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Diurnal Tidal Currents in the Gulf of Tonkin and the Qiongzhou Strait: Regime Characterization with Implications for Navigation and Tidal-Stream Energy

This study characterizes the Gulf of Tonkin's strongly diurnal tidal regime and surface kinetic energy by integrating multi-source data to map spatial variability, explain the significant gap between theoretical and realized power density through standing-wave dynamics, and identify the Qiongzhou Strait as the sole location meeting tidal-stream energy thresholds while highlighting the regime's impact on navigation.

Original authors: Manh Hung Nguyen, Gia Huy Dinh

Published 2026-07-24

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Manh Hung Nguyen, Gia Huy Dinh

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the ocean not as a flat, blue sheet, but as a giant, breathing drum. When the moon and sun pull on this drum, the water doesn't just rise and fall; it sloshes back and forth, creating waves of water that move horizontally. These moving currents are called tidal currents. In most places on Earth, the water sloshes twice a day, like a pendulum swinging back and forth. But in some special spots, the shape of the ocean floor and the coastline act like a tuning fork, making the water slosh only once a day. This is called a diurnal (daily) regime.

Why does this matter? Because moving water carries energy. If we can catch that energy, we can spin turbines to make electricity for our homes. However, to build these turbines, engineers need to know exactly how fast the water moves and where. They also need to know this for ships; a fast current can push a giant cargo ship off course or make it impossible to enter a shallow port. The big question scientists have been asking is: "In the Gulf of Tonkin, a massive bay in Southeast Asia, how strong are these currents, and can we trust the maps we currently have to find the best spots for energy?"

The Ocean's One-Day-Long Cycle

In the Gulf of Tonkin, the ocean has a very peculiar personality. While most of the world's oceans rise and fall twice a day, this bay mostly does it just once. The authors of this study, Manh Hung Nguyen and Gia Huy Dinh, decided to map out this "one-a-day" rhythm across the entire bay to see what it means for ships and for clean energy.

Think of the Gulf of Tonkin like a giant, shallow bathtub that is open at one end. When the water sloshes in, it hits the far end (the "head" of the bay) and bounces back. Because the bathtub is just the right size and depth, the water resonates—like a singer hitting the perfect note to make a glass shatter. This resonance makes the water rise and fall with huge force at the far end, but the water doesn't move very fast there. Instead, the water rushes in and out fastest at the entrance and through narrow channels, even though the water level doesn't change much there. It's a bit like a see-saw: when one end goes up high, the other end moves fast, but not very high.

The Great Map Mystery

The researchers wanted to create the first complete map of this "one-a-day" rhythm. They used a mix of satellite data, computer models, and real-world measurements from tide gauges (which are like rulers stuck in the water to measure height).

They found that the "one-a-day" pattern isn't just a small trick near the shore; it dominates almost the entire northern half of the bay. They drew a line across the map at about 19°N latitude. North of this line, the ocean is purely diurnal. In the very northern interior, the rhythm is so strong that the "Form Factor" (a number scientists use to measure how dominant the daily tide is) reaches a massive 12.7. To put that in perspective, a number below 0.25 means "twice a day," and anything above 3 means "once a day." So, 12.7 is a very loud, very clear "once a day" signal.

The "Fake" Currents and the Real Ones

Here is where the study gets really interesting. Scientists have two main ways to guess how fast the water moves:

The "Tide Model" method: This looks at how high the water rises and uses a simple math formula (like a recipe) to guess the speed. It assumes the water moves like a wave traveling freely.
The "Reanalysis" method: This uses giant computer simulations of the ocean that include wind, temperature, and currents, but they often only give us a "daily average" speed.

When the researchers compared these two methods, they found a shocking difference. The "Tide Model" method suggested the water had an average power of 121.35 W/m² (watts per square meter). The "Reanalysis" method suggested only 2.86 W/m². That is a difference of about 42 times!

At first, you might think the computer model was wrong or that the simple math was too optimistic. But the authors dug deeper and found the truth: Both were right, but they were measuring different things.

