Tropical and mid-latitude teleconnections interacting with the Indian summer monsoon rainfall: a theory-guided causal effect network approach

Giorgia Di Capua,Ramesh Vellore,Marlene Kretschmer,Reik V Donner,Raghavan Krishnan,Bart Van Den Hurk,Dim Coumou

doi:10.5194/esd-11-17-2020

Abstract

Abstract. The alternation of active and break phases in Indian summer monsoon (ISM) rainfall at intraseasonal timescales characterizes each ISM season. Both tropical and mid-latitude drivers influence this intraseasonal ISM variability. The circumglobal teleconnection observed in boreal summer drives intraseasonal variability across the mid-latitudes, and a two-way interaction between the ISM and the circumglobal teleconnection pattern has been hypothesized. We use causal discovery algorithms to test the ISM circumglobal teleconnection hypothesis in a causal framework. A robust causal link from the circumglobal teleconnection pattern and the North Atlantic region to ISM rainfall is identified, and we estimate the normalized causal effect (CE) of this link to be about 0.2 (a 1 standard deviation shift in the circumglobal teleconnection causes a 0.2 standard deviation shift in the ISM rainfall 1 week later). The ISM rainfall feeds back on the circumglobal teleconnection pattern, however weakly. Moreover, we identify a negative feedback between strong updraft located over India and the Bay of Bengal and the ISM rainfall acting at a biweekly timescale, with enhanced ISM rainfall following strong updraft by 1 week. This mechanism is possibly related to the boreal summer intraseasonal oscillation. The updraft has the strongest CE of 0.5, while the Madden–Julian oscillation variability has a CE of 0.2–0.3. Our results show that most of the ISM variability on weekly timescales comes from these tropical drivers, though the mid-latitude teleconnection also exerts a substantial influence. Identifying these local and remote drivers paves the way for improved subseasonal forecasts.

Highlights

The Indian summer monsoon (ISM) is crucial for Indian society, which receives 75 % of its total annual rainfall during the summer months June through September (JJAS)
We assess whether the two-way interaction between the circumglobal teleconnection pattern and the monsoon trough” (MT) rainfall, as hypothesized by Ding and Wang (2005, hereafter DW2005), is reproduced using our causal effect networks (CENs) analysis
We apply causal discovery algorithms to analyse the influence of global middle and upper tropospheric fields on weekly ISM rainfall and study the twoway causal links between the mid-latitude circulation and ISM rainfall, together with tropical drivers and ISM intraseasonal variability

Summary

Introduction

The Indian summer monsoon (ISM) is crucial for Indian society, which receives 75 % of its total annual rainfall during the summer months June through September (JJAS). An important feature of this wave train is the 200 hPa Central Asian High, located to the east of the Caspian Sea, i.e. over the same region used to define the circumglobal teleconnection pattern, which may trigger positive rainfall anomalies over the northern ISM region by modifying the easterly vertical shear that drives the ISM circulation and its related effect on moist dynamic instability in the ISM region This wave train generated in the North Atlantic might aid in modulating the alternating active and break conditions over central India (Ding and Wang, 2007; Krishnan et al, 2009; Saeed et al, 2011).

Data and methods

Causal effect networks

Response-guided causal precursor detection

Results

Intraseasonal variability and tropical influence on the monsoon circulation

Discussion and conclusions