Causal Drivers Behind Enhanced Rainfall Activity OverNorthern Indian

The Western (WHF) and Eastern Himalayan foothills (EHF) are two densely populated regions that experience extreme precipitation events during the Indian summer monsoon (ISM) season lasting typically from June to September [1, 2]. Therefore, a better understanding of the processes controlling ISM intraseasonal variability is of great relevance.

In our present work we identify and quantify causal relationships at short lead-times (three to nine days) between characteristic remote and local climate patterns and the precipitation over the WHF and EHF. More specifically we apply the so-called response-guided causal precursor detection (RGCPD) scheme that builds on the Peter and Clark momentary conditional dependence (PCMCI) algorithm [3]. The employed method is based on concepts of information theory and statistical mechanics, and allows to identify strongly interdependent climate patterns associated with the ISM and to distinguish between spurious and truly causal links. Finally, causal effect networks (CENs) visually summarise the identified causal links between different variables, indicating the directionality, time lag and magnitude of the causal effect.

Our analysis reveals that WHF rainfall variability is influenced by mid-latitude teleconnections such as the circumglobal teleconnection index and seems to be driven by similar precursors and time scales as the precipitation over central India [4]. In contrast, CENs indicate that the EHF rainfall is characterised by faster dynamics compared to the WHF and whilst it is also driven by mid-latitude teleconnections, a different set of atmospheric processes appears to play a major role in its variability. Specifically, a unique and strong causal connection to the tropical western Pacific is revealed, manifesting itself in the geopotential height at 500 hPa and the mean sea-level pressure. A thorough analysis of this signal indicates a Gill-type response to a heat sink over the equatorial Pacific, that may be associated with the Madden-Julian oscillation (MJO) and suggests a link between suppressed MJO phases and enhanced rainfall activity over the EHF region. Thus, our analysis hints to a connection between break spells of the ISM, where large parts of the Indian landmass experience reduced precipitation activity, and enhanced rainfall activity over the EHF region.

References
[1] Vellore, R., et al., On the anomalous precipitation enhancement over the Himalayan foothills during monsoon breaks, Clim. Dynam. 43, 2009-2031 (2014).
[2] Vellore, R., et al., Monsoon - extratropical circulation interactions in Himalayan extreme rainfall, Clim. Dynam. 46, 3517-3564 (2016).
[3] Runge, J., Causal network reconstruction from time series: From theoretical assumptions to practical estimation, Chaos 28, 075310 (2018).
[4] Di Capua, G., et al., Tropical and mid-latitude teleconnections interacting with the Indian summer monsoon rainfall: a theory-guided causal effect network approach, Earth Syst. Dyn., 11, 17-34 (2020).