海洋环流与波动重点实验室知识库

ENSO（El Niño-Southern Oscillation）是热带太平洋最大的年际气候变率模态，对全球气候、海陆生态系统以及社会经济等方面有十分重要的影响。近几十年来，ENSO的复杂性越来越突出，主要表现为CP-El Niño（central Pacific El Niño）事件的频繁发生。与传统的EP-El Niño（eastern Pacific El Niño）事件相比，CP-El Niño作为一种新型El Niño事件，其在时空演变过程和对全球气候的影响方面有很大的差异。因此，深入研究CP-El Niño的生成过程及机制对未来更好地预测ENSO有着重要指导价值和现实意义。

前人的大量研究表明，风应力异常结构对CP-El Niño的生成和演变具有重要作用。然而，不同的CP-El Niño事件所对应的风应力异常也是不同的，这使得前人的研究无法避免过度概括其中真正起作用的风的时间和位置。

本文基于德国版海洋环流和气候估算系统（German Estimating the Circulation and Climate of the Ocean，GECCO）的伴随敏感性分析方法，首次给出了理论上可以使Niño-4区海表温度升高的有利风扰动（favorable wind perturbations, FWPs），包括其方向、水平结构、位置和发生在CP-El Niño峰值月份（12月）之前的具体月份，以及其促进CP-El Niño形成的机制。FWPs在CP-El Niño发展年份上半年主要为赤道中太平洋的东风扰动和赤道南北两侧以及热带西太平洋的西风扰动；FWPs在CP-El Niño发展年份下半年主要为热带太平洋西部和中部的西风扰动。本研究创新性地揭示出上半年FWPs位于赤道中太平洋沿南北方向强度递减的东风扰动可有利于CP-El Niño的形成。中太平洋沿南北方向强度递减的东风扰动，可产生强的反气旋式风扰动旋度，有利于激发赤道外下降罗斯贝波（DR）。DR波向西传播并在西边界反射为下降开尔文波（DK），在CP-El Niño顶峰时期左右到达赤道中太平洋附近，其伴随的纬向流异常所产生的纬向平流反馈可以有效促进Niño-4区海表变暖。同时，海洋-大气耦合实验结果表明，一方面，上半年赤道中太平洋的独特东风结构导致冷异常并且激发Bjerknes反馈；另一方面，由于Gill响应，该东风结构在赤道外强的风应力旋度导致赤道外暖异常，激发DR波向西传播，引起热带西北太平洋的海表变暖。DR波在西边界反射为DK波，引起赤道西-中太平洋海表变暖。热带西北太平洋和赤道西中太平洋的海表暖异常诱发西风异常，通过Bjerknes反馈不断向东发展。由于Gill响应诱发的（和后续的Bjerknes反馈诱发的）热带西太平洋海表变暖强于Bjerknes反馈诱发的热带东太平洋的海表变冷，最终导致了CP-El Niño的生成。

另外，通过与CP-El Niño事件中的实际风异常对比，我们发现有不少CP-El Niño事件在上半年具有与FWPs类似的风异常结构，即赤道中太平洋的东风异常或在赤道中太平洋南北两侧有很强的风异常梯度（即有利于形成反气旋式风应力旋度）。这进一步说明，上半年独特的东风结构可能是CP-El Niño发生的一个重要因素。

同时，该类CP-El Niño事件下半年位于热带太平洋西部和中部的西风异常也与FWPs相一致。因此，本研究获取的FWPs可以为分析风驱动CP-El Niño的生成和演变过程提供有用的指导。

ENSO (El Niño-Southern Oscillation) is the largest interannual climate variability mode in the tropical Pacific, and has important impacts on the global climate, marine and terrestrial ecosystems, and social economy. In the past few decades, the complexity of ENSO has become more and more prominent, mainly manifested by the frequent occurrence of CP-El Niño (central Pacific El Niño) events. Compared with the traditional EP-El Niño (eastern Pacific El Niño), CP-El Niño, as a new type of El Niño event, has great difference in its spatio-temporal process and impact on global climate. Therefore, the in-depth study of the generation process and mechanism of CP-El Niño is of great guiding value and practical significance for better prediction of ENSO in the future.

Many previous studies have shown that wind stress anomalies play an important role in the generation and evolution of CP-El Niño. However, the wind stress anomalies corresponding to different CP-El Niño events are also different, which makes it hard for previous studies to avoid overgeneralizing the timing and location of the winds that indeed matter.

Based on the adjoint sensitivity method of GECCO (German Estimating the Circulation and Climate of the Ocean), the favorable wind perturbations (FWPs) which can theoretically increase sea surface temperature (SST) in Niño-4 region are given for the first time, including direction, horizontal structure, location, the specific month before the peak month of CP-El Niño (December), and their mechanism of promoting the generation of CP-El Niño. In the first half of the developing year of CP-El Niño, FWPs are mainly easterly perturbations over the central equatorial Pacific and westerly perturbations over the northern and southern sides of the equator and the western tropical Pacific; FWPs are mainly westerly perturbations over the western and central tropical Pacific in the second half of the developing year of CP-El Niño. This study innovatively reveals that the easterly perturbations with decreasing intensity along the north-south direction of FWPs over the central equatorial Pacific in the first half year is conducive to the generation of CP-El Niño. Easterly perturbations with decreasing intensity along the north-south direction in the central equatorial Pacific can produce strong anticyclonic wind perturbations curl, which is conducive to the excitation of off-equatorial downwelling Rossby (DR) waves. DR waves propagate westward and reflect as downwelling Kelvin (DK) waves at the western boundary, and reach the central equatorial Pacific around the peak period of CP-El Nino. The zonal advection feedback generated by the accompanying zonal flow anomalies can effectively promote the sea surface warming in the Niño-4 region. Meanwhile, the ocean-atmosphere coupling experiments results show that, on the one hand, the unique easterly wind structure over the equatorial central Pacific in the first half year results in cold anomaly and triggers Bjerknes feedback. On the other hand, due to Gill’s response, the strong wind stress curl of the easterly wind structure off the equator results in warm SST anomalies off the equator, which triggers the DR waves propagating westward, causing the SST warming in the northwestern tropical Pacific. DR waves reflect as DK waves at the western boundary, causing SST warming in the equatorial western-central Pacific. Those warm SST anomalies in the northwestern tropical Pacific and western-central equatorial Pacific induce westerly anomalies, which develop eastward through the Bjerknes feedback. Since the Gill’s response- (and the following Bjerknes feedback-) induced SST warming in the western tropical Pacific is stronger than Bjerknes feedback-induced SST cooling in the eastern tropical Pacific, the CP-El Nino is ultimately generated.

In addition, by comparing with the real wind anomalies in CP-El Niño events, we find that many CP-El Niño events have wind anomalies similar to FWPs in the first half of the year, that is, easterly anomalies in the central equatorial Pacific or strong positive wind anomalies gradient in the north and south sides of the central equatorial Pacific. This further indicates that the unique easterly wind structure in the first half of the year may be an important factor in the occurrence of CP-El Nino.

Meanwhile, the westerly anomalies of CP-El Niño events over the western and central tropical Pacific in the second half of the year are consistent with FWPs. Therefore, FWPs obtained in this study can provide useful guidance for analyzing the generation and evolution of wind-driven CP-El Niño.