The "Reanalysis" data was like looking at a video of a runner and only checking their position once every 24 hours. If you only check where a runner is once a day, you miss all the fast running they did in between! Because the daily average smooths out the fast-moving tides, it makes the energy look tiny. The "Tide Model" math, on the other hand, was trying to guess the speed based on the water height. But in this "bathtub" bay, the water height and water speed are out of sync. Where the water is highest (at the far end), it moves slowly. Where the water is lowest (at the entrance), it moves fast. The simple math formula didn't know this, so it guessed wrong in some places and right in others.

The Sweet Spot for Energy and Ships

So, where is the real energy hiding? The study found that the strongest currents are not where the water is highest, but where the water is squeezed through narrow gaps.

The superstar location is the Qiongzhou Strait, a narrow channel between Hainan Island and the mainland. Here, the water rushes through at speeds approaching 1.3 m/s (about 2.5 knots). This is the only place in the entire Gulf where the water moves fast enough to be considered a good candidate for tidal energy turbines.

However, there is a catch. Because this is a "one-a-day" tide, the water only rushes in and out once every 24 hours. In most places, ships get two chances a day to enter a port (high tide twice). Here, they get only one. If a ship misses that single window, it has to wait a whole day. This makes planning for ships much trickier.

What This Means for the Future

The authors conclude that if you want to find tidal energy in this part of the world, you cannot just look at daily weather reports or average ocean maps; you need data that captures the hour-by-hour rush of the tide. They also warn that using simple math to guess current speeds from water height can be dangerous in places like this, because the "bathtub" effect changes the rules.

The Qiongzhou Strait is the only spot that passes the first test for tidal energy, but it comes with a unique schedule: one big rush a day, not two. For the people living and working in the Gulf, understanding this rhythm is the key to keeping ships safe and maybe, one day, turning the ocean's daily breath into electricity.

Technical Summary: Diurnal Tidal Currents in the Gulf of Tonkin and the Qiongzhou Strait

Problem Statement
The Gulf of Tonkin represents a unique hydrodynamic environment where the diurnal tidal constituents ( $K_1$ and $O_1$ ) dominate over the semi-diurnal $M_2$ , a condition attributed to the basin's near-resonant response at diurnal frequencies. Despite the well-established nature of this regime at individual coastal stations, there has been a lack of spatially continuous descriptions characterizing the tidal regime across the entire basin. Furthermore, the distribution of surface current kinetic energy remains poorly constrained, particularly regarding the discrepancy between estimates derived from global tide models and those from ocean reanalyses. This gap hinders accurate resource screening for tidal-stream energy and the precise scheduling of maritime navigation in a region with heavy shipping traffic, including the critical Qiongzhou Strait.

Methodology
This study characterizes the diurnal regime and surface current energy using only open-access global datasets, avoiding the need for a regional hydrodynamic model. The methodology integrates:

Tidal Harmonic Constants: Derived from the EOT20 empirical global tide model (0.125° resolution) to synthesize elevations and first-order current estimates.
Current Data: Daily surface currents from the GLORYS12 reanalysis (2020) and hourly operational surface currents from the CMEMS SMOC product (Nov 2020–Oct 2021).
Validation: Comparison of synthesized elevations against hourly tide-gauge records from Hon Dau (Vietnam, 1995) and Beihai (China, 1997).
Analysis:
- Calculation of the Form Factor ( $F$ ) to map the tidal regime classification spatially.
- Application of a shallow-water progressive-wave scaling ( $U = A_\eta \sqrt{g/h}$ ) to estimate tidal currents from EOT20 elevations.
- Harmonic analysis (UTide) of hourly current records at four representative sites to compare constituent amplitudes ( $K_1, O_1, M_2$ ) against the progressive-wave scaling.
- Computation of mean surface power density ( $P = \frac{1}{2}\rho \langle |U|^3 \rangle$ ) for both the scaled tidal currents and the reanalysis currents.

