La Plata basin precipitation variability in spring: role of remote SST forcing as simulated by GCM experiments
La Plata basin precipitation variability in spring: role of remote SST forcing as simulated by GCM experiments
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拉普拉塔盆地春季降水变化：GCM 实验模拟的远程海温强迫的作用
作者：
A. Cherchi;A. Carril;C. Menéndez;L. Zamboni
利用相同海温年际变化（SST）和不同初始条件下的9个实验，研究了热带海洋对拉普拉塔盆地（LPB）地区降水变率的调节作用。该集合的结果与20世纪用海洋-大气耦合模式进行的实验进行了比较，它们具有相同的大气成分。南美洲降水的旋转经验正交函数分析表明，两个试验都真实地捕捉到了春季变率的主导模式。其主成分（RPC1）与全球海温和大气场相关，确定了与El Niño南方涛动及其大尺度遥相关的模式。总的来说，这种模式在热带南太平洋得到了很好的模拟，主要是在整体中，但在其他海洋地区则没有或太弱。耦合模式试验表明，副热带南大西洋海气相互作用对LPB降水与海温之间的关系有重要作用。当对RPC1与观测值一致的集合成员进行海表温度和大气场的复合分析时，模式和数据之间的对应关系得到了很大的改善：这种改善依赖于通过仅对统计上相似的成员进行平均来避免气候噪声。此外，结果表明，由于大气内部变率，存在高度的不确定性。从模型和数据RPC1比较中选择的个别年份的分析显示，LPB多雨春季之间存在有趣的差异。例如，1982年，对应于强厄尔尼诺Niño年，代表了一个清晰的例子，一个明显的波列从太平洋中部传播并与另一个来自热带东部南印度洋的波列合并。2003年是LPB春季多雨的一个例子，不是由远端海温强迫直接驱动的。在这种情况下，内部变率起主导作用，因为模式不能重现正确的局部降水模式。
An ensemble of nine experiments with the same interannually varying sea surface temperature (SST), as boundary forcing, and different initial conditions is used to investigate the role of tropical oceans in modulating precipitation variability in the region of La Plata Basin (LPB). The results from the ensemble are compared with a twentieth-century experiment performed with a coupled ocean-atmosphere model, sharing the same atmospheric component. A rotated empirical orthogonal functions analysis of South America precipitation shows that the dominant mode of variability in spring is realistically captured in both experiments. Its principal component (RPC1) correlated with global SST and atmospheric fields identifies the pattern related to El Niño Southern Oscillation and its large-scale teleconnections. Overall the pattern is well simulated in the tropical southern Pacific Ocean, mainly in the ensemble, but it is absent or too weak in other oceanic areas. The coupled model experiment shows a more realistic correlation in the subtropical South Atlantic where air-sea interactions contribute to the relationship between LPB precipitation and SST. The correspondence between model and data is much improved when the composite analysis of SST and atmospheric fields is done over the ensemble members having an RPC1 in agreement with the observations: the improvement relies on avoiding climate noise by averaging only over members that are statistically similar. Furthermore, the result suggests the presence of a high level of uncertainty due to internal atmospheric variability. The analysis of some individual years selected from the model and data RPC1 comparison reveals interesting differences among rainy springs in LPB. For example, 1982, which corresponds to a strong El Niño year, represents a clean case with a distinct wave train propagating from the central Pacific and merging with another one from the eastern tropical south Indian Ocean. The year 2003 is an example of a rainy spring in LPB not directly driven by remote SST forcing. In this case the internal variability has a dominant role, as the model is not able to reproduce the correct local precipitation pattern.