Journal of Geophysical Research-Oceans最新文献

Ocean fronts are dynamic regions that significantly influence marine biogeochemical processes and climate change. Despite their importance, there is limited research on the dynamics of particulate organic matter (POM) at these fronts, limiting our understanding of carbon burial mechanisms in such environments. Here, we investigated the seasonal physicochemical parameters and stable isotopes of POM in the Beibu Gulf to explore how ocean fronts affect POM fate. During non-frontal periods, notably summer and fall, microbial consumption of POM is minimal due to strong stratification, resulting in partial mineralization in middle and deep waters only. In contrast, frontal formations during winter enhance the decomposition of POM. In the northern frontal region with shallower water, intense vertical mixing induces upwelling of nutrient-rich deeper water, stimulating phytoplankton growth and enhancing the production of fresh POM. This is accompanied by significant decomposition (94%) of fresh POM, driven by oxygen replenishment from photosynthesis and vertical mixing. In the eastern frontal region, with its deeper waters, restricted nutrient availability limits phytoplankton growth, resulting in more aging POM. However, decomposition still reaches 73% due to active vertical mixing. Therefore, regardless of the occurrence of phytoplankton blooms within frontal areas, decomposition is identified as a prevailing process, reducing the potential for marine carbon burial and challenging traditional theories that depict ocean fronts solely as carbon sinks. This comprehensive study sheds light on the biological pump mechanisms active within ocean fronts and highlights their potential role as significant carbon sources.

Internal tides (ITs) in the Indonesian Seas were largely investigated and held responsible for strong water mass transformation and intense surface cooling. Here, we evaluate the ITs' impact on chlorophyll-a through a coupled INDESO ocean-biogeochemical model which is compared with in situ data and satellite products. The results show that explicit tides' inclusion within the model improves the representation of chlorophyll-a and nutrients. Previous studies highlighted that tides at spring-neap cycle cool the surface water by 0.2°C. Our current results show increases of chlorophyll-a by 0.2 up to 5 × 10⁻⁷ mg Chl m⁻³ (in log10) at ITs' generation sites (Sangihe, Ombai, Banda, and Halmahera Straits) and over the shallow Australian plateau and Java Sea, where barotropic tidal friction is high (Zaron et al., 2023, https://doi.org/10.5194/os-19-43-2023). In addition, maxima of chlorophyll-a concentration have a spring-neap tides pulse in good agreement with ocean color images. We use INDOMIX in situ vertical diffusivities in a 1D diffusion model to explain the biogeochemical tracers' transformation within the Halmahera Sea and to estimate the nutrients' turbulent flux. We find an associated increase in new production of ∼25% of the total and an increase in mean chlorophyll-a of ∼30%. These findings support the idea of enhanced surface mixing capable of providing cold and nutrient-rich water favorable for the phytoplankton growth. Hence, we confirm the key role of ITs in shaping vertical distribution and variability of chlorophyll-a, along with nutrients and oxygen, in the Indonesian archipelago at the hotspots of intensified mixing where strong ITs are found.

High hydrostatic pressure in deep-sea environments potentially impacts microbial community diversity, the structure of cellular components and functions. The specific characteristics of aerobic methanotrophs originating from deep-sea environments and their responses to local pressure fluctuations in terms of community diversity and methane oxidation potential remain unexplored. This study investigates subsurface sediments rich in aerobic methanotrophs from the natural gas hydrate-bearing region in the Shenhu area, Northern South China Sea. By conducting aerobic oxidation of methane (AeOM) incubation experiments under various environmental pressures up to 10 MPa, the study aims to elucidate differences in microbial community diversity and AeOM rates. The results show a profound impact of pressure on both the taxonomic composition of bacterial and methanotrophic communities and their capacity for methane consumption. The key aerobic methanotrophs, that is, Methylococcales, exhibit a gradual decrease in composition as pressure rises. Accordingly, their AeOM rates also show a significant negative correlation with pressure (r = 0.986, P < 0.01). The composition of three dominant methanotrophic genera, that is, unclassified_Methylococcaceae, Methylobacter, and Methylocaldum, exhibited irregular fluctuations under varying pressure conditions, with the lowest abundance observed at 2 MPa. Our study also shows that unclassified_Methylococcaceae is the primary methanotroph that exhibits the main response to pressure changes in marine environments.

