On the dynamical organization of temperature variability over South Asia

Abstract

Temperature extremes are identified as threshold-defined events, but their intensity, persistence, and warm-cold asymmetry are governed by the dynamical processes that shape the tails of regional temperature distributions. This thesis develops a diagnostic framework for understanding temperature variability, using a South Asian pre-monsoon (March-June) heatwave hotspot. The analysis focuses on March-April, when near-surface temperature changes are strongly linked to resolved atmospheric advection. Lower-tropospheric dry static energy (DSE) advection over 600-900 hPa is used as a dynamical bridge between synoptic circulation and near-surface temperature variability.

The thesis first establishes that lower-tropospheric DSE advection is closely related to daily near-surface temperature tendencies and is embedded within vertically coherent synoptic disturbances. The DSE-advection tendency is then decomposed into quasilinear and nonlinear Reynolds components. The tendency distribution is primarily organized by quasi-linear interactions between transient anomalies and the climatological background state, while the tails require nonlinear anomaly–anomaly interactions. Interpretable machine-learning techniques like decision-trees and regression help identify advective regimes associated with growth and decay of pre-existing lower-tropospheric DSE anomalies. The thesis next examines how nonlinear advection modifies the quasilinear tendency distribution and shapes skewness. Although nonlinear terms are strongly active in both tails, their components partly compensate across zonal, meridional, and vertical components.

Weak skewness can therefore coexist with strong nonlinear tail activity. This shows that scalar distribution moments should be decomposed to yield dynamically important tail interactions. Finally, the thesis shows that cold and warm near-surface anomalies decay through structurally different pathways. Cold decay is associated with anomalous vertical motion acting on a lower-tropospheric DSE anomaly gradient, while low-level nonlinear horizontal advection helps maintain near-surface cold anomalies under relatively stable stratification. Warm decay exhibits weaker vertical contrast, low-level nonlinear horizontal advection that weakens the anomaly, and rapid thermodynamic adjustment.

Together, these results show that regional temperature extremes are dynamically organized regions of a probability distribution, shaped by synoptic circulation, nonlinear advection, orographically modified transport pathways, and stability-modulated contrast between the lower and upper troposphere.