Journal of Geophysical Research-Oceans最新文献

Increased meltwater runoff from glaciers may drive localized ocean acidification and impact carbon dioxide (CO₂) uptake in the coastal ocean. However, the paucity of carbonate system observations from continental shelves receiving inputs from glaciers limits our understanding of cryosphere-ocean connectivity. Here, we contrast meltwater impacts on seawater carbonate chemistry and stable isotopes (δ¹³C-DIC) off marine- and land-terminating glacier outflows off Iceland. On the shelf outside a marine-terminating glacier, glacial meltwater reduced the seawater buffer capacity of receiving surface waters through dilution of total alkalinity, and increased CO₂ uptake through salinity-driven drawdown of pCO₂. Primary production acted as a counterbalance to the lowered [TA-DIC]. On the shelf area receiving meltwater from large glacial river deltas, CO₂ uptake was almost halved and the saturation state of aragonite was 0.2 units lower than on the marine-terminating glacier shelf. Reduced net autotrophy due to higher turbidity and upwelling of low-pH deep waters off the delta-dominated shelf likely explain those differences. The diverging carbonate dynamics on the two shelves build on previous observations that land-terminating glaciers can reduce the buffer capacity as well as CO₂ uptake potential of nearshore surface waters in comparison to marine-terminating glaciers. The future retreat of many marine-terminating glaciers onto land is likely to modify how meltwater will impact coastal seawater carbonate chemistry.

Tropical cyclones (TCs) induce complex three-dimensional ocean thermal responses, but how pre-existing mesoscale eddies modulate this remains inadequately assessed. Using idealized simulations, we demonstrate that while cyclonic and anticyclonic eddies (AEs) dramatically alter the spatial pattern of subsurface cooling, the net, vertically integrated ocean heat content change is remarkably insensitive to their presence. This insensitivity, evidenced by a consistent sea level anomaly reduction, arises because eddy-induced surface heat flux anomalies are minor compared to the total TC-driven ocean heat loss. Nevertheless, eddies critically mediate a subsurface temperature redistribution. Upper-ocean cooling is enhanced within AEs but attenuated within Cyclonic eddies (CEs), which aligns with previous observational findings. In contrast, anomalous warming or cooling occurs in the vicinity of eddy edges. A heat budget analysis reveals that advection, driven by eddy-induced anomalous vertical velocities, dominates the sustained thermal response. These vertical motions are well explained by the interaction of TC wind stress with eddy vorticity following Stern's (1965, https://doi.org/10.1016/0011-7471(65)90007-0) theoretical framework. Our findings demonstrate that pre-existing eddies primarily act as key mediators of the ocean's three-dimensional thermal structure under TC forcing, but do not fundamentally alter the net water-column heat loss, highlighting their role in redistributing, rather than determining, TC-induced heat changes.

The continental shelf off the northwest coast of North America experiences seasonal upwelling driven by summertime southward winds. Nutrient-rich, low-oxygen water is upwelled onto the shelf from a depth of about 150 m seaward of the continental shelfbreak. Though nutrients fuel a productive food web, respiration of organic matter can lead to hypoxic conditions (dissolved oxygen < 61 μmol/kg ∼ 1.4 ml/l) near the seafloor and sometimes reaching to over half of the lower water column. We compare data collected inshore of the 200-m isobath during the summers of 2021–2024 to determine the magnitude and spatial distributions of near-bottom hypoxic conditions on the shelf and whether these patterns are similar year-to-year. We found widespread hypoxia across the shelf during the summers of 2021 (52% of the shelf), 2022 (28%), 2023 (29%) and 2024 (39%). The largest (smallest) area of near-bottom hypoxia was 17,079 (9,196) square kilometers in 2021 (2022), when upwelling winds were particularly strong (weak). The spatial distribution of near-bottom dissolved oxygen exhibits similar patterns across years reinforcing the importance of oceanographic processes in setting those patterns. We investigate relationships between hypoxia and possible forcing factors including dissolved oxygen levels of upwelled source waters—declining over time—and cumulative upwelling-favorable wind over the upwelling season. There is some evidence for stronger cumulative upwelling-favorable seasonal winds to drive more hypoxia on the shelf. This implies that should upwelling-favorable winds continue to increase due to climate change, more hypoxia will occur over the US Pacific Northwest continental shelf.

