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Free keywords:
causal effect networks
ENSO teleconnections
Himalayas
monsoon Hadley circulation
regional Walker circulation
summer monsoon precipitation
Abstract:
We perform a causal analysis to quantify the direct causal links on intraseasonal time-scales associated with the influence of El Niño–Southern Oscillation (ENSO) teleconnections on summer monsoon Himalayan precipitation (SMHP) and two key coupled regional circulations, which are the monsoon Hadley circulation (MHC) and the component of the regional Walker circulation (RWC) over the Himalayas. This is achieved by using a causal effect network (CEN) approach to show graphically the direction and strength of linkages between four monthly climate indices representing ENSO, SMHP, RWC, and MHC from 1940 to 2022 with a time lag of one month. Depending on the complexity of the CENs constructed, the results show causal effect (CE) values as follows: (i) ENSO to SMHP of 0.33 to 0.44 (i.e., a one standard deviation increase in ENSO causes a decrease in SMHP of between 0.33 and 0.44 standard deviations with a lag of one month), (ii) ENSO to MHC of 0.12 to 0.15, and (iii) ENSO to RWC of 0.17 to 0.20. These are interpreted as an increase in ENSO (analogous to the El Niño phase) causing a substantial decrease in Himalayan precipitation, as well as weakening the key regional circulations. Further results showed CE values from MHC to SMHP of 0.32 to 0.43 and from RWC to SMHP of 0.30 to 0.37. These are interpreted as a strengthening of both the regional Walker circulation and the monsoon Hadley circulation causing a substantial increase in Himalayan precipitation. Finally, the approach was used to clarify the coupled interactions between RWC and MHC, which showed a CE value of 0.14 to 0.17 from RWC to MHC and from MHC to RWC. This suggests positive feedback between the two regional circulations.
