Monsoon low‐level jet (MLLJ) is an important component of Asian summer monsoon circulation, which is responsible for large‐scale distribution and variability of rainfall over the Indian subcontinent. The evolution of MLLJ characteristics prior to heavy rainfall days and its changes which lead to instances of heavy rainfall over the south‐western coast of India are studied. The European Centre for Medium Range Weather Forecasts (ECMWF) re‐analysis data and India Meteorological Department (IMD) rainfall data during 1979–2007 were used for this study. It is found that the MLLJ core speed, westerly wind depth, zonal water vapour flux, horizontal wind shear and cyclonic vorticity show a steady increase towards the day of heavy rainfall and gradually decrease thereafter. The intensification of these parameters not only brings in abundant moisture but also triggers convection, thereby resulting in the formation of heavy rainfall during monsoon season. The large‐scale advection of moisture brought about by strengthening of MLLJ is shown to be a prerequisite for heavy rainfall over the Indian subcontinent. An interesting result of this study is that heavy rainfall days may have some predictability by continuous monitoring of the progression of MLLJ.

India experienced a heavy rainfall event in the year 2013 over Uttarakhand and its adjoining areas, which was exceptional as it witnessed the fastest monsoon progression. This study aims to explore the causative factors of this heavy rainfall event leading to flood and landslides which claimed huge loss of lives and property. The catastrophic event occurred from 14th to 17th June, 2013 during which the state received 375% more rainfall than the highest rainfall recorded during a normal monsoon season. Using the high resolution precipitation data and complementary parameters, we found that the mid-latitude westerlies shifted southward from its normal position during the intense flooding event. The southward extension of subtropical jet (STJ) over the northern part of India was observed only during the event days and its intensity was found to be increasing from 14th to 16th June. The classical theory of westward tilt of mid-latitude trough with height, which acts to intensify the system through the transfer of potential energy of the mean flow, is evident from analysis of relative vorticity at multiple pressure levels. On analysing the North Atlantic Oscillation (NAO), negative values were observed during the event days. Thus, the decrease in pressure gradient resulted in decrease of the intensity of westerlies which caused the cold air to move southward. During the event, as the cold air moved south, it pushed the mid-latitude westerlies south of its normal position during summer monsoon and created a conducive atmosphere for the intensification of the system.

The life cycle and the large‐scale factors driving extreme heavy rainfall events over the south west coast of India are studied. The extreme rainfall events are linked to the development of monsoon depressions and the associated large‐scale dynamics. Strengthening of these parameters intensifies the monsoon low‐level circulation over the Arabian Sea and the west coast via steepened meridional pressure gradient. The intensification of the low‐level jet stream speed and its extension in the vertical causes an increase in the humidity flux in the lower and midtroposphere. The consequent ascending motion is from the midtroposphere to the upper troposphere, resulting in the formation of deep convective cloud clusters over the west coast and eastern parts of the Arabian Sea. This results in the incidence of extreme heavy rainfall over the south west coast of India. It is observed that during days of extreme rainfall, the direction of wind in the lower troposphere tends to be almost perpendicular to the Western Ghats favouring a strong orographic lift. The extreme rainfall events over the south west coast do not necessarily occur during the active cycle of monsoon intraseasonal oscillation, but are linked to the north westwards propagating monsoon depressions. We show that the signatures of extreme rainfall can be observed in several meteorological variables developing over different parts of the monsoon region. A synergistic analysis of these variables may help in the accurate and timely prediction of these events.