How interactions between tropical depressions and western disturbances affect heavy precipitation in South Asia

Kieran M R Hunt,Reinhard K H Schiemann,Andrew G Turner

doi:10.1175/mwr-d-20-0373.1

Kieran M R Hunt, Reinhard K H Schiemann + Show 1 more

Open Access

https://doi.org/10.1175/mwr-d-20-0373.1

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Abstract

AbstractInteractions over South Asia between tropical depressions (TDs) and extratropical storms known as western disturbances (WDs) are known to cause extreme precipitation events, including those responsible for the 2013 floods over northern India. In this study, existing databases of WD and TD tracks are used to identify potential WD–TD interactions from 1979–2015; these are filtered according to proximity and intensity, leaving 59 cases which form the basis of this paper. Synoptic charts, vorticity budgets, and moisture trajectory analyses are employed to identify and elucidate common interaction types among these cases. Two broad families of interaction emerge. Firstly, a dynamical coupling of the WD and TD, whereby either the upper- and lower-level vortices superpose (a vortex merger), or the TD is intensified as it passes into the entrance region of a jet streak associated with the WD (a jet-streak excitation). Secondly, a moisture exchange between the WD and TD, whereby either anomalous moisture is advected from the TD to the WD, resulting in anomalous precipitation near the WD (a TD-to-WD moisture exchange), or anomalous moisture is advected from the WD to the TD (a WD-to-TD moisture exchange). Interactions are most common in the post-monsoon period as the subtropical jet, which brings WDs to the subcontinent, returns south; there is a smaller peak in May and June, driven by monsoon onset vortices. Precipitation is heaviest in dynamically-coupled interactions, particularly jet-streak excitations. Criteria for automated identification of interaction types are proposed, and schematics for each type are presented to highlight key mechanisms.

Highlights

The devastating 2013 north India floods were caused by unusually heavy rainfall after a trough in the uppertropospheric westerlies interacted with a strong monsoon low pressure system (Mishra 2015; Chevuturi and Dimri 2016)
There is significant precipitation associated with the tropical depressions (TDs), in agreement with what we found in section 3b(1), that a western disturbances (WDs) takes only a fraction of the TD moisture flux, so the latter is still associated with significant precipitation
The 59 cases could be broadly separated into two main families: dynamical coupling and moisture exchange

Summary

Introduction

The devastating 2013 north India floods were caused by unusually heavy rainfall after a trough in the uppertropospheric westerlies interacted with a strong monsoon low pressure system (Mishra 2015; Chevuturi and Dimri 2016). The convergence/ascent charts (Fig. 10b) show that moisture from the WD was advected straight toward the region of ascent-driven moisture flux convergence to the west-northwest of the TD This is near the expected location of ascent and maximum rainfall for monsoonal TDs, which is southwest of the center where it is driven by quasigeostrophic forcing arising from the interaction of the TD vortex and background monsoonal shear (Rajamani and Rao 1981; Boos et al 2015). These figures suggest that, at least in this case, the moisture contribution from the WD was small but nonnegligible. We refer to this as a satellite interaction

Climatological statistics of WD–TD interactions

Toward an automated framework

Findings

Discussion

Conclusions