The role of synoptic circulations in lower-tropospheric DSE variability over a South Asian heatwave hotspot

Abstract. We examine the role of the synoptic-scale circulation in the distribution of day-day changes of 600–900 hPa dry static energy (DSE) in a heatwave hotspot in northwest South Asia. Using a combination of linear regression and decision trees, we identify how the quasilinear (mean-eddy) and nonlinear (eddy-eddy) components of the flow contribute to different parts of this distribution. We show that the presence of synoptic eddies leads to strong correlations in the quasilinear components due to quasigeostrophy, allowing us to identify periods of upper tropospheric eddy activity. We show that the synoptic eddies induce a zonal quasilinear component which plays an important role in governing the magnitude and sign of DSE changes. Nonlinear components are observed to play an important role in the tails of this distribution, and we show that the specific nonlinear components that are involved depends on the phase of growth or decay of DSE and the background DSE anomaly. We identify energetically distinct configurations involved in the tails of this distribution, and identify eddy configurations corresponding to each of these energetic configurations. Our analysis thus provides a discrete set of "regimes" which can be used to classify extreme DSE changes, and provides a more nuanced approach to compositing extreme events which is sensitive to the dynamics underpinning each event.