Dominant patterns of interaction between the tropics and mid-latitudes in boreal summer: causal relationships and the role of timescales
Dominant patterns of interaction between the tropics and mid-latitudes in boreal summer: causal relationships and the role of timescales
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DOI：
10.5194/wcd-1-519-2020
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发表时间：
2020-10-15
影响因子：
--
通讯作者：
Coumou, Dim
中科院分区：
文献类型：
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作者：
Di Capua, Giorgia;Runge, Jakob;Coumou, Dim
热带对流活动是北方半球中纬度天气可预测性的一个来源。在冬季，厄尔尼诺-南方涛动（ENSO）是热带和热带外地区可预测性的主要来源，但它在夏季的作用要小得多，确切的遥相关路径还没有得到很好的理解。在这里，我们评估热带对流如何在不同的季节内时间尺度与中纬度夏季环流相互作用，以及ENSO如何影响这些相互作用。首先，我们应用最大协方差分析（MCA）热带对流活动和中纬度位势高度场之间的相互作用，以确定占主导地位的模式。第一个MCA模态将南亚季风与中纬度环全球遥相关型联系起来。第二个MCA模态将热带西北太平洋夏季风与中纬度北太平洋高压中心的5波型联系起来。我们发现，MCA模式是相当不敏感的选定的季节内的时间尺度从每周到4周的数据。为了研究这些模式与其他大气场之间的潜在因果关系，我们在不同的时间尺度上应用因果发现方法PCMCI。PCMCI通过消除自相关、间接联系和共同驱动因素的混杂效应，扩展了标准相关分析。一般来说，热带和中纬度地区之间存在双向因果关系，但因果关系的强度和符号有时取决于时间尺度。我们引入因果图，显示区域特定的因果效应，从每个MCA模式。这些地图证实了相互作用的主要模式，并突出显示了与热带对流联系最密切的特定中纬度地区。总的来说，已确定的因果遥相关型只受到厄尔尼诺/南方涛动的轻微影响，热带中纬度联系仍然相似。尽管如此，拉尼娜现象加强了南亚季风，在中纬度地区产生了更强的反应，而在厄尔尼诺年，太平洋模式得到加强。这项研究铺平了道路的过程为基础的验证北半球夏季遥相关（分）季节预报模式和气候模式，因此，努力改善分季节预测和气候预测。
Tropical convective activity represents a source of predictability for mid-latitude weather in the Northern Hemisphere. In winter, the El Nino-Southern Oscillation (ENSO) is the dominant source of predictability in the tropics and extratropics, but its role in summer is much less pronounced and the exact teleconnection pathways are not well understood. Here, we assess how tropical convection interacts with mid-latitude summer circulation at different intra-seasonal timescales and how ENSO affects these interactions. First, we apply maximum covariance analysis (MCA) between tropical convective activity and mid-latitude geopotential height fields to identify the dominant modes of interaction. The first MCA mode connects the South Asian monsoon with the mid-latitude circumglobal teleconnection pattern. The second MCA mode connects the western North Pacific summer monsoon in the tropics with a wave-5 pattern centred over the North Pacific High in the mid-latitudes. We show that the MCA patterns are fairly insensitive to the selected intra-seasonal timescale from weekly to 4-weekly data. To study the potential causal interdependencies between these modes and with other atmospheric fields, we apply the causal discovery method PCMCI at different timescales. PCMCI extends standard correlation analysis by removing the confounding effects of autocorrelation, indirect links and common drivers. In general, there is a two-way causal interaction between the tropics and mid-latitudes, but the strength and sometimes sign of the causal link are timescale dependent. We introduce causal maps that show the regionally specific causal effect from each MCA mode. Those maps confirm the dominant patterns of interaction and in addition highlight specific mid-latitude regions that are most strongly connected to tropical convection. In general, the identified causal teleconnection patterns are only mildly affected by ENSO and the tropical mid-latitude linkages remain similar. Still, La Nina strengthens the South Asian monsoon generating a stronger response in the mid-latitudes, while during El Nino years the Pacific pattern is reinforced. This study paves the way for process-based validation of boreal summer teleconnections in (sub-)seasonal forecast models and climate models and therefore works towards improved sub-seasonal predictions and climate projections.