Key Contributions and Results

Spatially Continuous Regime Characterization:
The study presents the first spatially continuous Form Factor map for the Gulf of Tonkin. The analysis reveals that diurnal dominance is a basin-scale condition, not merely a coastal feature. The $F=3$ contour (separating mixed mainly diurnal from diurnal regimes) crosses the basin near 19°N, with $F$ values reaching a maximum of 12.7 in the northern interior.
Validation of Synthesized Tides:
The EOT20 harmonic synthesis was validated against independent gauge records. At Hon Dau, the synthesis achieved a Nash–Sutcliffe efficiency (NSE) of 0.916 and a Willmott skill score of 0.980. Constituent amplitudes for the dominant diurnal terms ( $K_1, O_1$ ) matched observations to within 3–5%, with a total vector difference of approximately 7% across eight resolvable constituents.
Quantification of the Energy Estimation Gap:
A significant discrepancy was identified between mean surface power density estimates: 2.86 W/m² from the daily GLORYS12 reanalysis versus 121.35 W/m² from the EOT20 progressive-wave scaling (a ~42-fold difference). The study demonstrates that this gap is not a simple pointwise frictional ratio but results from two physical factors:
- Temporal Averaging: Daily averaging in reanalysis products removes most tidal-band variance, suppressing power density by factors of 3.8–8.9 compared to hourly data.
- Standing Wave Structure: The Gulf acts as a quarter-wavelength resonator. The progressive-wave scaling assumes velocity and elevation are in phase, but in a standing wave, velocity maxima occur near the nodal entrance (where elevation is low), and elevation maxima occur at the antinodal head (where currents are weak). Consequently, the scaling overestimates currents near the northern head (by factors of 2.3–2.6) and underestimates them near the entrance and the Qiongzhou Strait (by factors of 0.1–0.2).
Identification of the Qiongzhou Strait as a Resource Hotspot:
Harmonic analysis of hourly currents at the Qiongzhou Strait (P1) shows combined diurnal amplitudes approaching 1.3 m/s. This is the only area in the Gulf meeting the first-order screening thresholds (0.5 m/s) for tidal-stream energy. The study identifies this location as the sole candidate for a Stage-1 assessment under IEC (2015) guidelines.
Seasonal Variability:
The study highlights that the two energy reservoirs (monsoon-driven circulation and tidal currents) peak in different seasons. The reanalysis (monsoon-driven) peaks in autumn, while the scaled tidal power peaks in winter, reflecting the semi-annual modulation of declinational diurnal tides.

Significance and Claims
The paper claims that the relationship between astronomical tidal forcing and realized surface kinetic energy in the Gulf of Tonkin is governed by the standing-wave structure of the near-resonant diurnal oscillation. The primary physical result is that the ratio between progressive-wave scaling and realized tidal amplitudes is spatially organized, serving as a diagnostic for the basin's resonant mode rather than a uniform frictional correction.

Practically, the study establishes that:

Navigation: The single daily high-water cycle and the concentration of strong currents in the Qiongzhou Strait dictate specific scheduling windows for deep-draft vessels, distinct from semi-diurnal regimes.
Energy Screening: Daily reanalysis products are fundamentally unsuitable for tidal-stream resource screening as they remove the resource itself. Accurate screening requires hourly products with explicit tidal forcing or validated harmonic synthesis.
Resource Potential: While the Gulf possesses a strong diurnal regime, viable tidal-stream energy is confined to the Qiongzhou Strait approach, where channel amplification and standing-wave dynamics combine to produce currents near technological cut-in speeds.

The authors conclude that the study provides a necessary physical reconciliation of different data sources, offering a robust basis for navigation planning and a realistic, albeit localized, assessment of tidal energy potential in the Gulf of Tonkin.

The Ocean's One-Day-Long Cycle

The Great Map Mystery

The "Fake" Currents and the Real Ones

The Sweet Spot for Energy and Ships

What This Means for the Future

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