Long-term Climatological Understanding of the Rossby Wave Breaking Events Over the Indian sub-continent.

The synoptic-scale Rossby waves propagating at the upper levels in the extratropical regions have a significant effect on the surface weather patterns. More importantly, when these baroclinic waves break they can trigger extreme weather conditions. Therefore, the Rossby wave breaking (RWB) events have been studied extensively in the extratropical and mid-latitude regions. However, notwithstanding the evidence of increasing trends in extremes, studies are elusive on the contribution of RWB over the extratropical Indian region. To address these shortcomings, we implemented a contour searching algorithm for the first time over the Indian region(5-40oN, 55-105oE) and identified the RWB events using a long-term ERA5 Reanalysis dataset (1979-2021). We detected nearly 500 RWB events over 43 years time period and analyzed their seasonal, interannual, and decadal variability. Our results have shown that there has been a significant increasing trend in terms of frequency and intensity for the last two decades. Moreover, the spatial distribution of RWB occurrences is prominent over the northwest part of the study region during winter. Further, it is noted that the background wind changes within a season have strong control on the location of RWB in the upper troposphere. In addition, intense RWB events, defined based on the depth of PV-intrusion within the troposphere, are stronger during winter relative to other seasons. Therefore, the findings of this study may contribute to an improved understanding of Rossby wave dynamics in triggering extreme weather and may find application in climatic studies on extremes in a warming climate.