The study, published in the journal Ocean Dynamics, draws on continuous wave measurements recorded between 2017 and 2019 by the coastal moored buoy CB06, operated by the National Institute of Ocean Technology under India’s Ministry of Earth Sciences. The buoy sits in shallow water at a depth of just 16 metres, close enough to the shore that its readings are directly relevant to coastal engineering, port operations, erosion management and navigation. Rather than treating the measured waves as a single undifferentiated field, the researchers applied a wave steepness algorithm to each recorded wave spectrum, a technique that exploits the fundamental physical difference between young, steep wind seas and mature, low-steepness swells. By sorting the energy in every spectrum into these two categories, the team could track the significant wave height of the swell component and the significant wave height of the wind sea component separately, and follow how each evolved through three distinct seasonal windows: the pre-monsoon months of February through May, the southwest monsoon months of June through September, and the post-monsoon northeast monsoon months of October through January.

The technical logic behind the separation is deceptively simple. Wind seas, generated by local winds, tend to be relatively steep because the waves are still growing under active forcing; swells, having left their generation region behind, lose steepness as they disperse and travel. Steepness-based partitioning therefore acts as a physical fingerprinting tool, allowing each directional wave spectrum measured by the buoy to be sliced into a swell part and a wind sea part. From these partitions the researchers derived Hm0s, the significant wave height attributable to swells, and Hm0w, the significant wave height attributable to wind seas, and then examined interannual variations across the three-year record. This decomposition matters because the two components carry different information: wind seas signal what the local atmosphere is doing right now, while swells preserve a memory of winds that blew days earlier, sometimes on the other side of the basin.

The headline finding of the analysis is that swells dominate the wave field off Chennai. Across all three years, the total significant wave height, Hm0, correlated more strongly with the swell component than with the wind sea component, confirming that the character of the sea at this location is set primarily by long-period waves arriving from distant generation areas rather than by locally born wind waves. This is consistent with a broader understanding of the North Indian Ocean, where the wave climate along the Indian east coast is shaped substantially by swells propagating from the Southern Indian Ocean and from the Bay of Bengal itself. For coastal practitioners, the implication is significant: design conditions, sediment transport estimates and coastal flood assessments off Chennai cannot be built on local wind statistics alone, because the largest and most consistent share of wave energy arrives as swell.

What elevates the study beyond a climatological description is the year that sits at its centre. The 2017 to 2019 window happened to bracket a major climate event: in 2019, a strong El Niño-Southern Oscillation episode coincided with one of the strongest positive phases of the Indian Ocean Dipole on record, a coupled ocean-atmosphere pattern in which the western Indian Ocean becomes unusually warm relative to the east. These modes are known to reorganise winds and rainfall across the Indo-Pacific, but their fingerprints on the partitioned wave climate of the Bay of Bengal had been harder to pin down from direct measurements. The Chennai buoy record caught those fingerprints clearly.

During the 2019 southwest monsoon, the wind field over the study area showed an increased occurrence of winds blowing from between 180 and 270 degrees, a southwesterly bias consistent with the large-scale circulation anomalies that a strong positive Indian Ocean Dipole tends to impose on the region. More strikingly, during the pre-monsoon period of 2019, the researchers observed unusual southeasterly winds, a departure from the patterns seen in 2017 and 2018 that coincided with the evolving El Niño conditions. The wave record responded in kind. The anomalous southwesterly winds during the 2019 monsoon were accompanied by an increased occurrence of young swells, waves that had recently left their generation area and had not yet fully matured, alongside a reduction in the annual swell percentage. In other words, the reorganised wind field did not merely strengthen local waves; it altered the age and origin structure of the swell population itself.

The pre-monsoon season told the opposite story. As El Niño conditions developed, the study recorded more swell-dominated conditions during the pre-monsoon months, with the swell share of wave energy rising relative to the preceding years. The contrast between a windier, more wind-sea-rich monsoon and a swell-rich pre-monsoon in the same year illustrates how a single climate event can push the wave climate in different directions at different times of the year, depending on how it reshapes the regional wind field and the swell pathways feeding the coast. For the Chennai coast, this means that climate teleconnections are not an abstract background factor but an active modulator of the day-to-day wave conditions that beaches, breakwaters and fishing communities actually experience.

The most quantitatively dramatic result concerns the wind sea component during the 2019 monsoon. The occurrence of wind sea significant wave heights exceeding 0.5 metres increased by 25 percent relative to 2017 and by 24 percent relative to 2018. In a shallow 16-metre water column, wind seas of that scale are far from trivial: they contribute directly to nearshore turbulence, sediment stirring and the wave-induced stresses that drive coastal erosion, a persistent problem along the Chennai shoreline. A quarter-century-scale jump in the frequency of such conditions within a single anomalous year demonstrates how quickly the shallow-water wave regime can shift under the influence of basin-scale climate variability, and how important it is for coastal models and operational forecasting systems to account for interannual climate modes rather than relying solely on a mean seasonal climatology.

