Influence of southern hemispheric upper troposphere potential vorticity intrusion events on the southwest monsoon rainfall

The possible influence of potential vorticity (PV) intrusion events in the southern low-latitude upper troposphere over the longitude region 0–90°E during June–September on the southwest monsoon rainfall (SWMR) over India is investigated using ERA-interim potential vorticity data and high resolution gridded IMD (India Meteorological Department) rainfall data. It is found that unlike in the northern hemisphere, the PV intrusion events are more frequent in the southern low-latitudes in the longitude region 0–90°E during June–September and they are present irrespective of the phase of the El Nino Southern Oscillation (ENSO). More PV intrusion events into the southern hemispheric low-latitude upper troposphere are observed during the negative SWMR anomaly years. The cross-equatorial flow is observed to be weak during intrusion events. The deep penetration of PV into the lower troposphere in southern hemisphere reverses low-level wind direction from north to south during or the next day of the PV anomaly events. This reversal of low-level wind leads to a break in the SWMR, which can be inferred from the positive time mean removed OLR (>10 W/m 2 ) during high PV event days. It is suggested that the reversal of low-level wind induced by PV intrusion event may have prevented moisture transport to India, resulting in break spell and hence reduced rainfall.