Critical role of boreal summer North Pacific subtropical highs in ENSO transition

The quasi-biennial (QB)-type El Niño-Southern Oscillation (ENSO), showing a fast phase transition from El Niño to La Niña, is closely related to the variability of the North Pacific subtropical high (NPSH) and western North Pacific subtropical high (WNPSH) during summer. Here, we show that the NPSH plays a key role in the fast ENSO transition. The QB-type ENSO is associated with both strengthened WNPSH and NPSH during the boreal summer. By contrast, the low-frequency-type ENSO, which occurs in a typical period of 3–7 years, displays an enhanced WNPSH but weakened NPSH. The stronger El Niño tends to generate a more intensified WNPSH from spring to summer, leading to the initial decay of El Niño via the modulation of easterly wind in the western Pacific. On the contrary, the NPSH has greater linkage with the decaying El Niño process after the boreal summer. Therefore, the coupled pattern of WNPSH–NPSH is important in changing ENSO phase from El Niño to La Niña. The NPSH causes sea surface temperature cooling over the subtropical Northeastern Pacific. The resultant subtropical cooling induces anomalous anticyclone west of the reduced heating, which generates the strengthening of trade winds south of the anticyclone. Consequently, this process contributes to tropical central Pacific cooling and the rapid transition of El Niño to La Niña. This study hints that the QB-type ENSO could be significantly linked to a tropics-midlatitudes coupled system such as an in-phase pattern between WNPSH and NPSH. The results are useful for improvement of ENSO prediction.