Influences of the Slow‐Decaying and Fast‐Decaying El Niño on the Tropical Easterly Jet

ABSTRACT As one of the key components of the Asian summer monsoon system in boreal summer, the upper‐tropospheric tropical easterly jet (TEJ) is significantly modulated by the El Niño‐Southern Oscillation (ENSO) on the interannual time scale. In this study, we report how the El Niño decay rate prominently controls the TEJ's behaviours from the post‐El Niño spring to summer. For the slow‐decaying (SD) El Niño cases, a warmer‐than‐usual sea surface temperature anomaly (SSTA) pattern in the tropical central‐eastern Pacific sustains from the mature winter to decaying summer. Consequently, the associated equatorial Kelvin wave response will decelerate the TEJ in its outflow and core regions in early summer. Moreover, the SD El Niño SSTA pattern induces a pair of anomalous upper‐tropospheric anticyclones over the western Pacific, where the TEJ usually inflows in the shape of double easterly jets. These anticyclone responses will strengthen the south branch of double easterly jets and weaken the north one branch. The fast‐decaying (FD) El Niño cases experience a phase transition into a La Niña‐like cooling pattern in the decaying phase. The FD El Niño‐induced atmospheric circulation is almost a mirror to the counterpart induced by the SD El Niño cases in early summer. That is, the FD El Niño cases aid in a weaker‐than‐normal south branch of double easterly jets and a stronger‐than‐normal north branch in the western Pacific, as well as an acceleration of the entire TEJ. These results foreground the significant influences of the El Niño decaying pace on the TEJ in early summer and indicate that the temporal diversity in the ENSO events should be considered for further understanding of the ENSO impacts on the Asian summer monsoon system.