A 126-year interactive map and timeline of the Indian summer monsoon, by meteorological subdivision, anchored on the 2015 drought.
India's monsoon, year by year
126 years of summer monsoon performance, by meteorological subdivision. The all-India number is a fig leaf; the variance is the story.
Each cell on the ribbon is one monsoon (June–September), 1901 to 2026, coloured by its all-India departure from the long-period average. The faint line running across the ribbon is the 11-year rolling mean — the long-term climate signal underneath the year-to-year noise. The strip beneath shows the El Niño / La Niña state entering each season (Dec–Feb Niño 3.4 index), 1950 onwards. The map toggle on the right switches between the summer monsoon and the March–May pre-monsoon, for testing whether the pre-season was a precursor. Click any year — or use the arrow keys — to scrub. The 2026 cell is IMD's first-stage forecast, released 13 April 2026.
Monsoon anomaly · year by year · 1901–2026
ENSO state entering the season (Dec–Feb Niño 3.4 index, ONI · 1950–2026)
190119251950197520002026
−40%−20%0+20%+40%
—
—
—
Monthly all-India
Pre-monsoon (Mar–May)
—
ENSO entering season
—
IMD first forecast
—
Actual outcome
—
Subdivision anomalies
Hover any subdivision · 36 IMD meteorological subdivisions · boundaries: IMD Pune
Methodology
Sources of data
- Monsoon & pre-monsoon anomalies, 1901–2017 — monthly subdivisional rainfall from the Pai et al. 2014 compilation (IMD Pune), against a 1971–2017 baseline. The all-India figure is rainfall-weighted across the 34 mainland subdivisions.
- Monsoon & pre-monsoon anomalies, 2018–2024 — extracted directly from IMD's Annual Climate Summary PDFs (
imdpune.gov.in/cmpg/Product/Annual_Climate_Summary/), via pdfplumber/pymupdf. Includes the full 36-subdivision table and the all-India seasonal + monthly text. - 2025 all-India + monthly — IMD's 2 October 2025 end-of-season Southwest Monsoon press release.
- 2023 monthly Jun–Sep — backfilled from IMD's Monthly Climate Summary PDFs (
Monthly_Clim_Summary_{6..9}_2023.pdf); the 2023 ACS itself omits the monthly breakdown. - ENSO state — Oceanic Niño Index (ONI), 3-month running mean of ERSSTv5 Niño 3.4 SST anomalies, from NOAA CPC. The "entering season" value is the DJF (December–February) mean, which is the state known to IMD when it issued its April forecast.
- First-stage IMD forecasts — IMD April press releases. The 2021 value was verified against the source PDF (
lrfapr21_presrelease_15apr.pdf); 2018–2020, 2022, 2024 are from widely-reported IMD statements that cross-check across multiple outlets. 2026 is the 13 April 2026 release. - Officially declared droughts — IMD's all-India drought year list (anomaly <-10% with >20% of area affected).
Exceptions & methodological seams
- Pai-vs-IMD aggregation gap — the all-India figure here is rainfall-weighted across 34 mainland subdivisions (excluding Andaman & Nicobar and Lakshadweep), which differs from IMD's official area-weighted percentage by typically 2–4 percentage points for years where both are reported.
- Baseline seam at 2017/2018 — the 1901–2017 series uses a consistent 1971–2017 reference; 2018–2024 use IMD's own normals, which themselves shifted across the period (1961–2010 in earlier reports, 1981–2010 mid-period, 1971–2020 from 2023 onward). Adds ~1–3 pp uncertainty to the join — read the two segments as adjacent rather than strictly continuous.
- 2018 precision loss — the 2018 ACS PDF rounded the all-India monsoon and pre-monsoon figures to whole percentages (91% and 92.6% of LPA), so 2018's
aiandpre_aiare coarser than neighbouring years.
What's missing
- 2025 subdivision detail (
sdandpre_sd) — neither the October 2025 end-of-season press release nor the December regional reports contain a national subdivision table. Awaits the 2025 Annual Climate Summary PDF (typically Jan–Feb of the following year). The map will show 2025 as no-data when selected. - 2025 first-stage IMD forecast — issued April 2025, not yet integrated.
- 2026 actual outcome — known October 2026. Until then 2026 displays only IMD's first-stage forecast (92% of LPA).