A Continuing Increase of the Impact of the Spring North Pacific Meridional Mode on the Following Winter El Nino and Southern Oscillation

This study reveals that the impact of the spring North Pacific meridional mode (PMM) on the following -winter El Nin similar to o-Southern Oscillation (ENSO) shows a continuing increase in the past. A comparative analysis is conducted for the high-and low-correlation periods to understand the factors for the strengthened impact of the PMM. The spring PMM-related sea surface temperature (SST) and atmospheric anomalies over the subtropical northeastern Pacific propa-gate southwestward to the tropical central Pacific via wind-evaporation-SST feedback in the high-correlation period. The tropical SST and atmospheric anomalies further develop to an ENSO-like pattern via positive air-sea interaction. In the low-correlation period, SST and atmospheric anomalies over the subtropical northeastern Pacific related to the PMM cannot extend to the deep tropics. Therefore, the spring PMM has a weak impact on ENSO. The extent to which the PMM-related SST and atmospheric anomalies extend toward the tropics is related to the background flow. The stronger mean trade winds in the high-correlation period lead to an increase in the air-sea coupling strength over the subtropical northeastern Pacific. As such, the spring PMM-related SST and atmospheric anomalies can more efficiently propagate southwestward to the tropical Pacific and exert stronger impacts on the succeeding ENSO. In addition, the southward shifted intertropical convergence zone in the high-correlation period also favors the southward extension of the PMM-related SST anomalies to the tropics and contributes to a stronger PMM-ENSO relation. The variation and its formation mechanism of the spring PMM-winter ENSO relationship appear in both the observations and the long historical simula-tion of Earth system models.

Automated summary

This study finds that the impact of the spring North Pacific meridional mode (PMM) on the following-winter El Niño-Southern Oscillation (ENSO) has been increasing. The study compares high- and low-correlation periods to understand the factors behind the strengthened impact of PMM. In the high-correlation period, PMM-related sea surface temperature (SST) and atmospheric anomalies propagate southwestward to the tropical central Pacific via wind-evaporation-SST feedback, resulting in a stronger ENSO-like pattern. In the low-correlation period, PMM-related anomalies do not extend to the deep tropics, leading to weaker impact on ENSO.