Abstract
The monsoon breaks in the core monsoon region of India, lasting a few days or sometimes even up to 2 weeks, are periods of low or no rainfall. Our analysis of breaks shows they are characteristically preceded by an upper-tropospheric transient blocking high over north India, which extends to low levels. The formation of this blocking high is found to be due to the penetration of mid-latitude baroclinic waves into lower latitudes. Importantly, this transient blocking high is weaker than the seasonal high at this level. The weakening of the high pressure due to the presence of this transient blocking leads to a sinking motion relative to the pre-break or post-break period, which in turn leads to a filled-up surface low, and an eventual reduction in daily rainfall.
The blocking high is associated with the convergence of Eliassen-Palm fluxes, which causes the upper-level zonal westerlies to decelerate and holds the blocking high in place without being advected. Also, we find that the resonance and amplification of planetary Rossby waves play a vital role in the block formation over the northwest of India. This sequence of events, the formation of upper-level blocking high due to resonance of planetary Rossby waves at mid-latitudes followed by the break and a relative sinking over the core monsoon region, also explains the North and South American summer monsoon breaks. The entire life cycle of monsoon break formation and reversal to normal monsoon can be seen as a barotropic adiabatic cycle, analogous to the well-known midlatitude index cycle. We propose these oscillations as "Monsoon Index Cycles."