Hidden Ocean Heat: Lakshadweep Sea Heatwaves Strike Hardest Below the Surface

Beneath the turquoise lagoons of the Lakshadweep Sea, a silent crisis is unfolding far out of sight of satellites and beachgoers. A new study published in Climate Dynamics by Neethu Chacko of the National Remote Sensing Centre in Kolkata reveals that marine heatwaves in this biodiverse stretch of the Arabian Sea do not simply warm the ocean’s skin. They plunge deep into the water column, where they grow longer, more intense, and more dangerous to marine life than their surface signatures suggest. The research, which spans more than four decades of observations from 1982 to 2025, offers one of the most detailed portraits yet of how extreme ocean warming behaves beneath the waves in a region that hosts some of India’s most sensitive coral reef ecosystems.

Marine heatwaves are defined as periods when ocean temperatures exceed a seasonally varying threshold for at least several consecutive days, and they have earned a reputation as some of the most disruptive climatic events in the sea. Globally, these events have been linked to mass coral bleaching, collapses in fisheries, shifts in species distributions, and economic losses for coastal communities. The Arabian Sea is one of the fastest-warming ocean regions on the planet, and the Lakshadweep Sea, an archipelago of coral atolls off India’s west coast, sits squarely within this warming bullseye. Yet while scientists have extensively catalogued surface marine heatwaves using satellite sea surface temperature records, the subsurface story has remained largely unwritten, particularly for the tropical Indian Ocean.

To fill that gap, Chacko combined high-resolution satellite sea surface temperature data from NOAA’s OISST archive with the Copernicus GLORYS ocean reanalysis, which reconstructs three-dimensional ocean conditions, and atmospheric fields from the ERA5 reanalysis. This combination allowed the study to track heatwave events not just at the surface but layer by layer through the upper ocean, from the wind-stirred mixed layer down through the thermocline, the zone of rapid temperature decrease that separates warm surface waters from the cold depths. The analysis applied standard marine heatwave metrics, including frequency, duration, and cumulative intensity, at each depth level, painting a vertical cross-section of extreme warming that surface-only studies cannot provide.

The headline finding is striking: the Lakshadweep Sea experiences roughly two to three surface marine heatwaves each year, typically lasting twelve to eighteen days, and all the key surface metrics are trending upward. But the real drama happens below. Although heatwave frequency decreases with depth, both the duration and the intensity of events increase, peaking within the thermocline. The maximum cumulative intensity was found between 80 and 100 meters, identifying this narrow band of the water column as the epicenter of subsurface heatwave activity. In other words, the ocean’s most punishing thermal stress in this region is not at the surface where monitoring is easiest, but in the twilight zone where light-loving reef organisms and thermally sensitive plankton live.

Even more concerning is the severity comparison. Surface marine heatwaves in the Lakshadweep Sea are typically classified as moderate or strong on the standard severity scale. Subsurface events, by contrast, are often more severe than their surface counterparts, meaning the temperature anomalies relative to their local baselines are larger and the events more punishing for organisms at depth. This asymmetry has profound implications. Coral reefs in Lakshadweep depend not only on water temperature at the reef surface but on the thermal structure of the entire water column, and deep-dwelling species, larval stages, and subsurface chlorophyll maxima may experience thermal stress that no satellite could ever detect.

The study’s composite analysis of atmospheric and upper-ocean conditions during heatwave onset and decline reveals the machinery behind these events. During the onset phase, coherent changes appear in net heat flux, mixed layer depth, stratification, and wind stress curl. A shallower mixed layer and enhanced stratification can trap heat in a thin near-surface layer, allowing temperatures to spike rapidly, while shifts in wind stress curl influence how heat is redistributed horizontally and vertically. These atmospheric and oceanic ingredients interact in ways that precondition the sea for extreme warming, echoing findings from other regions such as the Yellow Sea and eastern Australia, where stratification has been shown to play a key role in heatwave development.

The ocean heat budget analysis adds quantitative weight to this picture. Chacko found that the warming during the onset of heatwave events is driven by a combination of surface heat fluxes, horizontal advection of warm waters, and other unresolved processes that the reanalysis cannot fully capture. During the decline phase, however, the story changes: heat loss is dominated by horizontal advection, meaning that currents sweeping cooler water into the region are the primary mechanism that ends these events. This distinction matters for forecasting, because it suggests that predicting the end of a Lakshadweep heatwave requires understanding regional circulation, not just local weather patterns.

The findings place the Lakshadweep Sea within a growing global recognition that marine heatwaves are fundamentally three-dimensional phenomena. Recent international studies have documented bottom marine heatwaves along North American continental shelves, subsurface intensification off southeastern Australia, and hidden heatwaves beneath the surface of the global ocean. A global assessment published in Nature Climate Change in 2023 warned that marine biodiversity is exposed to prolonged and intense subsurface heatwaves that surface observations systematically miss. The Lakshadweep study extends this message to the northern Indian Ocean, where the combination of strong seasonal monsoon forcing, eddy activity, and rapid basin-wide warming creates a particularly dynamic environment for extreme events.

For the Lakshadweep archipelago itself, the stakes could hardly be higher. The atolls support hard coral communities whose diversity and spatial structure have been documented in studies of islands such as Agatti, and coral bleaching episodes worldwide have repeatedly followed marine heatwave events. Thermal stress concentrated in the 80 to 100 meter band may affect organisms and ecological processes that reef monitoring programs, which typically focus on shallow depths, never observe until damage is already done. Subsurface heatwaves could also influence the vertical distribution of chlorophyll and plankton in the region, with cascading effects through the food web to fish populations that support local livelihoods. As the study notes, thermal stress within ecologically sensitive subsurface layers can have profound implications for marine ecosystems that surface observations alone cannot adequately capture.

The broader lesson of this research is a call to rethink how we monitor and prepare for ocean extremes. Satellites see only the skin of the sea, and as this study demonstrates, that skin can be a misleading guide to what lies below. Sustained subsurface observing systems, high-resolution ocean reanalyses, and heat budget diagnostics will be essential tools as the Arabian Sea continues to warm at one of the fastest rates on Earth. For the coral atolls of Lakshadweep, and for the millions of people whose coasts border the Indian Ocean, understanding the hidden vertical structure of marine heatwaves may prove just as important as tracking the heat we can see from space.

Subject of Research: Subsurface marine heatwaves, their vertical structure and drivers in the Lakshadweep Sea of the Arabian Sea

Article Title: Subsurface marine heatwaves in the Lakshadweep Sea: characteristics, vertical structure, and drivers

Article References: Chacko, N. (2026). Subsurface marine heatwaves in the Lakshadweep Sea: characteristics, vertical structure, and drivers. Climate Dynamics, 64(10), Article 417. https://doi.org/10.1007/s00382-026-08370-2

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08370-2

Keywords: marine heatwaves, Lakshadweep Sea, Arabian Sea, subsurface ocean warming, thermocline, coral reefs, ocean heat budget, Climate Dynamics, ocean stratification, Indian Ocean warming, sea surface temperature, marine ecosystems