What causes the strengthening of the tropical easterly jet around the late 1990s?

Previous studies documented a dramatic weakening of the tropical easterly jet (TEJ) since the mid-20th century and attributed it to the decreasing meridional temperature gradient (MTG) between the Tibetan Plateau and the Indian Ocean. It has recently been reported that the TEJ core intensity experienced a recovery around the late 1990s, but the underlying causes remain unclear. The present study shows that this recovery arises from a combination of multi factors, with enhanced convective activity over the Indian Peninsula (INP) and sea surface temperature (SST) variability in the tropical Pacific as two prominent contributors. Idealized experiments with a baroclinic model demonstrate that the INP convective heating can excite an upper-tropospheric Rossby-wave response that accelerates the easterly wind in TEJ core region. Ocean pacemaker experiments further indicate that the tropical Pacific SST variability exerts a dominant influence on the large-scale TEJ changes through modulating the large-scale MTG and divergent circulations, whereas the tropical Indian Ocean warming acts to inhibit the strengthening and the Atlantic SST variability has negligible effects. These results point out the joint influence of multi factors on driving the decadal recovery of the TEJ intensity around the late 1990s, which can help advance our understanding of mechanisms governing the decadal variability of the TEJ.