Ocean conditions are known to play a critical role in the intensification of tropical cyclones (TCs). However, the relative roles of ocean temperature and salinity stratification for ocean mixing and TC-induced sea surface temperature (SST) cooling have remained unclear. Furthermore, there has been limited quantification of which factors, in terms of TC characteristics and prestorm ocean state, are the most important in controlling the amount of cooling. To investigate the mechanisms that control the amount of ocean cooling under a TC in the open ocean, we use a one-dimensional mixed layer model initialized with a variety of realistic oceanic profiles and forced with different simulated tropical cyclones. We then compare our findings to observations and reanalyses. Results consistently show that the thermodynamic effect (changes in the vertical temperature gradient with the density gradient held constant) is 2–3 times that of the mixing effect (changes in density stratification with temperature stratification held constant) and that translation speed and storm size are the two most important factors for SST cooling, followed by temperature stratification. These results emphasize the often overlooked role of storm size, which increases the residence time of high winds. We also investigate the potential predictability of TC-induced SST cooling using TC and ocean predictors in a linear regression model. It is found that this simple method, trained on observations, performs as well as more complex methods.

The Surface Water and Ocean Topography (SWOT) observatory is a complex system and contains unique systematic errors not in historical nadir altimeter observations. The errors contain expected shapes in the cross-track direction, and functional coefficients describing the shapes are correlated in the along-track direction. Existing nadir altimeter observations enable comparison to the SWOT observations. In the approach here, the SWOT error power spectral density in the along-track direction indicates the SWOT residual systematic errors are larger than nadir observed signal down to scales of 1,430 km in the PIC data and 2,500 km in the PGC data. The results guide considerations in SWOT processing and error removal.

Tropical cyclones (TCs) often undergo track turning when moving over the ocean. However, the influence of track turning on TC-ocean interactions remains little explored. This study systematically investigates sea surface temperature (SST) cooling and TC intensification during TC track-turning stages in global TC-active basins during 1998–2022. Globally, turning TCs induce stronger SST cooling than straight-moving TCs (e.g., −1.53°C vs. −1.08°C for categories 1–2), expand cooling area by 40%–110%, and greatly reduce cooling asymmetry for left-turning (right-turning) TCs in the Northern (Southern) Hemisphere. The translation speed of turning TCs is 1.5 m s⁻¹ slower compared to straight-moving TCs. Numerical experiments demonstrate that the enhanced cooling is attributed to the combined effect of track turning and accompanying slow translation speed. The enhanced cooling effectively suppresses storm intensification of turning TCs. The intensification rate for straight-moving versus turning TCs is 2.98 versus 0.06 m s⁻¹ per 24 hr for categories 1–2. As turning angle increases, cooling magnitude increases and intensification rate decreases. The probability of rapid intensification for turning TCs is about one-third lower than that for straight-moving TCs. Consequently, TCs with smaller turning angles are more likely to develop into intense TCs. Operational forecast models underforecast turning angles of turning TCs and thus overforecast TC intensity with forecast errors increasing with turning angle. This study demonstrates that TC track-turning stages play a crucial role in modulating TC intensification via an oceanic pathway, highlighting that improving track turning forecast will contribute to enhancing TC intensity forecast accuracy.

Estuaries in the northern California current system (NCCS) experience seasonally reversing wind stress, which is expected to impact the origin and properties of inflowing ocean water. Wind stress has been shown to affect the source of estuarine inflow by driving alongshelf currents. However, the effects of vertical transport by wind-driven Ekman dynamics and other shelf and slope currents on inflow are yet to be explored. Variations in inflow to two NCCS estuarine systems, the Salish Sea and the Columbia River estuary, were studied using particle tracking in a hydrodynamic model. Particles were released in a grid extending two degrees of latitude north and south of each estuary every two weeks of 2017 and tracked for sixty days. Inflow was identified as particles that crossed the estuary mouths. Wind stress was compared with initial horizontal and vertical positions and physical properties of shelf inflow particles. Inflow to the Salish Sea came from Vancouver Island and Washington slope water upwelled through canyons during upwelling-favorable wind stress, and from Washington slope water or Columbia River plume water during downwelling-favorable wind stress. Inflow to the Columbia River estuary came from Washington shelf bottom water during upwelling-favorable wind stress and Oregon shelf surface water during downwelling-favorable wind stress. For both estuaries, upwelling-favorable wind stress direction was significantly correlated with a denser and deeper shelf inflow source north of the estuary mouth. These results may help predict the source and properties of inflow to estuaries in other regions with known wind or shelf current patterns.

