Dynamics of Heatwave Intensification over the Indian Region

TL;DR

The paper addresses the intensification of heatwaves over the Indian region by linking a moist heatwave mode (Mode 2) to midlatitude Rossby-wave teleconnections and regional forcing from the Bay of Bengal. It combines observational analyses of Mode 2 with a simplified, non-divergent barotropic vorticity model and a regression step to estimate surface temperature, humidity, and heat stress from vorticity patterns. The key finding is that intensification arises from the superposition of extratropical Rossby waves and a regional tropical source, with background flow and initial wave structure governing persistence and amplification. The work provides a dynamical mechanism connecting large-scale atmospheric variability to humid heat stress over India and highlights regime-dependent responses under climate-change scenarios.

Abstract

In a warming world, heatwaves over India have become intense and are causing severe health impacts. Studies have identified the presence of amplified Rossby waves and their association with the intensification of heatwaves. Earlier studies have identified two dominant modes of temperature variability in India and their possible role in the development of dry (mode 1) and moist (mode 2) heatwaves. These modes are associated with midlatitude Rossby waves intruding over the Indian region. However the role of regional forcing and the teleconnection behind the intensification of the heatwaves over India is missing. The present study has analyzed the dynamical mechanisms for the regional intensification of the circulation features associated with the dominant moist heatwave mode (mode 2). Considering the predominant barotropic nature of the observed circulation features of the mode, a simple barotropic vorticity equation model forced with extratropical and regional vorticity sources is used to understand the intensification of the heat waves. It was found that a wave response initiated by a cyclonic vorticity over the Bay of Bengal superimposes with the mid-latitude anticyclonic vorticity generated Rossby waves intruding over India. This superimposition results in the amplification and persistence of the anticyclonic vorticity phase over the Northwest Indian region, leading to the intensification of circulation. It was also found that the barotropically forced intensified circulation leads to the intensification of the heat stress. Under a climate change scenario, different circulation regimes, characterized by zonal stationary wave number and jet speed, which can favor the intensification are also identified.

Figures (39)

• Figure 1: Spatial pattern of (a) EOF Mode 1 and (b) EOF Mode 2 of the April-May detrended surface mean temperature obtained for a period from 1951-2020 (c) Time series of the principal components (PCs) obtained by projecting temperature anomaly onto EOF modes from 1980-2020 and their long-term trend. The white box in Fig.1a represents Region-1 defined in Sec. 2.
• Figure 2: (a) Spatial composite of relative vorticity anomaly (s-1) at 200 hPa for active mode days (PC >1.0) from lag -7 to lead 1 day with respect to active mode days. (b) Spatial composite of surface temperature anomaly (⁰C) for active mode days in the same way as shown in Fig 3a. The star represents the statistically significant regions.
• Figure 3: Composite of the standardized anomaly of relative vorticity at 200 hPa and maximum temperature for (a) Active mode days (PC > 1.0) and (b) Extreme mode days (PC > 2.0) over Region 1 (marked in lag=0 of Fig 3b as a white box) during different lags during April-May from 1980-2020. Percentage number of grid points at different lags with maximum temperature > 40 ⁰C for (c) Active mode and (d) Extreme mode days. (e) Vertical profile of area-averaged observed relative vorticity over Region 1 for active mode days
• Figure 4: Composite of sea surface temperature anomalies (⁰C; shaded contour) superimposed with 1000-300 hPa vertically integrated moisture flux (IMF; vectors) anomalies ($\mathrm{ms}^{-1}\,\mathrm{g}\,\mathrm{kg}^{-1}$) for (a) Active mode days (PC > 1.0) and (b) Extreme mode (PC > 2.0) days during April-May from 1980-2020. The anomalies are calculated with respect to daily climatology from 1980-2020. The statistically significant regions of the SST composite are given in Fig S3.
• Figure 5: (a) Vertical profile of the relative vorticity anomaly (s$^{-1}$) composite for those days when PC 2 $>$ 1.0 for two regions, Source Region 1 (25--5$^{\circ}$W; 50--70$^{\circ}$N) and Source Region 2 (0-20$^{\circ}$N; 80--95$^{\circ}$E). (b) Set of background initial conditions: zonal mean wind ($\bar{u}$; m/s), vorticity perturbation ($\zeta^{\prime}$) and the locations of the vorticity (s$^{-1}$) forcing F1 (Source Region 1) and F2 (Source Region 2) applied in the equivalent barotropic model for the experiments. (c) Mean zonal wind configurations $\bar{u}(j_{\max})$ used for sensitivity experiments TS (z, $j_{\max}$) and IS (z, $j_{\max}$).