In the tropical Pacific, freshwater flux (FWF) at the air-sea interface is a crucial atmospheric forcing that affects the ocean variability. FWF influences sea surface salinity (SSS) and buoyancy flux (QB), thereby modulating upper-ocean stratification and dynamics. However, most ocean models typically represent FWF influences only through the SSS pathway, while neglecting its influence through the QB pathway. The distinct impacts of these two pathways on oceanic physical fields and ENSO asymmetry remain insufficiently understood. Using a layered ocean model, we systematically investigate the respective contributions of FWF variations through the SSS and QB pathways to upper-ocean physics. The results show that the SSS pathway dominates interannual sea surface temperature (SST) variability and substantially amplifies the amplitude of SSS anomalies. In contrast, the QB pathway primarily modulates mixed layer depth and reduces its interannual variability. We further examine the role of FWF in shaping asymmetry of ENSO amplitude and quantify the contribution of the two pathways. During cold and warm ENSO events, the relative roles of the two pathways in modulating SST and their dynamical origins differ substantially. During El Niño, the redistribution of upper-ocean heat is mainly controlled by vertical mixing driven by QB anomalies. Weakened QB anomalies shoal the mixed layer, suppress heat exchange between the surface and subsurface ocean, and thereby promote SST. In contrast, during La Niña, salinity changes induced by SSS pathway play a dominant role. This process increases surface seawater density, weakens ocean stratification, and enhances the mixing and diffusion of heat between the surface and subsurface layers, further reducing SST. This contrast may arise from the positive relationship between the amplitudes of QB and SST. FWF anomalies affect ENSO through two distinct pathways, regulating heat transport between the mixed layer and the underlying ocean and thereby enhancing the asymmetry of ENSO Amplitude.These findings highlight the distinct roles of FWF forcing and its two influence pathways in regulating tropical Pacific ocean–atmosphere interactions. This study advances our understanding of the mechanisms through which FWF affects the asymmetry of ENSO amplitude and provides insights that may improve the characterization and prediction of ENSO under a warming climate.
Under summer monsoon, upwelling develops around the Zhoushan Islands (ZSI) in the East China Sea, strongly influencing regional primary production and fisheries. Using a high-resolution circulation model, we investigated the general flow patterns and examined the respective roles of tides and winds in modulating the intensity and pathways of upwelling around the ZSI. The simulations revealed that summer flows were primarily tide-driven, characterized by a strong horizontal along-front jet and vertical cross-front secondary circulation. The along-front jet enhanced the cross-shore velocity gradient within the interior of water column, thereby strengthening the horizontal flow convergence and resulting in farther-upward increase of vertical velocity, namely, deviation of the upwelling pathway to the interior. Moreover, tide-induced uplift also occurred with weakened bottom stress and increased Slope Burger number. Nevertheless, the influence of weak summer monsoon was also significant and varied with depth. Specifically, weak northwestward winds declined the horizontal flow convergence, further suppressing the upwelling in the upper layer. However, winds enhanced the upwelling in the deep layer, for the addition of wind-induced upwelling peaking near the top of the thin bottom boundary layer. Furthermore, upwelling locations and intensities inferred from SST differed substantially from those diagnosed from hydrodynamic processes, likely for intense tidal mixing over very shallow regions and energetic jets within narrow channels. Therefore, the integrated upward transport over specified areas or vertical velocities away from islands may provide more robust metrics for quantifying the changes in coastal upwelling.
In ocean numerical modeling, the Localized Sigma Coordinates with Shaved Cell (LSC²) is employed to eliminate the spurious mixing phenomenon induced by conventional σ-coordinates. However, its traditional Depth-Dependent Adjustment Scheme (DDAS) relies heavily on user experience for parameter configuration, which poses certain challenges when applied to complex terrain. In this paper, Slope-Dependent Adjustment Scheme (SDAS) is designed to optimize the configuration of LSC², addressing spurious mixing problems in steep terrain. The maximum vertical coordinate slope is established as an objective constraint, achieving topography-adaptive grid configuration through horizontally diffusive iteration and slope-dependent adjustment between adjacent nodes. An idealized experiment with an isolated seamount reveals significant differences in slope of the vertical grid and stratification variation among SDAS, DDAS, and SZ grid. Results demonstrate that SDAS effectively controls the slope of vertical grid, reducing the mean vertical-grid slope by 40.6% and 70.7% compared to DDAS and SZ-500 grid. Furthermore, stratification changes are substantially decreased, with the reduction of stratification decay rate reaching 17.2% and 59.6% respectively. In a practical simulation over the seas surrounding Taiwan, the simply configured SDAS achieves comparable accuracy in simulated temperature profiles and a similar level of numerical dissipation to DDAS, while outperforming the traditional SZ grid. Comprehensive comparisons confirm the advantage of SDAS in resolving the limitations of DDAS and SZ grid, and provide a simpler yet effective configuration scheme.
Bathymetry is a primary control on estuarine hydrodynamics. This control is particularly relevant in engineered estuaries, where anthropogenic ship channels often impose sharp depth contrasts that can alter hydrodynamics and challenge numerical representation. Using the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM) for channel-shoal Mobile Bay, Alabama, in the northern Gulf of America, this study tested three vertical grids: Pure Sigma (Pure S), Hybrid Sigma-Geopotential (SZ) and Localized Sigma Coordinates with Shaved Cells (LSC2); two Shapiro filtering strategies; and two turbulence implementations: SCHISM’s internal Generic Length-Scale (GLS) setup and a General Ocean Turbulence Model (GOTM) coupled setup. The objective was to evaluate configuration sensitivity in salinity representation with emphasis on sharp channel-shoal transitions. Within the staged experimental design, vertical grid choice produced the largest differences in salinity skill. SZ showed most balanced performance by retaining saline water within the channel corridor and limiting spurious mixing across the channel-shoal interface, where steep depth transitions coincided with strong salinity gradients. Targeted Shapiro filtering (TS), retained in the bay but removed from the bayhead delta, produced modest and localized improvements in modeled delta water levels and bayhead salinity transport. Turbulence sensitivity varied by subregion: the internal GLS setup showed better agreement over the shoals, whereas the GOTM-coupled setup showed better agreement in the channel, consistent with modeled differences in mixing, stratification, and stability-dependent diffusivity. Among the configurations tested for Mobile Bay, SZ-GLS TS provided the most practical compromise across the channel, shoals, and delta. The findings also highlight needs for improved grid and turbulence representations.
The reflection of internal wave beams by three-dimensional (3D) topography can redistribute wave energy laterally, but how this process depends on the criticality parameter (γ) remains insufficiently understood. In this study, numerical simulations were performed using OpenFOAM to investigate the evolution and reflection of internal wave beams over 3D seamounts for a range of γ. The results reveal that, as γ transitions from subcritical to supercritical, distinct convergent and divergent regions of internal wave beams emerge above the seamount. Energy in the convergent region increases nearly linearly with decreasing γ, reaching up to 3.47 times that in the divergent region. The associated dissipation can be up to 2.72 times the value under supercritical conditions, thereby increasing the fraction of energy dissipated over subcritical seamounts. Under subcritical conditions, the changes in spatial structure of internal waves induced by seamount give rise to a recirculating mean flow, whose strength decreases linearly with increasing γ. This mean flow, together with the boundary mean flow, promotes particle accumulation in the upper windward slope region near the crest and on the lee side of the seamount, increasing the local particle concentration by approximately 21% and 28%, respectively. This particle redistribution may contribute to the development of the previously reported “seamount effect.”
