Marine heatwaves in the Indian Ocean are changing India’s monsoon

For millions of people across India, the monsoon determines the availability of water for drinking, agriculture, energy production, and livelihoods. Yet predicting when, where, and how much rain will fall remains one of climate science's greatest challenges.

While scientists have long understood that the Indian Ocean plays a central role in powering the South Asian monsoon, a less visible phenomenon is increasingly attracting attention: marine heatwaves. These periods of unusually high ocean temperatures are becoming more frequent as the planet warms.

Emerging research now suggests that they do more than affect marine ecosystems. They can alter atmospheric circulation, influence monsoon behaviour, and affect rainfall distribution across the Indian subcontinent. 

The Indian Ocean is warming faster than ever

The year 2024 was the hottest on record for both land and ocean temperatures. Recent evidence indicates that the Indian Ocean, the primary source of moisture for the South Asian monsoon, is warming faster than any other tropical ocean basin.

This warming has important implications for India's water security. The Indian Ocean supplies much of the moisture that fuels monsoon rainfall, making changes in ocean temperatures closely linked to rainfall variability and intensity.

While researchers have extensively studied long-term ocean warming, less attention has been paid to short periods of extreme ocean heating and their influence on the monsoon. A recent study by Joseph L. and colleagues in Frontiers in Climate sheds new light on how marine heatwaves in the North Indian Ocean interact with large climate systems and shape monsoon variability. The findings offer valuable insights for improving monsoon predictability and strengthening water management in a changing climate.

What are marine heat waves?

Marine heat waves (MHWs) are extreme rises in ocean temperature for an extended period of time. They can occur at different locations in the ocean, and their magnitude and frequency have been increasing over the last couple of decades. For example, the number of marine heatwave events has doubled since 1982 with the total number of days having increased by 50% over the last century. They are expected to increase further in the future due to climate change. 

Marine heat waves can harm ecosystems and affect the survival of marine species, lead to mass mortality events in marine waters, increase the incidence of harmful algal blooms and extreme weather events such as cyclones and heavy rainfall events. They can also have a negative impact on the livelihoods of fishing communities who depend on the waters for their livelihoods. More importantly, they are also known to impact the onset and strength of the monsoon. It thus becomes crucial to understand how they influence the rains and can affect water availability in India.

Studies show that MHWs in the North Indian Ocean are intensifying, although they differ in regional focus and metrics. However, all these studies compare today’s ocean temperatures to what was “normal” decades ago using a fixed baseline, inform Joseph, L., et al. in their paper. Since the ocean has warmed steadily, almost every hot spell looks extreme according to studies that use this metric. To deal with this drawback, the authors use a new moving baseline or detrended approach that adjusts for long-term warming so that only genuine short-term spikes get counted as MHWs. The study explores the variability, drivers, and monsoon impacts of MHWs in the North Indian Ocean using the moving baseline approach to detect episodes of sea surface temperature spikes that can be counted as MHWs over the period 1982–2024.

How marine heatwaves influence the Asian monsoon

The study finds that marine heatwaves do not occur in a single pattern. Instead, they emerge in distinct scenarios that influence monsoon circulation and rainfall differently.

Scenario 1: Basin-wide marine heatwaves weaken the monsoon

In the first scenario, marine heatwaves develop across much of the North Indian Ocean, particularly in the Arabian Sea. These basin-wide heatwaves generate high-pressure anomalies over the ocean. Because the summer monsoon depends on a pressure difference between the Indian Ocean and the Indian subcontinent, these conditions weaken the southwesterly monsoon winds. The resulting reduction in surface winds lowers evaporation, reduces cloud cover, and promotes further warming of the upper ocean.

These marine heatwaves are strongly associated with the mature phase of El Niño and often occur when the monsoon transitions from an active phase to a break phase. Rainfall patterns during these events typically show drier conditions across western and central India and north of the Bay of Bengal, while wetter conditions occur in the eastern equatorial Indian Ocean.

Scenario 2 and Scenario 3: A seesaw of ocean heat

The second and third scenarios resemble the Indian Ocean Dipole, a climate phenomenon marked by shifting ocean temperatures between the Arabian Sea and the Bay of Bengal.

In Scenario 2, marine heatwaves develop in the Bay of Bengal while being suppressed in the Arabian Sea. In Scenario 3, the pattern is reversed, with heatwaves developing in the Arabian Sea and reduced warming in the Bay of Bengal. These shifts influence where atmospheric moisture accumulates and where rainfall is likely to occur.

The role of ENSO and monsoon variability

The study highlights strong links between marine heatwaves, the El Niño Southern Oscillation (ENSO), and Monsoon Intraseasonal Variability (MISO). Basin-wide marine heatwaves that tend to weaken the monsoon occur more frequently during El Niño years. In contrast, Scenario 2, characterised by warming in the Bay of Bengal, is more common during La Niña years and generally supports monsoon development.

The negative phase represented by Scenario 3, where marine heatwaves occur mainly in the Arabian Sea, shows a weaker connection with ENSO. MISO also influences these patterns. During Scenario 1 and Scenario 2 events, southern India often experiences wetter conditions while northern India tends to become drier. This rainfall distribution is consistent with known MISO phases. Scenario 3, however, is linked to widespread dryness across much of the Indian subcontinent.

What happens when marine heatwaves end?

One of the study's most interesting findings concerns the period immediately after marine heatwaves dissipate. The end of basin-wide marine heatwave events is associated with increased rainfall across north-west and north-east India, as well as Bangladesh and Myanmar.

Similarly, the termination of Scenario 2 events is followed by enhanced rainfall over central India. The end of Scenario 3 events leads to increased rainfall across southern and central India. Researchers suggest that these delayed effects occur because monsoon winds strengthen again after a break phase, releasing heat and moisture that had accumulated over the ocean during the marine heatwave. The termination of marine heatwaves is therefore linked with renewed monsoon activity, increased evaporation, and enhanced moisture transport into the atmosphere.

What this means for water security

The findings show that Indian Ocean warming, particularly in the form of marine heatwaves, can significantly influence rainfall across South Asia by increasing evaporation and altering atmospheric circulation.

The study points to a hierarchical relationship between major climate drivers. ENSO appears to establish the broader background conditions in the Indian Ocean, while MISO influences the timing, location, and termination of marine heatwaves.

These interactions can determine whether ocean warming develops into a marine heatwave and how that heatwave subsequently affects monsoon rainfall. For India, these insights are particularly important. As monsoon behaviour becomes increasingly uncertain, understanding marine heatwaves could improve seasonal forecasts and help anticipate shifts in rainfall distribution.

Reading the ocean to understand the monsoon

The future of India's water security depends not only on understanding what happens in the atmosphere but also on understanding what happens beneath it. This study demonstrates that marine heatwaves are not isolated ocean events. They are closely connected to the climate systems that govern India's rainfall. By influencing monsoon winds, evaporation, and moisture transport, these short-lived bursts of ocean warming can shape where and when rain falls across the country.

For agricultural planners, water resource managers, and disaster response agencies, this growing understanding offers an important opportunity. Better knowledge of marine heatwaves and their interaction with ENSO and MISO could improve monsoon forecasting and help prepare for uneven rainfall patterns, whether in the form of drought conditions in some regions or flooding in others.

As climate change continues to warm the oceans, learning to read these signals from the sea may become essential for managing water on land.