Present and future of the South Asian summer monsoon’s rainy season over Northeast India

Saha, Prolay; Mahanta, Rahul; Goswami, B. N.

doi:10.1038/s41612-023-00485-1

Cited by 7 publications

References 38 publications

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Indian summer monsoon rainfall response to two distinct evolutions of La Niña events

Sharma,

Ratna,

Pai

et al. 2024

Intl Journal of Climatology

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This study examines the asymmetry in the Indian summer monsoon rainfall (ISMR) response over India and its four homogeneous regions to two distinct types of temporal evolution in La Niña. We have shown this uneven response by analysing the large‐scale dynamics over tropical Indo‐Pacific region for the period 1951–2022. We have identified two types of La Niña events during monsoon season (June–September) based on whether they evolved from El Niño or La Niña from preceding boreal winter season (December–February). India receives significantly more (less) rainfall during La Niña years, when it was preceded by El Niño (La Niña) in the preceding winter. We further observed the spatial diversity of rainfall over India with a northeast–southwest dipole pattern. When La Niña years were preceded by El Niño, positive surface pressure anomaly over west‐north Pacific, low‐level westerlies and moisture transport favoured the rainfall over south peninsula and west‐central India. Whereas moisture divergence associated with anomalous lower‐tropospheric anticyclone over west‐north Pacific suppressed the rainfall over Indo‐Gangetic plains. However, when La Niña years were preceded by La Niña in winter, the absence of westerlies and weak moisture transport subdued the rainfall over south peninsula and west‐central India. At the same time, moisture convergence and a greater number of monsoon depressions favoured rainfall over north‐west India. This study also looked at how well eight Copernicus Climate Change Service (C3S) models predicted ISMR and SST for two types of La Niña with April initial conditions during the period 1993–2016. Models were able to capture the spatial pattern of SST anomalies over Indo‐Pacific Ocean, but all models could not capture the spatial pattern of ISMR. However, in terms of intensity, six out of eight models could predict more (less) ISMR when it was preceded by El Niño (La Niña), coinciding with the observed anomaly.

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Indian summer monsoon rainfall response to two distinct evolutions of La Niña events

Sharma,

Ratna,

Pai

et al. 2024

Intl Journal of Climatology

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Dynamics of May ‘onset’ of Indian summer monsoon over Northeast India

Das,

Goswami,

Mahanta

et al. 2024

Quart J Royal Meteoro Soc

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Acknowledging the prolonged duration of the rainy season in Northeast India (NEI) compared to Central India, the official onset of the Indian summer monsoon over NEI is traditionally marked around 5 June, with May rainfall categorized as ‘pre‐monsoon’. However, our study reveals that May rainfall in NEI occurs in active/break spells driven by persistent synoptic‐scale systems, contributing to a significant monsoon heat source during this period. Through an objective analysis, we determine that the climatological ‘onset’ in NEI actually occurs around 18 May, with withdrawal around 14 October, resulting in an extended rainy season of approximately 150 days. The enigma of the May onset, while the Intertropical Convergence Zone remains proximate to the equator, is addressed by identifying a conducive climate in May. This climate is characterized by low‐level cyclonic vorticity over the region, influenced by the interannual strengthening (weakening) by the Atlantic Niño (El Niño). The introduction of potential vorticity from extratropical transient Rossby waves in May significantly amplifies low‐level cyclonic vorticity by 3–4 times, acting as a catalyst for the monsoon onset in NEI. Furthermore, a 5–6 times intensification of northward moisture transport from the Bay of Bengal sustains the monsoon heat source post onset. The May onset is made feasible by the uplifting of low‐level cyclonic winds, facilitated by the horseshoe‐shaped orography around the Brahmaputra valley. This process is complemented by an increased local moisture content resulting from evapotranspiration. Our findings challenge conventional notions by demonstrating that a component of Indian summer monsoon rainfall in NEI is not directly related to the Intertropical Convergence Zone. This challenges the fundamental definition of the South Asian Monsoon and calls for a reassessment of the prevalent belief that the Indian monsoon season is confined to June–September in NEI.

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Integration of Indigenous Traditional Knowledge and AI in Hurricane Resilience and Adaptation

Chakravarty,

Gattupalli

2024

Hurricane Risk

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Present and future of the South Asian summer monsoon’s rainy season over Northeast India

Cited by 7 publications

References 38 publications

Indian summer monsoon rainfall response to two distinct evolutions of La Niña events

Indian summer monsoon rainfall response to two distinct evolutions of La Niña events

Dynamics of May ‘onset’ of Indian summer monsoon over Northeast India

Integration of Indigenous Traditional Knowledge and AI in Hurricane Resilience and Adaptation

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