The study also adds to a growing body of work showing that the Indian Ocean’s wave climate is tightly coupled to its leading climate modes, including ENSO and the Indian Ocean Dipole, which modulate wind patterns, swell generation and wave propagation pathways across the basin. Earlier research has linked these modes to wave climate variability in the eastern Arabian Sea and to high-swell events along the Indian coast, but direct, partitioned measurements from a shallow-water buoy off the east coast provide a particularly vivid confirmation. Because the data come from a long-running, quality-controlled moored buoy network maintained by the National Institute of Ocean Technology, the record offers the kind of continuous, in situ validation that satellite altimeters and numerical wave models alone cannot always provide in the complex nearshore environment.

The practical consequences reach well beyond academic interest. Chennai is a major port city with dense coastal infrastructure, an eroding shoreline and a large population exposed to marine hazards. Wave climate information that distinguishes swells from wind seas directly improves the inputs to shoreline change models, breakwater design criteria, sediment budget studies and navigational safety assessments. The finding that a strong positive Indian Ocean Dipole year can simultaneously boost wind sea occurrences during the monsoon and swell dominance before it suggests that seasonal and interannual wave forecasts tailored to climate mode outlooks could become valuable tools for coastal managers. As climate variability and change continue to reshape the Indian Ocean’s winds and waves, the humble buoy off Chennai, watching the sea separate its local storms from its far-travelled swells, is helping to write the baseline against which those future changes will be measured.

Subject of Research: Wind sea and swell partitioning in the shallow-water wave climate off Chennai and its modulation by ENSO and the Indian Ocean Dipole

Article Title: Wind sea and swell characteristics in the wave climate off Chennai

Article References: Janakiram, R., Latha, G., Balamurugan, R., & Jena, B. K. (2026). Wind sea and swell characteristics in the wave climate off Chennai. Ocean Dynamics, 76(10), Article 99. https://doi.org/10.1007/s10236-026-01856-x

Image Credits: AI Generated

DOI: 10.1007/s10236-026-01856-x

Keywords: moored buoy, wind seas, swell, wave climate, wave spectra, Bay of Bengal, ENSO, Indian Ocean Dipole, significant wave height, Chennai coast, monsoon, coastal oceanography

The research, led by John K. Lix and R. Sajeev of the Department of Physical Oceanography at Cochin University of Science and Technology, together with Ramasamy Venkatesan of the Indian Institute of Technology Mandi, draws on high-resolution observations from a moored OMNI buoy stationed at 15°N, 68.9°E in the east-central Arabian Sea, close to Nanauk’s track. The buoy recorded air temperature, pressure, humidity, wind speed and direction at 3-meter height, along with temperature and salinity at discrete depths from 1 to 500 meters, every hour. This rare in-situ time series allowed the team to reconstruct exactly how the upper ocean responded as the depression that would become Nanauk passed overhead.

Nanauk’s story began on 10 June 2014, when a depression formed over the east-central Arabian Sea under favorable conditions: sea surface temperatures above 31°C, tropical cyclone heat potential of roughly 60 to 80 kJ cm⁻², strong low-level vorticity and convergence, and upper-level divergence. The system strengthened into a deep depression and then a cyclonic storm by the early hours of 11 June, peaking with maximum sustained winds near 23.1 m s⁻¹, a minimum central pressure of 986 mb, and a radius of maximum wind of about 37 km. Vertical wind shear during the monsoon onset phase ultimately curbed its growth, and the storm weakened by 13 June.

The upper-ocean response to a tropical cyclone is traditionally divided into a forced stage, when strong winds within the radius of maximum wind directly drive mixed-layer currents, and a relaxation stage, dominated by near-inertial currents generated as winds subside. During the forced stage, two mechanisms cool the mixed layer: Ekman-pumping-driven upwelling and entrainment mixing, in which shear instability at the base of the mixed layer stirs colder thermocline water upward into the warm surface layer. Because cooling at the surface reduces the enthalpy flux — the sum of latent and sensible heat that fuels a cyclone — sea surface cooling acts as a negative feedback that can weaken storms.

