Quick answer: A marine heatwave is a spell of unusually warm sea-surface temperature that stays above the 90th percentile of the local daily climate record. It is in the news because INCOIS, Hyderabad, flagged marine heatwave conditions across the Arabian Sea, Bay of Bengal and wider Indian Ocean in early November 2026, and its 6 November 2026 update showed heatwave conditions over nearly 80 per cent of the Arabian Sea.
Why marine heatwaves are in the news
The Indian National Centre for Ocean Information Services (INCOIS), which works under the Ministry of Earth Sciences, issued marine heatwave warnings for the Arabian Sea, the Bay of Bengal and the Indian Ocean around 5 November 2026. Most affected waters were in the “Watch” and “Alert” bands, meaning temperatures up to about 1°C above the threshold. INCOIS also issued a swell surge warning for both coasts for the following two days, with waves of one metre or more expected. The warning covered Gujarat, Maharashtra, Goa, Karnataka, Kerala, Tamil Nadu, Puducherry, Andhra Pradesh, Odisha, West Bengal, Lakshadweep and the Andaman and Nicobar Islands, with a specific danger alert for Car Nicobar.
The INCOIS Marine Heatwave Advisory Services (MAHAS) page, updated for 6 November 2026, shows how widespread the warm water is.
Key facts: where the sea is running hot (INCOIS, 6 November 2026)
| Region | Area under marine heatwave | Category |
|---|---|---|
| Arabian Sea | 79.69% | Moderate to Extreme |
| South China Sea | 74.37% | Moderate to Extreme |
| Persian Gulf | 68.60% | Moderate to Extreme |
| Bay of Bengal | 62.53% | Moderate to Extreme |
| Rest of Indian Ocean | 38.20% | Moderate to Extreme |
| Red Sea | 23.23% | Moderate |
| Lakshadweep coast | 100% | Moderate to Extreme |
| Nicobar coast | 97.58% | Moderate to Extreme |
| Karnataka / Kerala coasts | 94.64% / 94.59% | Moderate to Extreme |
- Sharp rise since August: INCOIS’s bulletin of 17 August 2026 had put only 14 per cent of the Arabian Sea under “Watch” and 2 per cent under “Alert”.
- Earlier spell in April: In late April 2026, about 22 per cent of the Arabian Sea was under Watch, 9 per cent under Alert and 5 per cent under Warning, according to reports on an INCOIS advisory.
How INCOIS measures a marine heatwave
INCOIS compares daily sea-surface temperature from satellite-based OISST data with the 90th percentile of daily climatology for each spot. When water stays above that mark for a prolonged period, it counts as a marine heatwave. The service then grades intensity in three steps:
| Category | Temperature above the 90th-percentile threshold |
|---|---|
| Watch | 0 to 0.5°C |
| Alert | 0.5 to 1°C |
| Warning | More than 1°C |
MAHAS divides its study area into six ocean basins (Arabian Sea, Bay of Bengal, Persian Gulf, Red Sea, southern Indian Ocean and South China Sea) and 14 sectors off Indian states and island territories. Daily advisories are rolled into a weekly bulletin for fishermen, coastal managers and researchers.
A marine heatwave differs from long-term ocean warming: warming is a slow rise in the average, while a heatwave is a sudden spike on top of it.
Why it matters for India’s coasts
- Coral bleaching: Stressed corals expel the algae that live in their tissue and turn white. Reefs of Lakshadweep, the Gulf of Mannar and the Andaman and Nicobar Islands are exposed when nearby waters stay hot.
- Fisheries: INCOIS notes that heatwaves can shift where fish gather and when they breed, which hits the catch of small-scale fishers.
- Food chain: Disturbance to plankton, the base of the marine food chain, can lower overall ocean productivity.
Exam angle: how NDA, CDS and CAPF can ask this
- Questions on Indian ocean-information bodies are common: know that INCOIS is in Hyderabad and works under the Ministry of Earth Sciences.
- Coral reef locations in India (Gulf of Mannar, Gulf of Kachchh, Lakshadweep, Andaman and Nicobar) are frequently tested in NDA and CDS geography.
- Expect conceptual questions on the difference between swell waves, storm surge and tsunamis in the oceanography portion.
Practice MCQs
Q1. The Marine Heatwave Advisory Services (MAHAS) are run by:
(a) IMD, New Delhi (b) INCOIS, Hyderabad (c) NIO, Goa (d) ISRO, Bengaluru
Answer: (b) — INCOIS, under the Ministry of Earth Sciences, runs MAHAS.
Q2. In the INCOIS scale, an “Alert” means sea temperature is above the 90th-percentile threshold by:
(a) 0–0.5°C (b) 0.5–1°C (c) 1–2°C (d) more than 3°C
Answer: (b) — Watch is 0–0.5°C, Alert 0.5–1°C and Warning above 1°C.
Q3. Which coastal sector showed 100 per cent marine heatwave coverage on 6 November 2026?
(a) Gujarat (b) West Bengal (c) Lakshadweep (d) North Tamil Nadu
Answer: (c) — the entire Lakshadweep coastal sector was under heatwave conditions.
Q4. Coral bleaching occurs mainly because corals:
(a) absorb excess salt (b) expel the symbiotic algae living in their tissue (c) are covered by sediment (d) lose their calcium skeleton overnight
Answer: (b) — heat stress makes corals expel the algae, leaving them white.
Frequently asked questions
What is a marine heatwave?
It is a prolonged period when sea-surface temperature stays above the 90th percentile of the local daily climate record. INCOIS uses this threshold to issue its advisories.
Which agency issues marine heatwave alerts in India?
INCOIS, Hyderabad, an institution under the Ministry of Earth Sciences, issues them through its Marine Heatwave Advisory Services (MAHAS).
How bad is the November 2026 marine heatwave?
On 6 November 2026, INCOIS recorded heatwave conditions over 79.69 per cent of the Arabian Sea and 62.53 per cent of the Bay of Bengal, with the Lakshadweep sector fully affected.
How is a marine heatwave different from ocean warming?
Ocean warming is a slow rise in average temperature over decades. A marine heatwave is a short-term spike above normal levels that can stress marine life suddenly.
Also read on NCA: Kariyachalli Island, Gulf of Mannar | Operation Sagar Bandhu and cyclones | Monthly defence current affairs PDFs | NDA coaching in Chandigarh
Sources: INCOIS MAHAS, INCOIS MHW Bulletin, 17 August 2026, Report on INCOIS warning (5 November 2026), The Indian Witness (April 2026). Facts as reported up to 8 November 2026.
Featured image background (representative): Matt Kieffer / Wikimedia Commons, CC BY-SA 2.0.
