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what causes the Indian summer monsoon and its mechanism

Rahul PalRahul Pal·researched on Researchly·June 18, 2026Try free
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Indian Summer Monsoon: Causes & Mechanisms

The Indian summer monsoon (ISM) rainfall interannual variability is known to be strongly linked to El Niño–Southern Oscillation (ENSO), with this linear relationship being the primary factor controlling interannual variation in ISM precipitation1. Empirical evidence further supports that the interdecadal variation of the Indian monsoon rainfall (IMR) is strongly correlated with the interdecadal variations of various indices of ENSO, and the interannual variances of both IMR and ENSO indices vary in phase and follow a common interdecadal variation2

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1
Unraveling the role of the western North Pacific circulation anomaly in modulating Indian summer monsoon rainfall variability beyond ENSO.Son Jun-Hyeok, Franzke Christian L E et al.2024Scientific reports
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2
Indian Monsoon–ENSO Relationship on Interdecadal TimescaleV. Krishnamurthy, B. N. Goswami2000Journal of Climate
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During El Niño, two off-equatorial surface anticyclones dominate Asian–Australian monsoon anomalies: one over the south Indian Ocean (SIO) and the other over the western North Pacific (WNP) . The monsoon anomalies associated with La Niña resemble those during El Niño, but with cyclonic anomalies instead .

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Despite ENSO's primacy, many outlier cases exist, posing challenges to seasonal prediction1. Jun-Hyeok et al. (2024) show these deviations can be attributed to anomalous atmospheric pressure patterns in the WNP region1

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1
Unraveling the role of the western North Pacific circulation anomaly in modulating Indian summer monsoon rainfall variability beyond ENSO.Son Jun-Hyeok, Franzke Christian L E et al.2024Scientific reports
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  • An anticyclonic circulation anomaly over WNP causes easterly winds toward the Indian subcontinent → positive precipitation anomalies with stronger low-level moist convergence.
  • A cyclonic circulation over WNP decreases ISM precipitation.
Linear baroclinic model simulations further support that the WNP circulation pattern can serve as an independent forecasting factor for precipitation over India, particularly for July and September1

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Xie et al. (2008)1

identify a critical post-ENSO persistence mechanism: significant climate anomalies persist through the summer after El Niño dissipates, including:

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Indian Ocean Capacitor Effect on Indo–Western Pacific Climate during the Summer following El NiñoShang‐Ping Xie, Kaiming Hu et al.2008Journal of Climate
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  • Tropical Indian Ocean (TIO) SST warming
  • Increased tropical tropospheric temperature
  • An anomalous anticyclone over the subtropical northwest Pacific
  • Increased mei-yu–baiu rainfall over East Asia
The TIO warming acts like a capacitor, anchoring atmospheric anomalies over the Indo–western Pacific. It increases tropospheric temperature via moist-adiabatic adjustment in deep convection, emanating a baroclinic Kelvin wave into the Pacific1. In the northwest Pacific, this Kelvin wave induces northeasterly surface wind anomalies, triggering suppressed convection and an anomalous anticyclone through Ekman divergence1

.

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Gautam et al. (2023)1

show that soil moisture plays an active role in modulating ISM at sub-seasonal scales:

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Intraseasonal Oscillation of Land Surface Moisture and its role in the maintenance of land ITCZ during the active phases of the Indian Summer MonsoonPratibha Gautam, Rajib Chattopadhyay et al.2023arXiv
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  • Soil moisture shows intraseasonal oscillation similar to rainfall
1.
  • During active monsoon phases, maximum positive soil moisture anomaly is found over North West India
1.
  • Soil moisture serves a pre-conditioning role during active phases over the monsoon core zone, with the mechanism depending on the region's soil type and climate classification
1.
  • Soil moisture feedback affects the boundary layer Moist Static Energy (MSE) and, consequently, rainfall
1

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Bollasina et al. (2011)1find that South Asia underwent a widespread summertime drying during the second half of the 20th century, attributable mainly to human-influenced aerosol emissions. The drying results from a slowdown of the tropical meridional overturning circulation, which compensates for the aerosol-induced energy imbalance between the Northern and Southern Hemispheres1

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1
Anthropogenic Aerosols and the Weakening of the South Asian Summer MonsoonMassimo Bollasina, Yi Ming et al.2011Science
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Suvarna et al. (2021)1

show that ISMR deficits prevail for two years after moderate and large (VEI > 3) tropical volcanic eruptions (this is not the case for extra-tropical eruptions). The mechanism involves:

1
The role of tropical volcanic eruptions in exacerbating Indian droughts.Fadnavis Suvarna, Müller Rolf et al.2021Scientific reports
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  • Tropical volcanic eruptions strengthen El Niño and weaken La Niña conditions, further enhancing Indian droughts
1.
  • Eruptions induce an El Niño-like warming in the central Pacific via Kelvin wave dissipation
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  • Solar dimming caused by the volcanic plume additionally reduces Indian rainfall
1

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Chen-Yi et al. (2025)1show that ASM intensity is primarily driven by solar activity and tropical ocean–atmosphere circulation, with different phase combinations of the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) also influencing ASM variations1

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1
Evolution of the Asian summer monsoon and regional karst ecological environment since the middle ages in Southwest China.Wang Chen-Yi, Li Jun-Yun et al.2025Scientific reports
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╔══════════════════════════════════════════════════════════════════════════════════╗ ║ INDIAN SUMMER MONSOON (ISM) — CAUSAL MECHANISM PIPELINE ║ ╚══════════════════════════════════════════════════════════════════════════════════╝