In recent decades, the Greenland ice sheet has been losing ice and contributing substantially to global sea level rise. Approximately half of this recent loss is due to glacier acceleration, increasing the calving of icebergs into the ocean. This process has been linked with increased ocean heat content on the continental shelf, yet the pathways delivering this heat into Greenland's fjords and its interactions with fjord-scale processes modulating glacier ice loss are still unclear. In this study, we use a series of numerical ocean model configurations to examine feedbacks between ocean circulation, subglacial discharge, submarine glacier melt, and ice mélange in Kangerlussuaq Fjord—a major fjord system where Greenland's third-largest glacier terminates. We find that subglacial discharge is a major control on ocean properties, increasing the up-fjord advection of deep warm water more than 10-fold over fjords without discharge and modulating ocean temperature on the continental shelf near the fjord mouth. Further, discharge-driven upwelling increases ice mélange melt 3-fold, revealing that subglacial discharge is an important control on mélange melt, particularly in the summer when submarine glacier melt and subsequent glacier retreat is highest. These results suggest that subglacial plume activity contributes to the strong correlation between mélange thickness and retreat noted in previous studies and may contribute to extensive future retreat at Kangerlussuaq Glacier.

Ocean water clarity, influenced by marine chlorophyll concentration, significantly alters the distribution of shortwave radiation in the water column. This work aims to assess the effects of varying chlorophyll on the upper-ocean physical properties and their subsequent impact on the atmosphere, using a coupled ocean-atmosphere regional model for the Mediterranean and Black Seas. We performed 11-year (2011–2021) twin-simulation experiments based on different chlorophyll concentrations to estimate the penetration of solar radiation in the ocean. The first simulation used a monthly climatology field of chlorophyll concentrations derived from satellite observations, while in the second experiment, the chlorophyll concentration was kept constant at 0.05 $��m�g��m^{�-�3�}�$, representing clear water conditions. Results show that radiative heating driven by chlorophyll amplifies the seasonal cycle of temperature in the upper layers, leading to increased surface warming in summer and surface cooling in winter. Also, higher surface chlorophyll contributes to cooling in subsurface layers throughout the year due to its shading effect. The temperature response to chlorophyll variations is controlled by the mixed layer depth and a balance between (a) direct near-surface radiative heating due to the chlorophyll absorption and (b) indirect cooling resulting from vertical turbulent mixing processes with subsurface waters. The atmosphere moderates the seasonal sea surface temperature (SST) response caused by chlorophyll differential heating primarily through changes in latent heat flux. Ultimately, our simulations suggest that increased surface chlorophyll concentrations enhance the Mediterranean overturning circulation, highlighting the necessity of incorporating realistic optical forcing into regional climate modeling studies.

Using temperature and salinity profiles from the Argo repository, distributions of water volume on temperature-salinity plane (so-called volumetric T-S diagrams or TSV plots) are compiled for the upper 2,000-m layer of the Atlantic, Indian, and Pacific oceans. The study is focused on Central Waters (CWs) that originate at the surface in the Subtropical Convergence and occupy the permanent thermocline within subtropical gyres. The South and North Atlantic, South Indian, and western North Pacific CWs (SACW, NACW, SICW, and WNPCW, respectively) are shaped as steep-sided ridges or narrow strips of elevated volume on the T-S plane (tight T-S relationship). In contrast, the western South Pacific, eastern South Pacific, and eastern North Pacific CWs (WSPCW, ESPCW, and ENPCW, respectively) do not have a tight T-S relationship and appear as still elongated but relatively wide poorly structured low elevations on TSV plots. Central Waters with tight T-S relationship are bordered from the poleward side by strong eastward baroclinic currents. The tight T-S relationship is found only in a denser part of CWs that outcrops within the strong eastward baroclinic currents and at higher latitudes. We hypothesize that enhanced isopycnal stirring and further diapycnal mixing of thermohaline irregularities in the strong meandering eddy-producing eastward currents substantially contribute to tightening the T-S relationship in CWs. By searching for same T-S characteristics in the main thermocline and at the surface, it was confirmed that CWs originate at the surface in the Subtropical Convergence in early spring and late winter.