A strait, defined as a narrow waterway connecting two seas or oceans between adjacent landmasses, serves not only as a critical maritime corridor and shipping hub with strategic military importance but also plays a pivotal role in global oceanographic processes. As crucial conduits linking distinct marine basins, straits regulate regional and even global thermohaline transport and material cycling through their complex water exchange patterns, circulation structures, and energy transfer mechanisms. For instance, the Mediterranean Overflow via the Gibraltar Strait significantly influences the Atlantic Meridional Overturning Circulation (Hernández-Molina et al., 2014; Swingedouw et al., 2019), while water exchange through the Bering Strait governs the physico-chemical and biological environments of both the Arctic and North Pacific Oceans (Woodgate et al., 2005; Grebmeier et al., 2006; Woodgate et al., 2012). Even in smaller-scale systems like the Qiongzhou Strait, intense tidal currents directly shape the unique “butterfly delta” submarine geomorphology at its eastern and western outlets (Ni et al., 2014).

Residual flow refers to the time-averaged, non-tidal components of oceanic flow that persist after filtering out high-frequency motions such as tides, inertial oscillations, and wind-driven surface waves. Residual flow in straits, emerging as net water transport from the interplay of multiple dynamic processes, serve as both a manifestation of complex hydrodynamic interactions and a key linkage connecting various oceanographic processes. The spatiotemporal characteristics of residual flow fundamentally control suspended particulate transport pathways, pollutant dispersion ranges, and nutrient cycling efficiency within straits. In strongly tidal straits, residual flow dominates the formation of sediment “trapping zones” through long-term cumulative effects, thereby modifying submarine topography and posing navigational challenges (Van Maren and Gerritsen, 2012; Ni et al., 2014; Ai et al., 2024). Notably, the Taiwan Strait acts as a conduit for wintertime transport of East China Sea water masses to the northeastern South China Sea under northeast monsoon forcing, significantly impacting local marine environments (Chen et al., 2020; Li et al., 2024) and consequently influencing fishery resource distribution patterns. These mechanisms underscore the irreplaceable scientific value of studying residual currents in straits for understanding marine material transport, ecosystem stability, and anthropogenic impacts.

The Qiongzhou Strait, situated between the Leizhou Peninsula of Guangdong Province and Hainan Island, China ( Figure 1a ), serves as the sole natural conduit for water exchange between the northern South China Sea and the Beibu Gulf, and ranks among China’s three major straits. This east-west oriented strait extends approximately 80 km in length, with a minimum width of 19.4 km, averaging 44 m in water depth and reaching a maximum depth of 114 m. A 33-km-long tidal channel with water depths exceeding 60 m dominates the central strait, exhibiting asymmetric topography across its northern and southern flanks: the northern flank is relatively gentle whereas the southern side is steeper. During summer, the Qiongzhou Strait is dominated by the southerly monsoon ( Figure 1b ). Distinct thermal regimes characterize its eastern and western entrances: cold waters occupy the eastern entrance due to the development of coastal upwelling, while the western entrance is governed by vertically well-mixed warm waters resulting from intense tidal mixing combined with strong solar radiation ( Figure 1b ; Hu et al., 2003; Bai et al., 2016; Lin et al., 2016; Bai et al., 2020a, 2022).

