Figure 1. Schematic diagram representing the factors contributing to the spatial dipole pattern of below-normal and above-normal rainfall during ELLA and LALA episodes. The oceanic and atmospheric conditions over the Indo-Pacific region are shown. Over the ocean, the red and blue shading represents a positive and negative SST anomaly, respectively. The thickness of the red (upward motion) and blue (subsidence) arrows represents the strength of Walker circulation. The location of monsoon depressions (MD) from the Bay of Bengal (BoB) and the strength of low-level south-westerly flow are just illustrations and not meant to be precise.

Indian Summer Monsoon Rainfall (ISMR) has a significant negative simultaneous correlation with El Niño Southern Oscillation (ENSO), which associates La Niña with above-normal rainfall over the Indian region. However, as all the La Niña years are not the same, this study examines two distinct types of evolution in La Niña and the associated asymmetry in the response of the ISMR. The two types of La Niña events during the Indian summer monsoon season (June–September) were identified based on whether they evolved from El Niño or La Niña from the preceding boreal winter season (December–February) and termed as ELLA and LALA, respectively. The results showed the asymmetric response of ISMR by analyzing the large-scale dynamics and the role played by the tropical basin Interactions between the Pacific Ocean and the Indian Ocean for the period 1951–2022. It was observed that India receives significantly more and less rainfall during ELLA and LALA events, respectively, and ISMR has a spatial diversity with a northeast-southwest dipole pattern.

When La Niña years were preceded by El Niño (ELLA), positive surface pressure anomaly over the west-north Pacific, low-level westerlies and moisture transport from the north Indian Ocean favoured rainfall over the south peninsula and west-central India. In contrast, moisture divergence associated with anomalous lower tropospheric anticyclones over the west-north Pacific suppressed the rainfall over Indo-Gangetic plains. When La Niña preceded La Niña years in winter (LALA), the absence of westerlies and weak moisture transport from the north Indian Ocean subdued rainfall over the south peninsula and west-central India. At the same time, moisture convergence and a greater number of monsoon depressions over the Bay of Bengal favoured rainfall over northwest India. This study also looked at how well eight coupled climate models from Copernicus Climate Change Service (C3S) predicted ISMR and sea surface temperature (SST) for ELLA and LALA events with April initial conditions during the period 1993–2016. Models could capture the spatial pattern of SST anomalies, but most of the models could not capture the spatial dipole-pattern of ISMR. Regarding rainfall intensity, six out of eight models could predict more and less ISMR during ELLA and LALA, respectively, coinciding with the observed anomaly.

(Submitted by Satyaban Ratna, member of CLIVAR Climate Dynamics Panel and Monsoons Panels, edited by Agus Santoso) 

The full article can be accessed from https://doi.org/10.1002/joc.8588

List of Authors: Tanu Sharma, Satyaban B. Ratna, Damodara S. Pai, Arti Bandgar, Madhavan Rajeevan, Mrutyunjay Mohapatra, Onpathamkuniyil P. Sreejith, Krishnanand S. Hosalikar.

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Sharma, T., Ratna, S. B., Pai, D. S., Bandgar, A., Rajeevan, M., Mohapatra, M., Sreejith, O. P., & Hosalikar, K. S., 2024: Indian summer monsoon rainfall response to two distinct evolutions of La Niña events. International Journal of Climatology, 44, 4405–4427, https://doi.org/10.1002/joc.8588