On the seasonal prediction of the western United States El Nio precipitation pattern during the 2015/16 winter Climate Dynamics

A “typical” El Niño leads to wet (dry) wintertime anomalies over the southern (northern) half of the Western United States (WUS). However, during the strong El Niño of 2015/16, the WUS winter precipitation pattern was roughly opposite to this canonical (average of the record) anomaly pattern. To understand why this happened, and whether it was predictable, we use a suite of high-resolution seasonal prediction experiments with coupled climate models. We find that the unusual 2015/16 precipitation pattern was predictable at zero-lead time horizon when the ocean/atmosphere/land components were initialized with observations. However, when the ocean alone is initialized the coupled model fails to predict the 2015/16 pattern, although ocean initial conditions alone can reproduce the observed WUS precipitation during the 1997/98 strong El Niño. Further observational analysis shows that the amplitudes of the El Niño induced tropical circulation anomalies during 2015/16 were weakened by about 50% relative to those of 1997/98. This was caused by relative cold (warm) anomalies in the eastern (western) tropical Pacific suppressing (enhancing) deep convection anomalies in the eastern (western) tropical Pacific during 2015/16. The reduced El Niño teleconnection led to a weakening of the subtropical westerly jet over the southeast North Pacific and southern WUS, resulting in the unusual 2015/16 winter precipitation pattern over the WUS. This study highlights the importance of initial conditions not only in the ocean, but in the land and atmosphere as well, for predicting the unusual El Niño teleconnection and its influence on the winter WUS precipitation anomalies during 2015/16.

典型厄尔尼诺（El Niño）事件会导致美国西部（Western United States，WUS）的南部（北部）区域出现冬季偏湿（偏干）异常。然而在2015/2016年的强厄尔尼诺事件期间，美国西部的冬季降水分布却与这一典型（历史记录平均态）异常型大体相反。为探究这一反常现象的成因及其可预测性，我们采用耦合气候模式开展了一系列高分辨率季节预测试验。研究发现，当利用观测资料初始化海洋、大气、陆面各分量时，2015/2016年的异常降水分布在零预见期下具备可预测性。但仅利用海洋观测资料初始化时，该耦合模式无法再现2015/2016年的降水分布；不过仅依靠海洋初始条件，模式能够复现1997/1998年强厄尔尼诺事件期间美国西部的观测降水特征。进一步的观测分析表明，相较于1997/1998年，2015/2016年厄尔尼诺诱导的热带环流异常强度减弱了约50%。这一现象源于2015/2016年热带太平洋东（西）部的相对冷（暖）异常抑制了（增强了）东（西）热带太平洋的深对流异常。厄尔尼诺遥相关的减弱导致北太平洋东南部及美国西部南部区域的副热带西风急流出现弱化，进而引发美国西部2015/2016年冬季异常的降水分布。本研究强调，不仅海洋初始条件，陆面与大气的初始条件同样至关重要——这对于预测2015/2016年异常的厄尔尼诺遥相关及其对美国西部冬季降水异常的影响具有关键意义。