The Qiongzhou Strait is a strong tide-dominated channel. Within the strait, diurnal tidal constituents predominantly propagate east to west. At the western entrance, the amplitude of diurnal tides ( O 1 + K 1 2 ) reaches 70 cm, gradually decreasing eastward to 25 cm at the eastern entrance. The maximum phase lag occurs in the southwestern strait (with minimal phase lags at both entrances), where tidal waves from both ends interact. The phase speed of westward-propagating diurnal tides measures approximately 10 m/s, significantly faster than eastward-propagating diurnal constituents. In contrast, the semidiurnal M₂ constituent propagates exclusively west-to-east from the Beibu Gulf, with its amplitude increasing from 20 cm at the western entrance to 40 cm at the eastern entrance. The phase lag difference between the strait’s two ends is approximately 100° (equivalent to 6.5 hours), yielding a propagation speed of ~4.7 m/s, slower than diurnal tidal speeds. Both diurnal and semidiurnal tidal current ellipses align nearly parallel to the strait’s axis. Diurnal tidal currents peak in the 5–10 m water layer, exhibiting amplitudes of ~30 cm/s at the western entrance and up to 100 cm/s at the eastern entrance. Notably, near-bottom currents (within 3 m of the seabed) maintain substantial amplitudes of 30–50 cm/s. The M₂ semidiurnal currents display weaker intensities (10–20 cm/s), approximately one order of magnitude smaller than diurnal currents. Unlike diurnal constituents, semidiurnal currents show minimal vertical variability, with near-identical surface and near-bed current magnitudes (Shi et al., 2002; Chen et al., 2009; Zheng et al., 2024).

In coastal shallow seas, waves alongside tides are key hydrodynamic drivers (e.g., Gao et al., 2017, 2021, 2023, 2024). During summer, the southwest monsoon weakens as it encounters Hainan Island ( Figure 1b ), resulting in mild wind waves with disorganized directional distribution in Qiongzhou Strait. In contrast, the northeast winter monsoon directly impacts the strait. Waves from the open sea east of the strait also propagate unimpeded into the region, generating larger waves (~1 m height). Strong tidal currents in the strait further modulate wave dynamics. Current-induced convergence and wavenumber shifts can amplify significant wave height at the strait’s eastern entrance. Additionally, tidal flow regulates peak wave directions through current-induced refraction (Bai et al., 2020b).

The southwest monsoon governs the entire northwestern South China Sea, leading to the early assumption that summer water transport from the Beibu Gulf through the Qiongzhou Strait to the eastern shelf seas of the Leizhou Peninsula would generate eastward residual flow within the strait. However, Shi et al. (2002) demonstrated, using 37-year observational datasets of tidal elevation and currents combined with numerical modeling, that a persistent westward residual flow (10–40 cm/s) exists year-round in the strait. This flow contributes a summer water transport of 0.1–0.2 Sv into the Beibu Gulf. Subsequently, Yang et al. (2003) identified westward residual flow structures in the Qiongzhou Strait during summer using drifter bottle data. Employing a three-dimensional ECOM model, Chen et al. (2009) systematically investigated tidal-induced residual flow, wind-driven currents, and density-driven circulation. Their results confirmed that westward residual flow dominates the strait in both winter and summer, with tidal rectification generating westward residual currents of 5–30 cm/s. These currents fully suppress wind-driven eastward flows (5–15 cm/s), establishing tidal dynamics as the primary driver of the strait’s westward residual flow. Wang et al. (2014) analyzed summer Acoustic Doppler Current Profiler (ADCP) measurements, revealing meridional variability in the westward residual flow, with peak velocities reaching -0.34 m/s and a mean westward transport of 0.16 Sv. Recently, Zheng et al. (2024) implemented the SCHISM model to further elucidate residual flow dynamics in the Qiongzhou Strait. Their study quantified a mean summer westward residual flow of 0.2 m/s and identified that wind stress curl differences between the eastern and western strait entrances can induce transient eastward residual flows. According to Shi et al. (2002), summer westward transport through the Qiongzhou Strait accounts for approximately 44% of the Beibu Gulf’s water renewal. A series of studies (Wu et al., 2008; Ding et al., 2017; Yang et al., 2020; Zavala-Garay et al., 2022) have confirmed that this transport mechanism critically governs the formation of summer cyclonic circulation within the Beibu Gulf. Additionally, the summer westward residual flow significantly modulates thermal patterns at the western strait entrance (Bai et al., 2022) and influences seabed geomorphology and sediment deposition (Ni et al., 2014).

