热带北大西洋海温与降水关系的季节性特征及其物理机制

Seasonality of the SST-precipitation relationship over the tropical North Atlantic and possible mechanisms

  • 摘要: 基于英国哈得来中心提供的逐月海表温度资料、美国全球降水气候中心提供的逐月降水资料以及CMIP6历史模拟资料,文中探究了热带北大西洋海温异常指数(NTA)与局地降水异常关系的季节性特征及其在有无ENSO海温强迫下的差异,并分析了可能的物理机制。结果表明,NTA和局地降水异常的关系呈现出明显的季节性差异。春、夏季,该区域海温与降水呈显著正相关,局地海-气耦合强;相对而言,秋、冬季海-气耦合弱,局地降水对海温异常几乎没有响应。进一步研究表明,这种季节性变化特征主要是由热带北大西洋气候态海温及局地海温变率的季节循环导致。春季热带北大西洋气候态海温偏低,但该季的强海温变率使海温可突破对流阈值引发降水异常;夏季偏高的气候态海温使得该区域海-气耦合活跃;尽管秋季气候态海温也偏高,但是较小的海温变率使得局地降水响应减弱,易受外界噪声干扰;与夏季相反,冬季偏低的气候态海温导致该区域海-气耦合较弱。由于ENSO对春季NTA的显著影响,春季该区域在ENSO强迫下的局地海温异常更容易激发局地对流;而在其他季节,该区域海-气耦合程度受ENSO的影响较小。本研究的结果强调了春、夏季NTA对激发热带强对流和引起远程气候影响的重要作用,这对短期气候预测具有重要意义。

     

    Abstract: Based on the monthly Sea Surface Temperature (SST) data from the Met Office Hadley Centre, the Global Precipitation Climatology Project monthly precipitation data, and the historical simulations from Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models, the present work investigates the seasonality of the SST-precipitation relationship over the Tropical North Atlantic (NTA) and possible role of ENSO (El Niño-Southern Oscillation). It is found that the relationship of SST anomalies in the NTA with local precipitation exhibits a remarkable seasonality. During spring and summer, there is a significant positive correlation between SST and precipitation in this region, indicating a strong local ocean-atmosphere coupling. In contrast, in autumn and winter, the ocean-atmosphere coupling weakens significantly, and almost no significant precipitation response to SST is detected. Further analysis reveals that this seasonality is mainly associated with the seasonal cycle of the background SST and local SST variability in the NTA. Despite cooler background SST in spring, the strong SST variability during this season makes SST easy to exceed the convection threshold and thus induces precipitation anomalies. In summer, the warm background SST favors the enhanced local ocean-atmosphere coupling. The relatively weak SST variability in autumn weakens the local precipitation response, despite a relatively warm SST background. Different to that in summer, the cooler background SST in winter results in a weak ocean-atmosphere coupling. ENSO has a significant influence on spring SST anomalies in the NTA, while the Atlantic intertropical convergence zone is at its southernmost point in spring. As a result, the influence of ENSO on spring precipitation anomalies also shifts southerly, and strong local SST anomalies in spring are more likely to actively trigger local convective responses under ENSO forcing. However, in other seasons, the impact of ENSO on SST anomaly in the NTA is relatively small, and thus there is almost no difference in local ocean-atmosphere coupling in the tropical North Atlantic with or without ENSO SST forcing. These findings emphasize the critical role of spring and summer NTA SST anomalies in local convection and associated climate impacts, which is important for short-term climate prediction related to the NTA SST.

     

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