The Kuroshio intrusion into the South China Sea (SCS) through the Luzon Strait (LS) exhibits three different paths—looping, leaping, and leaking. In this study, a high-resolution Oceanic Regional Circulation and Tide Model is employed to investigate these pathways by calculating the Kuroshio SCS Index (KSI), coupled with a comprehensive diagnostic analysis of the vorticity balance to explore the controlling dynamic mechanisms. The results reveal that the KSI exhibits seasonal variations, with the leaping path predominantly occurring in summer, the looping path primarily prevailing in winter, and the leaking path mainly emerging in spring and autumn. The spatial distribution of the advection of geostrophic potential vorticity (APV) transport reveals two key regions within the LS that are critical to the Kuroshio intrusion into the SCS. The Joint Effect of Baroclinicity and Relief (JEBAR), serving as the dominating forcing term, balancing the APV term; its intensity is significantly affected by upstream Kuroshio transport (KT) via modulating the local horizontal density gradient: stronger KT homogenizes the water mass, weakening JEBAR and favoring the formation of the leaping path, while weaker KT enhances density–topography coupling, strengthens JEBAR, and promotes the formation of the looping path; intermediate conditions give rise to the leaking path. This study provides a general dynamic framework for understanding the multiple pathways of Kuroshio intrusion, offering insights into the circulation in the SCS.

Barrow Canyon is a major conduit through which Pacific-origin water enters the Arctic Basin. Mooring data acquired across the mouth of Barrow Canyon from 2000 to 2022 have enabled direct computation of seawater transports. No significant decadal trend in volume transport of Barrow Canyon throughflow was observed, although an upward trend of Bering Strait throughflow has been reported. Annual heat transport through the canyon varied widely, ranging from 0.93 to 7.05 TW, and larger values of heat transport occurred more often in the 2010s compared to the 2000s. The interannual variability of heat transport was not correlated with the Bering Strait heat transport, even though most of the Pacific water inflow through the Bering Strait flows along an eastern path toward Barrow Canyon during summer. Instead, year-to-year variation in Barrow Canyon heat transport was driven by variation in summertime sea ice coverage of the northeastern Chukchi Sea, because early ice clearance reduces sensible heat loss to thawing ice and increases direct warming of the surface water via insolation. Using sea surface temperature and wind data from the Chukchi Sea, we derived a proxy for estimating volume and heat transport through Barrow Canyon over the last 40 years. Estimated heat transport in the canyon doubled between 1980s and 2010s likely because of decreasing sea ice presence in summer. This change in heat transport has been sufficient to explain the increase in the heat content of Pacific summer water in the Canada Basin over the same interval.

In 2024 an anomalous region of low chlorophyll water covering ∼721,000 km² or 1.7% of North Atlantic surface area dominated the Northeast Atlantic. This feature formed during spring, remained identifiable as a region of low chlorophyll throughout the summer months in temperate and subpolar waters and, due to low opal ballasting potential of newly formed biomass, likely impacted ecosystem processes and carbon export fluxes across a wide sector of the Northeast Atlantic. In situ sampling along the southern edge of this region for 15 days in May–June encountered an unusually deep euphotic layer that shoaled rapidly from >80 to ∼40 m over subsequent days and low surface chlorophyll concentrations (<0.3 mg m⁻³) despite non-limiting nitrate and phosphate availability, though silicate was exhausted. Integrated net primary production rates within this feature ranged between 0.5 and 0.6 g C m⁻² d⁻¹, NO₃⁻ uptake rates between 0.8 and 1.3 mmol N m⁻² d⁻¹, and new production rates between 0.08 and 0.13 g C m⁻² d⁻¹; rates that on average were 45%–79% lower than rates outside of this feature. Integrated concentrations of particulate organic carbon (POC), nitrogen (PON) and phosphorous (POP) were up to 44%–63% lower than surrounding waters. We hypothesize that this region of low productivity may be the consequence of prolonged and anomalous warming of the wider North Atlantic throughout 2023–2024 leading to weaker mixing and preconditioning of the surface ocean during winter 2024 with implications for the resident phytoplankton community.