Current research methodologies for investigating residual flow in the Qiongzhou Strait primarily rely on two approaches: in situ data analysis and ocean numerical modeling (Shi et al., 2002; Yang et al., 2003; Chen et al., 2007; Yan et al., 2008; Chen et al., 2009; Wang et al., 2014; Zhu et al., 2014, 2015; Zavala-Garay et al., 2022; Zheng et al., 2024; Chen et al., 2025). However, both methodologies demand substantial human, material, and temporal resources while yielding limited long-term residual flow records. This constraint has directly contributed to significant discrepancies in residual flow intensity reported in previous studies focusing on different summer periods within the strait. To address these limitations, this study proposes an innovative algorithm for residual flow inversion based on the unique east-west thermal contrast (cooler eastern vs. warmer western sectors) and westward residual flow background during summer seasons. By solving a one-dimensional thermal balance equation using MODIS-derived sea surface temperature (SST) data, we achieved cost-effective and physically reasonable estimations of summer residual flow patterns in the Qiongzhou Strait from 2003 to 2022. The remainder of this paper is organized as follows: Section 2 describes the primary datasets employed in this study. Section 3 details the residual flow inversion algorithm derived from SST observations. Section 4 presents the estimated summer residual flow patterns and evaluates the reliability of the proposed methodology. Section 5 investigates the interannual variability mechanisms through selected representative years. Section 6 concludes with a summary of key findings and implications.

The summer climatological residual current for 2003–2022 in the Qiongzhou Strait was estimated using the aforementioned inverse calculation methodology. Figure 5 presents a comparative analysis between MODIS satellite-derived SST measurements (red line) and the optimal simulated SST distribution (thick blue dashed line) along control line AB. The optimal calculated SST demonstrates good agreement with the observed SST pattern along control line AB. The corresponding residual current U r e s i d u a l = − 0.130  m / s (negative values indicate westward direction) derived from this optimal SST solution drives a water transport of approximately 0.1102 Sv from the Qiongzhou Strait into the Beibu Gulf. This finding aligns with prior research documenting summer residual currents of 0.05–0.3 m/s (equivalent to 0.026–0.2 Sv transport) in the Qiongzhou Strait ( Table 1 ), thereby validating the reasonableness of our results.

Furthermore, summer westward residual flow in the Qiongzhou Strait during 2003–2022 was estimated using MODIS SST. Figure 6 compares the MODIS-observed summer SST (solid red lines) with the optimal simulated SST (blue dashed lines) along control line AB for individual years. The results reveal that most years exhibit a linear east-to-west increasing SST pattern along control line AB. Meanwhile, the optimally inverted SST generally aligns well with MODIS observations across most years, which further validates the rationality of both the one-dimensional thermal balance ( Equation 1 ) developed in this study and the residual flow inversion methodology. Notably, the thermal control line in the summer 2010 appears shorter than in other years due to missing MODIS SST data in that specific period.

Building upon these findings, Figure 7 quantitatively illustrates the temporal distribution of summer residual flow and associated volume transport in the Qiongzhou Strait from 2003 to 2022. The results reveal pronounced interannual variability in both residual current intensity and transport magnitude, aligning with the numerical simulations of Zheng et al. (2024), which suggest that interannual variability in local winds could lead to potential fluctuations in the intensity of summer residual currents. During the study period, the maximum westward residual flow occurred in 2018, recording values of -0.5040 m/s (velocity) and 0.4271 Sv (transport). In contrast, the minimum westward flow was observed in 2016, with corresponding estimations of -0.0640 m/s and 0.0542 Sv, respectively.

Figure 7 demonstrates that the summer westward residual flow velocities in the Qiongzhou Strait during 2004, 2010, and 2018 consistently exceeded 0.3 m/s, significantly surpassing the maximum westward residual currents previously reported based on field measurements or three-dimensional circulation models (Shi et al., 2002; Chen et al., 2009; Zheng et al., 2024). Does this imply that our original SST-based inversion method for residual flow is unreliable?

The summer residual flow in the Qiongzhou Strait primarily comprises three components: (1) residual flow induced by tidal rectification over complex topography, (2) wind-driven residual currents, and (3) baroclinic-effect-driven residual currents caused by density gradients (Shi et al., 2002; Chen et al., 2009; Zhu et al., 2014, 2015; Zheng et al., 2024). On seasonal timescales, residual flow forced by tidal rectification remains relatively constant across years. Therefore, interannual variability in the strait’s residual flow is predominantly governed by wind-driven and baroclinic-effect-driven components. This variability can be validated using the GLORYS12V1 reanalysis product, which corroborates the interannual trends in our inverted residual flow.

