The research, led by Anjela A. Ilagan of the Philippine Atmospheric, Geophysical and Astronomical Services Administration (DOST-PAGASA) and the University of the Philippines Diliman, together with Olivia C. Cabrera and Marcelino Q. Villafuerte II, tackles a persistent blind spot in Philippine hazard science. Most previous investigations of the country’s storm surges have concentrated either on individual catastrophic events or on numerical simulations of specific tropical cyclones. What was missing was a systematic, observation-based reconstruction spanning the whole archipelago and many decades. Because the Philippines is made up of more than seven thousand islands with an extraordinarily long and complex coastline, local bathymetry, shelf width, and coastal orientation can dramatically alter how high the sea rises when a typhoon makes landfall. A national picture assembled from actual measurements, rather than models alone, is therefore an essential foundation for hazard assessment.

The team’s approach hinged on a deceptively simple but technically demanding manipulation of tide gauge data. Tide gauges record the total water level at the coast, which is a mixture of the predictable astronomical tide, longer-term sea level variations, and the meteorological contribution from winds and pressure changes associated with storms. To isolate the storm signal, the researchers first removed the tidal component using harmonic analysis with the UTide software package, which fits the known tidal constituents to the observed record and allows the tide to be subtracted out. What remains is the residual sea level, the non-tidal part of the water level that carries the fingerprint of storm-driven water pile-up. The raw residuals were then smoothed with a low-pass filter to suppress high-frequency noise, ensuring that short-lived fluctuations unrelated to genuine surge events would not contaminate the detection.

Identifying storm surge events from residuals required a detection threshold, and here the researchers made a methodologically important choice. Rather than applying a single fixed cutoff across the entire country and record, they used an annually varying threshold, which accounts for the fact that baseline sea levels and seasonal conditions differ from year to year and from station to station. This approach, consistent with skew surge methods used by established sea level monitoring facilities, allows the detection algorithm to remain sensitive to genuine surge events even in years or locations where the background water level is naturally higher. Applying this threshold to the de-tided records across multiple coastal stations, the team identified 133 distinct storm surge events over the 77-year study period, the most comprehensive instrument-based storm surge inventory ever assembled for the Philippines.

The statistics that emerged from this inventory carry a message with direct implications for coastal safety. The majority of detected storm surges, measured as pure surge residuals, reached heights of only 20 to 50 centimeters, and only about 5 percent of events exceeded one meter. On its face, that might suggest that surges in the Philippines are generally modest. But the picture changes sharply when the researchers examined storm tide, the combined water level produced when the storm surge coincides with the astronomical tide. In approximately 39 percent of events, the storm tide exceeded one meter, nearly eight times the proportion seen for surge alone. The lesson is unambiguous: the timing of a typhoon’s arrival relative to the tidal cycle can transform a manageable rise in water into a genuinely dangerous coastal flood. A 40-centimeter surge arriving at high tide can inundate areas that the same surge at low tide would leave untouched.

The single largest surge in the record was measured at Pasacao, in the province of Camarines Sur, during Typhoon Molave in 2020, when the water level anomaly reached 211 centimeters, more than two meters above the expected tide. Molave, known locally as Quinta, swept across the Bicol region in late October 2020, and the Pasacao observation stands as a stark reminder of what even a storm that is not among the most intense on record can deliver when its winds align favorably with coastal geography. The study’s case analyses of Molave and Super Typhoon Goni, which struck the same general region just days later in early November 2020, highlighted how the orientation of the coastline relative to the storm’s track and the phase of the tide jointly determine the severity of coastal impacts. Two storms hitting within a week of each other, with different tracks and tidal timings, produced markedly different surge outcomes.

Mapping the geographic distribution of the 133 events revealed a pronounced regional pattern that reflects the underlying physics of surge generation. High storm surges clustered along the coasts of Catanduanes, Camarines Sur, Eastern Samar, Cagayan, Batanes, and Palawan. What these disparate locations share is a shallow and wide continental shelf, a bathymetric configuration that favors surge amplification. When storm winds push ocean water across a broad, shallow shelf, friction with the seabed constrains the water’s vertical escape and forces it to pile up at the coast, a mechanism that deep water adjacent to steep coasts does not permit. Eastern Samar’s presence in this list will resonate with anyone familiar with Haiyan, whose catastrophic surge in Leyte Gulf was amplified by exactly this kind of shelf-driven shoaling. Equally telling is where surges were largely absent: Mindanao, the large southern island, registered few significant events, a pattern consistent with its exposure to a less typhoon-frequented portion of the western North Pacific basin.

Beyond its scientific findings, the database itself may prove to be the study’s most consequential product. A validated, observation-based catalogue of historical surge events spanning nearly eight decades provides exactly the kind of ground truth that storm surge forecasters, hazard mappers, and model developers need. Numerical surge models, from the high-resolution unstructured-mesh systems used in operational hurricane forecasting to global reanalyses of extreme sea levels, depend on historical observations for calibration and validation. Extreme value statistics, the branch of statistics used to estimate the probability of rare events such as hundred-year surges, likewise require long, homogeneous observational records to produce trustworthy return-period estimates. By anchoring such analyses in real tide gauge measurements from Philippine waters, the new inventory reduces the reliance on extrapolations from other coastlines and improves the credibility of national hazard assessments, evacuation planning, and coastal infrastructure design.

The study also arrives at a moment when the stakes of surge forecasting in the Philippines are rising. Mean sea level is climbing globally, and even a modest long-term rise raises the baseline upon which every future storm surge builds, effectively converting yesterday’s moderate surge into tomorrow’s damaging flood. The researchers’ use of an annually varying threshold partly reflects this shifting baseline, and their 1947-to-2024 record offers a valuable window into how surge occurrences have unfolded across a period of both changing climate and changing observation technology. The authors acknowledge the National Mapping and Resource Information Authority’s Hydrography Branch for providing the tide gauge data, and the work was carried out without dedicated external funding, a testament to the value of careful analysis of existing national observational assets.

For a country that sits squarely in the most active tropical cyclone basin on Earth, with roughly twenty storms entering its area of responsibility in a typical year, the ability to look back through 77 years of measured sea level data is more than an academic exercise. It is a way of letting the ocean itself testify about which coasts are most exposed, how badly the tide can conspire with the wind, and what the worst-case scenarios look like when the next Molave or Goni forms over the warm waters of the Pacific. The 133 events now catalogued in this inventory are, in effect, 133 rehearsals for future disasters, and the researchers hope that forecasters, planners, and coastal communities across the archipelago will use them to be better prepared when the next rehearsal becomes the real thing.

Subject of Research: Historical storm surge events in the Philippines reconstructed from tide gauge observations

Article Title: Investigating historical storm surge occurrences in the Philippines from tide gauge observations

Article References: Ilagan, A. A., Cabrera, O. C., & Villafuerte, M. Q., II (2026). Investigating historical storm surge occurrences in the Philippines from tide gauge observations. Natural Hazards, 122(18), Article 625. https://doi.org/10.1007/s11069-026-08360-x

Image Credits: AI Generated

DOI: 10.1007/s11069-026-08360-x

Keywords: storm surge, storm tide, Philippines, tide gauge, tropical cyclone, Typhoon Molave, Super Typhoon Goni, sea level, coastal flooding, hazard assessment, UTide, Natural Hazards

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