<p>We investigated the physical mechanism for late Indian Summer Monsoon onset over Kerala<br>(MOK). 14 early and 9 late onset years are selected based on the criteria when the onset is 5 days or<br>more prior and after normal onset date (i.e 1 st June according to India Meteorological Department)<br>respectively. Then, we perform composite analyses of mean May monthly and daily evolution during<br>early and late onset years to examine the differences in monsoon circulation features prior to the MOK.<br>We find that advection of Surface Air Temperature (SAT) from the northern to the southern China and<br>the eastern Tibetan Plateau (TP) plays an important role to modulate the MOK processes. In the late<br>onset years, more low-level jet (LLJ) from the Bay of Bengal (BOB) divert towards the east Asia before<br>the onset, which is due to an extension of the low sea level pressure and high SAT over the east Asia<br>(eastern TP, east-central China). This strengthens the low-level convergence and upper level divergence<br>over the eastern TP and southern China. As a result, a significant amount of moisture from the BOB<br>is transported towards the eastern TP and southern China. Thereby, a comparatively weaker LLJ and<br>deficit low-level moisture supply over the eastern BOB maintain the key roles in modulating the MOK<br>processes.</p>