┌─────────────────────────────────────────────────────────────────────────────────┐ │ EXTERNAL / BOUNDARY FORCINGS │ │ │ │ ☀ Solar Activity 🌋 Tropical Volcanic 💨 Anthropogenic Aerosols │ │ [eqt9q7r] Eruptions (VEI>3) (Human Emissions) │ │ │ [er8ebyz] [e18v9fs] │ │ │ │ │ │ │ ▼ ▼ ▼ │ │ Hemispheric Kelvin Wave + Tropical Meridional │ │ Temperature Solar Dimming ──► Overturning Circulation │ │ Gradient → El Niño-like Slowdown │ │ Changes SST Warming → ISM DRYING │ └──────────────────┬──────────────────────────────────┬──────────────────────────┘ │ │ ▼ ▼ ╔══════════════════════════════╗ ╔══════════════════════════════════════╗ ║ ENSO (PRIMARY DRIVER) ║ ║ INTERDECADAL MODULATORS ║ ║ [e1ek7bm][epqnw77] ║ ║ AMO + PDO phase combinations ║ ║ ║ ║ [eqt9q7r] ║ ║ El Niño → ISM suppression ║ ╚══════════════════════════════════════╝ ║ La Niña → ISM enhancement ║ │ ║ (cyclonic/anticyclonic ║ │ ║ anomaly pairs) ║◄────────────────────────┘ ╚═══════════╤══════════════════╝ │ ├─────────────────────────────────────────────────────┐ ▼ ▼ ╔══════════════════════════════════╗ ╔══════════════════════════════════╗ ║ SOUTH INDIAN OCEAN (SIO) ║ ║ WESTERN NORTH PACIFIC (WNP) ║ ║ ANTICYCLONE ║ ║ ANTICYCLONE/CYCLONE ║ ║ [e15zzxf] ║ ║ [e1ek7bm][e15zzxf] ║ ║ ║ ║ ║ ║ Peaks: El Niño growing summer ║ ║ Anticyclone → Easterly winds ║ ║ Decays: El Niño maturity ║ ║ → Moist convergence over India ║ ║ Impact: Indian Ocean, E.Africa ║ ║ → POSITIVE ISM anomaly ║ ║ India ║ ║ ║ ╚════════════╤═════════════════════╝ ║ Cyclone → NEGATIVE ISM anomaly ║ │ ║ Key months: July & September ║ ▼ ╚════════════════╤═════════════════╝ ╔══════════════════════════════════╗ │ ║ INDIAN OCEAN CAPACITOR EFFECT ║ │ ║ [e1pg1i5] ║◄───────────────────────────┘ ║ ║ ║ TIO SST Warming (post-ENSO) ║ ║ │ ║ ║ ▼ ║ ║ Moist-Adiabatic Adjustment ║ ║ in Deep Convection ║ ║ │ ║ ║ ▼ ║ ║ Baroclinic Kelvin Wave ║ ║ → Pacific Propagation ║ ║ │ ║ ║ ▼ ║ ║ NE Wind Anomalies in NW Pacific ║ ║ → Ekman Divergence ║ ║ → Anomalous NW Pacific ║ ║ Anticyclone ║ ║ → Mei-yu/Baiu rainfall ↑ ║ ╚════════════╤═════════════════════╝ │ ▼ ╔══════════════════════════════════════════════════════════════════════════════╗ ║ LAND-SURFACE FEEDBACK LAYER ║ ║ [eq1cbdm] ║ ║ ║ ║ Soil Moisture Intraseasonal Oscillation ║ ║ │ ║ ║ ├──► Pre-conditioning of Active Monsoon Phases ║ ║ │ (North-West India; depends on soil type & climate class) ║ ║ │ ║ ║ └──► Boundary Layer MSE Budget ║ ║ │ ║ ║ ▼ ║ ║ MSE Feedback ──► Modulates Rainfall Intensity ║ ╚══════════════════════════════════════════════════════════════════════════════╝ │ ▼ ╔══════════════════════════════════════════════════════════════════════════════╗ ║ ISM RAINFALL OUTPUT ║ ║ ║ ║ ┌──────────────────────────────────────────────────────────────────────┐ ║ ║ │ Interannual Signal ◄── ENSO (primary) + WNP anomaly (secondary) │ ║ ║ │ Intraseasonal Signal ◄── Soil moisture pre-conditioning (MSE) │ ║ ║ │ Interdecadal Signal ◄── AMO + PDO + Solar + ENSO variance │ ║ ║ │ Long-term Trend ◄── Aerosol-forced drying (20th century) │ ║ ║ │ Episodic Droughts ◄── Tropical volcanism (2-yr ISMR deficit) │ ║ ║ └──────────────────────────────────────────────────────────────────────┘ ║ ╚══════════════════════════════════════════════════════════════════════════════╝

Table
MechanismDirection of EffectTimescaleSource
ENSO (El Niño)Suppresses ISMInterannual
WNP anticycloneEnhances ISMInterannual (Jul/Sep)
WNP cycloneSuppresses ISMInterannual
SIO anticycloneModulates Indian Ocean/IndiaSeasonal[CITATION:e15zzx
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