The surface waters of the subarctic northeastern Pacific Ocean contain high concentrations of macronutrients such as phosphate and nitrate, which can potentially drive photosynthesis and biological uptake of carbon dioxide. Regional biological productivity, however, is limited by a lack of necessary micronutrients such as iron. In the Gulf of Alaska, there is a strong biogeochemical gradient from highly productive and nitrogen-limited coastal waters to the weakly productive and iron-limited open ocean. It has been hypothesized that the cross-shelf transport by ocean eddies transports the iron-rich coastal waters to the offshore regions. An eddy-permitting ocean biogeochemistry model is developed to simulate the role of mesoscale ocean eddies in shaping nutrient cycling and regional biological productivity. Model skills are first evaluated using in situ and satellite observations. Through a case study and sensitivity experiments, the physical and biogeochemical structures of the mesoscale eddies are examined, informing the mechanisms behind cross-shelf transport of dissolved iron. Anticyclonic eddies generated along the continental shelves during the late winter contain an elevated level of dissolved iron just below the surface of the mixed layer. The biological productivity in the late spring and early summer in the offshore regions is supported by these anticyclonic eddies within about 300 km from the coastline. A sensitivity experiment is performed to purposefully suppress the mesoscale features, showing a reduction in the biological productivity accompanied by a reduction in eddy activity in this region. Resolving mesoscale eddies is crucial for correctly representing the biological productivity in this region.

This study investigates the velocity and salinity structure of the large surface-attached, low-latitude, and microtidal plume of the Magdalena River (southern Caribbean Sea) during a period of high freshwater discharge and variable wind conditions. The plume was analyzed through observations at multiple transects along, across, and diagonal to the shoreline, using ADCP measurements and a CTD chain. Results show that the plume is very shallow, with a large aspect ratio of O(10⁴) and rapid changes in its extension and direction within hours in response to forcing variability. The plume's dynamics are primarily governed by the river momentum in the near-field, a competition between river momentum and wind stress in the mid-field, and the wind stress in the far-field, while the ambient ocean currents influence the plume only during mild winds. The velocity and salinity structures reveal a marked asymmetry between the downwind and upwind sides of the plume. Downwind, the plume is faster, narrower, and more mixed and remains supercritical beyond the near-field. Upwind, the interaction between opposing river momentum and wind-driven flows generates vortices and fronts. Finally, the plume Kelvin and Rossby numbers indicate that the Coriolis effect, often neglected in low-latitude coastal systems, also influences the plume dynamics when the system exceeds a critical horizontal extension. The findings are rationalized to provide insights into the dynamics of low-latitude wind-dominated river plumes.

The central California coast between San Francisco Bay (SFB) and Monterey Bay (MB) is an upwelling-dominated marine ecosystem with a coastal population of 8.5 million. Terrestrial nutrients enter the ocean via three primary pathways, representing natural and anthropogenic sources: (a) SFB net export across the Golden Gate Strait, (b) coastal rivers, and (c) municipal wastewater discharged to ocean outfalls. The consequences of these inputs on primary production, acidification, and hypoxia remain poorly understood. Here, we investigate these effects with a submesoscale-resolving biogeochemical ocean model. Simulations suggest that while terrestrial nutrient inputs collectively affect a broad region, stronger impacts occur in nearshore waters, increasing dissolved inorganic nitrogen by 11.4%, primary production by 6.5%, and chlorophyll concentration by 4.5% along a 15-km coastal band. While exchanges from the SFB dominate these effects, all sources, including coastal rivers and ocean outfalls, produce distinct, localized footprints. Subsurface oxygen and pH decline due to terrestrial nutrient loading, but these changes are small relative to vigorous upwelling and circulation. The resulting nutrient enrichment could promote conditions favorable for diatom growth, including toxigenic species such as Pseudo-nitzschia spp., creating an environment predicted to elevate the risk of domoic acid (DA) events. Model results indicate that chlorophyll concentrations exceed the threshold associated with elevated DA risk on 10%–45% more days under nutrient-enriched conditions. These findings highlight the need for expanded observational and modeling efforts to better understand the ecological consequences of terrestrial nutrient pathways and their anthropogenic contributions along the central California coast.

Air-sea gas exchange affects the biogeochemical cycling of trace gases such as CO₂ and dimethyl sulfide (DMS) on a global scale, thereby influencing Earth's climate. In nearshore regions, differences in wind fetch and surfactants are expected to have an impact on gas transfer velocity (k). Accurate determination of air-sea gas exchange in nearshore regions is crucial for assessing the efficacy of carbon dioxide removal (CDR) techniques, as many CDR methods are expected to be deployed in these regions. In this study, we used the ³He/SF₆ dual tracer technique to determine k and investigate factors that control air-sea gas exchange in a nearshore inland sea ecosystem, the coastal Baltic Sea. We found that k was, on average, about 39% lower than in other coastal and offshore regions at the same wind speed, with a more pronounced reduction at higher wind speeds and during developing wave conditions. Most of the wind speed/gas exchange parameterizations proposed for the Baltic Sea were found to overestimate k. The lower k was likely due to wind fetch limitation, wind-wave interactions, and the presence of surfactants.