Figure 8 illustrates the spatial distributions of summer wind anomalies and associated current anomalies in the northwestern South China Sea during 2004, 2010, and 2018. Cyclonic wind anomalies were observed in all three years, which intensified the western Guangdong coastal current. This enhanced flow transported additional water masses from the western Guangdong shelf into the Qiongzhou Strait, where they merged with the westward residual flow and entered the Beibu Gulf. This mechanism coherently explains the anomalously strong westward residual flow in the strait during these years and further validates the reliability of our inversion method. The wind field over the South China Sea exhibits high sensitivity to large-scale air-sea interactions. Previous studies have extensively documented and elucidated the linkage between South China Sea wind anomalies and ENSO events (e.g., Du et al., 2009, 2011; Xie et al., 2009, 2010; Chowdary et al., 2011; Jing et al., 2011). For instance, during the strong El Niño summer of 2015, pronounced anticyclonic wind anomalies occurred in the northwestern South China Sea. These anomalies strengthened wind-driven eastward residual currents in the strait (not shown), substantially offsetting the westward residual flow generated by tidal rectification. Consequently, the net residual flow in the strait weakened markedly, with westward residual velocities dropping to the lowest levels observed during the study period ( Figure 7 ). Note that, prior studies (e.g., Zheng et al., 2024; Chen et al., 2025) also highlighted that wind stress curl modulates residual flow intensity in the strait by altering the sea surface height gradient between its eastern and western ends. In summary, our SST-based inversion method for estimating residual flow in the Qiongzhou Strait is both physically sound and robust, yielding credible results.

The Qiongzhou Strait serves as the sole west-east conduit connecting the northern South China Sea shelf and the Beibu Gulf. During summer, westward water transport through the strait contributes approximately half of the Beibu Gulf’s water renewal volume. This mass exchange plays a decisive role in shaping the summer cyclonic circulation within the Beibu Gulf (e.g., Wu et al., 2008; Zavala-Garay et al., 2022) while modulating key processes including thermal patterns, submarine geomorphology, sediment transport, nutrient fluxes, and ecological dynamics in both the strait and the gulf (e.g., Ni et al., 2014; Bai et al., 2022; Lao et al., 2022; Geng et al., 2024).

Nevertheless, current investigations into residual flow in the Qiongzhou Strait remain constrained to two conventional approaches: in-situ observational analysis and numerical ocean modeling. Both methodologies demand substantial human, material, and temporal investments, yet fail to provide long-term residual current datasets. These limitations impede a comprehensive understanding of strait dynamics and broader marine environmental processes in the northwestern South China Sea.

To address these challenges, this study pioneers an innovative algorithm that inversely calculates summer residual flow within the strait using satellite-derived SST data. The algorithm capitalizes on two critical factors: the unique east-west thermal gradient (cooler eastern vs. warmer western sectors) across the strait during summer, and the persistent westward residual flow regime. Principal findings are summarized as follows:

It should be emphasized that the developed one-dimensional thermal balance equation for Qiongzhou Strait incorporates reasonable simplifications accounting for local bathymetry and summer dynamic/thermal conditions, particularly through the exclusion of vertical processes and meridional dynamic influences on SST evolution. Furthermore, our residual flux calculation assumes uniform cross-sectional distribution of the residual flow, despite documented spatial heterogeneity in both horizontal dimensions and vertical profiles (e.g., Shi et al., 2002; Chen et al., 2009; Zheng et al., 2024). While this methodological approach may introduce uncertainties in the current quantification, it does not fundamentally compromise the validity or practical utility of our inversion methodology for deriving residual flow from SST patterns. Future studies could adopt the methodological framework presented here to develop a vertically resolved two-dimensional thermal balance equation, thereby enhancing the precision of residual flow estimates.