorticity (PV) streamers serve to examine the flow condition leading to cold surges. Case studies suggest that several PV anomalies are related to cold surge episodes: (1) the potential vorticity unit (2‐PVU) isoline upstream of South America becomes progressively more distorted prior and during the cold surge episode, indicating a flow situation which is conducive for Rossby wave breaking and hence a flow which strongly deviates from zonality; (2) the initial stage of a cold surge episode is characterized by a northward bulging of high‐PV air to the east of the Andes, resulting in a PV streamer whose northern end reaches Uruguay and southeastern Brazil; the strong PV gradient on its western flank constitutes a flow configuration that induces and maintains the transport of sub‐Antarctic air toward the subtropics; (3) a distinct negative PV anomaly, a blocking, originates over the eastern South Pacific, upstream of the South America sector. A composite analysis of 27 cold surges is performed for stratospheric PV streamer frequency on several isentropic surfaces. It reveals that equatorward wave breaking over South America and the western South Atlantic represents an important potential component of the dynamics of intense cold surges. The indications are most pronounced around the isentropic levels of 320 K and immediately before the day with largest temperature drops over subtropical Brazil.