Climatology of Rossby Wave Breaking Over the Subtropical Indian Region

The weather of subtropical Indian region is frequently impacted by Rossby Wave Breaking (RWB) triggered dynamical changes in the atmosphere. However, studies are elusive on the comprehensive understanding of RWB events over the region. A Potential Vorticity (PV) contour searching algorithm based on existing literature, but implemented with more constraints appropriate for the Indian subcontinent, is used to detect the RWB events. We detect 513 RWB events during the analysis period (1979-2021) and utilize them to understand the RWB climatology and variability over the region. A significant increase in the frequency (about 20%) and intensity of RWB events noticed during the last two decades (1999-2018). The spatial distribution of RWB occurrences is prominent over the northwest parts of the Indian subcontinent during winter months. The most frequent breaking occurs in the wavenumbers ranging between 5 and 7, with stronger wave amplitudes and deeper tropospheric intrusions (similar to 650 hPa) during winter months that reach as low as 7.5 degrees N in latitude. Further, our analysis shows that RWB promotes extreme rainfall by generating instability in the atmosphere allowing enhanced moisture influx over the region. It is also noted that interannual-to-decadal variations in the number of RWB events are linked to sea surface temperature conditions in the Pacific ocean. The findings of this study would contribute to an improved understanding of RWB and associated wave dynamics in triggering extreme weather and may find applications in climate studies on extremes in a warming climate. Plain Language Summary Rossby waves are planetary scale waves that usually propagate over higher latitude regions. Sometimes, these waves break similar to waves breaking at the shoreline. This results in air mass transport from high latitudes to low latitudes and vice-versa. Also, the wave breaking destabilizes the underlying atmosphere and triggers extreme weather conditions. In this study, we have implemented an algorithm suitable to the subtropical Indian region to detect the Rossby Wave Breaking (RWB) events. The algorithm uses the high Potential vorticity (PV) contours which determine the flow of the upper atmosphere to identify the breaking events. Using the algorithm, we have detected more than 500 RWB events during the period 1979-2021 and reported the spatiotemporal variability of the RWB events over the study region. The results suggest that the number of events per year has increased in the last two decades. In addition, the vertical and latitudinal intrusions of the RWB events along with its wave properties suggest that the breakings are stronger in winter compared to other seasons. The RWB events trigger extreme weather events such as extreme rainfall events and these events are modulated by the conditions in the Pacific.