During Nanauk’s depression stage, the mooring recorded a striking and rapid sea surface temperature drop of about 1.9°C within 12 hours. The mixed layer, typically oscillating between 20 and 30 meters in depth under calm pre-monsoon conditions and diurnal solar cycling, deepened dramatically as wind stress intensified — from 35 to 70 meters between 11 and 12 June, while the depth of the 26°C isotherm plunged from 70 to 90 meters. Satellite sea-level anomaly data revealed an anticyclonic warm-core eddy on the right side of the track, where cooling was notably weaker, while stronger cold wakes formed on the left side — an asymmetry that is rare, since cyclone-induced cooling is usually strongest to the right of the track where wind-driven current shear is greatest.

To disentangle the physical processes behind the cooling, the team computed a mixed-layer temperature budget, separating the temperature tendency into net surface heat flux, vertical processes, and residual terms. Surface heat fluxes were estimated with the COARE 3.6b bulk algorithm using in-situ buoy data, while radiation terms came from the ERA5 reanalysis. Because entrainment velocity cannot be measured directly from a mooring, the researchers employed the one-dimensional Price-Weller-Pinkel (PWP) mixed-layer model, which simulates turbulent mixing using static stability, bulk Richardson number, and gradient Richardson number criteria, initialized with pre-storm buoy profiles and forced with observed meteorology.

The model and observations agreed remarkably well during the initial phase of direct wind forcing: from 00 to 07 UTC on 10 June, the mean difference between simulated and observed sea surface temperature was just 0.07°C, and temperature tendencies differed by only 0 to 0.05°C per hour. During this first phase, entrainment mixing dominated the cooling, contributing on average around 81% of the mixed-layer temperature change, with net surface heat flux contributing roughly 13% and the residual term about 6%. At one point, the entrainment term alone accounted for 77% of the observed cooling tendency. Average enthalpy fluxes during the forcing period reached −562 W m⁻², peaking at −648 W m⁻², with latent heat providing 80 to 95% of that loss.

Perhaps the most intriguing finding concerns why such strong entrainment occurred despite relatively weak depression-stage winds. The answer lies in the seasonal structure of the Arabian Sea. During the monsoon onset phase, the east-central basin features a shallow mixed layer — less than 30 meters — sitting atop an elevated temperature gradient at its base, roughly 1.5 to 1.7°C. This sharp stratification meant that even moderate wind-driven shear was sufficient to trigger shear instability, drive entrainment velocities peaking at 3 meters per hour, and pump cold water into the surface layer. Sensitivity experiments confirmed the robustness of this conclusion: switching the mixed-layer depth criterion changed entrainment estimates by only about 5% on average, though lowering the critical bulk Richardson number from 0.65 to 0.5 reduced the entrainment contribution by 17%.

The picture changed during the final phase of forcing, when the one-dimensional model captured only about half the observed cooling, pointing to significant contributions from three-dimensional processes such as wind-stress-curl-driven upwelling — visible as the upward tilting of the 26°C isotherm — horizontal advection, and inertial oscillations. These processes became dominant during the relaxation stage, when alternating upwelling and downwelling with a roughly 46-hour inertial period sustained the cold wake for days to weeks. The authors note that a full three-dimensional model would be needed to close the temperature budget during these later phases.

Crucially, the cooling — about 1.3°C during the direct forcing period — did not weaken the young storm. Post-forcing sea surface temperature at the mooring remained 29.6°C, far above the 26°C threshold needed to sustain a cyclone, so the ocean continued to supply ample heat and Nanauk went on to intensify. But the study carries a broader warning for forecasters: high sea surface temperature and high heat potential alone do not guarantee that a storm will escape oceanic negative feedback. A slow-moving cyclone crossing a shallow, sharply stratified mixed layer can still cool the sea substantially, chipping away at the enthalpy flux that sustains it — as Nanauk’s slow drift over its own cold wake during recurvature may have contributed to its dissipation. Accurately capturing this entrainment physics, the authors argue, requires dense in-situ measurements of temperature, salinity, and currents along storm tracks, and their explicit representation in coupled cyclone forecast models, particularly in the Arabian Sea where such process studies have been scarce.

Subject of Research: Entrainment mixing and mixed-layer cooling in the Arabian Sea during the depression stage of Cyclonic Storm Nanauk

Article Title: The role of entrainment mixing on the mixed layer cooling in the Arabian Sea during the depression stage of the Cyclonic Storm Nanauk

Article References: Lix, J. K., Sajeev, R., & Venkatesan, R. (2026). The role of entrainment mixing on the mixed layer cooling in the Arabian Sea during the depression stage of the Cyclonic Storm Nanauk. Discover Oceans, 3(1), Article 48. https://doi.org/10.1007/s44289-026-00162-0

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00162-0

Keywords: Arabian Sea, Cyclone Nanauk, entrainment mixing, mixed layer cooling, sea surface temperature, PWP model, air-sea interaction, tropical cyclone, moored buoy, ocean heat content, Richardson number, monsoon onset