Weak Winds, Strong Cooling: Entrainment Mixing Chilled the Sea Beneath Cyclone Nanauk

When a tropical cyclone churns across the ocean, the drama at the sea surface often steals the show. But beneath the towering clouds, a quieter battle unfolds between a warm, sun-warmed surface layer and the colder water lurking just below. A new study of Cyclonic Storm Nanauk, which formed over the Arabian Sea in June 2014, reveals that even during the earliest, weakest stage of a cyclone’s life, turbulent entrainment mixing can drive substantial cooling of the ocean’s mixed layer — with important consequences for how storms intensify or decay.

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