Impacts of the annual cycle of background SST in the tropical Pacific on the phase and amplitude of ENSO

The dominant annual cycle of sea surface temperature(SST)in the tropical Pacific exhibits an antisymmetric mode,which explains 83.4%total variance,and serves as a background of El Niño-Southern Oscillation(ENSO).However,there is no consensus yet on its anomalous impacts on the phase and amplitude of ENSO.Based on data during 1982-2022,results show that anomalies of the antisymmetric mode can affect the evolution of ENSO on the interannual scale via Bjerknes feedback,in which the positive(negative)phase of the antisymmetric mode can strengthen El Niño(La Niña)in boreal winter via an earlier(delayed)seasonal cycle transition and larger(smaller)annual mean.The magnitude of the SST anomalies in the equatorial eastern Pacific can reach more than±0.3◦C,regulated by the changes in the antisymmetric mode based on random sensitivity analysis.Results reveal the spatial pattern of the annual cycle associated with the seasonal phase-locking of ENSO evolution and provide new insight into the impact of the annual cycle of background SST on ENSO,which possibly carries important implications for forecasting ENSO.

基于1982–2022年资料分析,本文发现,热带太平洋海温年循环的主导模态为反对称模式,是ENSO发展的背景场.然而,其对ENSO相位和振幅的异常影响尚未可知.反对称模态异常可以通过Bjerknes反馈影响ENSO的年际变化,其正(负)异常可以通过更早(更晚)的季节循环转变时间和更大(更小)的年平均值增强冬季El Nino(La Nina)的强度.根据随机敏感性实验分析,与反对称模态变化有关的赤道中东太平洋海温异常可达±0.3℃以上.研究结果为背景海温年循环对ENSO的影响提供了新的见解,这可能对ENSO的预测具有重要意义.