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Previous studies found a tight connection between the tropical easterly jet (TEJ) and Indian summer monsoon rainfall (ISMR). Here we show that the TEJ-ISMR relationship is nonstationary and breaks down from 1994–2003 (epoch P2), in contrast to the significant positive correlation during the epoch P1 (1979–1993) and P3 (2004–2016). The breakdown of the TEJ-ISMR relationship concurs with the increased rainfall variability over the tropical eastern Indian Ocean (TEIO). The enhanced TEIO rainfall anomalies excite a significant lower-level cyclonic circulation that reduces the ISMR, meanwhile, strengthens the upper-level divergence and excites a pair of upper-level anticyclone to the west of the TEIO as Rossby wave responses, both accelerating the TEJ. Thus, the TEIO rainfall plays a more important role than the ISMR in the TEJ variability during P2, causing the breakdown of the TEJ-ISMR relationship. In contrast, a relatively weak amplitude of the TEIO rainfall during P1 and P3 was unable to change the positive TEJ-ISMR relationship. The changes in the TEIO rainfall variability is mainly attributed to the increased SST variability over the tropical southeastern Indian Ocean, but the cause of it remains elusive.

The upper-level tropical easterly jet (TEJ) is a crucial component of the summer monsoon system and tropical general circulation. The simulation and projection of the TEJ, however, have not been assessed. Here we evaluate models’ fidelity and assess the future change of the TEJ by utilizing 16 models that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). Most of the models can reproduce the TEJ reasonably well in terms of climatology, seasonal evolution, and interannual variability. Nevertheless, underestimation of the TEJ’s intensity and extent is identified, with the maximum bias occurring in the jet centers over the tropical Indian Ocean (IO) and the tropical eastern Pacific (EP). Under the shared socioeconomic pathway 5–8.5, the multimodel ensemble projects a remarkable reduction in the central TEJ intensity by about 18% over the IO and 77% over the EP toward the end of the twenty-first century. The mean intensity of TEJ will weaken by about 11%, and the extent will reduce by 6%, suggesting a significantly weakened upper-level monsoon circulation in the future climate. The projected El Niño–like warming pattern over the tropical Pacific may play a critical role in the future weakening of the TEJ via inducing suppressed rainfall over the tropical eastern IO and Central America. The model uncertainties in the projected TEJ changes may arise from the uncertainties in the models’ projected tropical EP warming. The sensitivity of future projections to model selection is also examined. Results show that the selection of models based on different physical considerations does not yield a significantly different projection.