Journal of Geophysical Research-Oceans最新文献

Intraseasonal variabilities (ISVs) of the western boundary currents (WBCs) east of Luzon Island were explored using acoustic Doppler current profiler (ADCP) measurements from three moorings at 18°N during 2018–2020. Besides the traditionally known surface-intensified ISV, subsurface-intensified ISV with a typical period of approximately 60 days was also detected in the currents, and the strongest signal appeared between 400 and 800 m. Further analysis indicates that they are highly associated with subsurface eddies. Based on their lifespan, subsurface eddies are classified into two categories: short-lived and medium-to long-lived eddies. The short-lived eddies are primarily generated locally near the eastern coast of Luzon Island, whereas the medium-to long-lived eddies are mainly generated away from the western boundary, in the region west of 135°E. Additional energy diagnosis suggests that baroclinic instability induced by the velocity shear of the North Equatorial Current (NEC)/subtropical countercurrent (STCC) system dominates the generation of medium-to long-lived subsurface eddies in the interior ocean, while barotropic instability and baroclinic instability play a comparable role in the generation of short-lived eddies near the eastern coast of Luzon Island.

Solar heating of the upper ocean is a primary energy input to the ocean-atmosphere system, and the vertical heating profile is modified by the concentration of phytoplankton in the water, with consequences for sea surface temperature and upper ocean dynamics. Despite the development of increasingly complex modeling approaches for radiative transfer in the atmosphere and upper ocean, the simple parameterizations of radiant heating used in most ocean models can be significantly improved in cases of near-surface stratification. There remains a need for a parameterization that is accurate in the upper meters and contains an explicitly spectral dependence on the concentration of biogenic material, while maintaining the computational simplicity of the parameterizations currently in use. Here, we assemble observationally-validated physical modeling tools for the key controls on ocean radiant heating, and simplify them into a parameterization that fulfills this need. We then use observations from 64 spectroradiometer depth casts across 6 cruises in diverse water bodies, 13 surface hyperspectral radiometer deployments, and broadband albedo from 2 UAV flights to probe the accuracy and uncertainty associated with the new parameterization. A novel case study using the parameterization demonstrates the impact of chlorophyll concentration on the structure of diurnal warm layers. The parameterization presented in this work will allow for better modeling of global patterns of sea surface temperature, diurnal warming, and freshwater lenses, without a prohibitive increase in complexity.

The Southern Ocean (SO) plays a crucial role in the process of sequestering heat and carbon dioxide from the atmosphere and transferring them to the deep ocean. This process is intricately linked to the formation of Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW), which are pivotal components of the Meridional Overturning Circulation (MOC) and have a substantial impact on the global climate balance. AAIW and SAMW take shape in specific regions of the Southern Ocean due to the influence of strong winds, buoyancy fluxes, and their effects, such as convection, the development of thick mixed layers, and wind-driven subduction. These water masses subsequently flow northward, contributing to the ventilation of the intermediate layers within the subtropical gyres. In this study, our focus lies on investigating the regional aspects of AAIW and SAMW transformation in CMIP6 models. We accomplish this by analyzing the relationship between the meridional transport of these water masses and air-sea fluxes, particularly Ekman pumping, freshwater fluxes, and heat fluxes. Our findings reveal that the highest transformation rates occur in the Indian sector of the Southern Ocean, with notable values also observed in the southeast Pacific and south of Africa. Additionally, we assess the potential changes in these formation regions under future scenarios projected for the end of the 21st century. Although the patterns of formation regions remain consistent, there is a significant decrease in the transformation process.

In Eastern boundary upwelling systems, such as the California Current System (CCS), seasonal upwelling brings low oxygen and low pH waters to the continental shelf, causing ocean acidification and hypoxia (OAH). The location, frequency, and intensity of OAH events is influenced by a combination of large-scale climatic trends, seasonal changes, small-scale circulation, and local human activities. Here, we use results from two 20-year long submesoscale-resolving simulations of the Northern and Southern U.S. West Coast (USWC) for the 1997–2017 period, to describe the characteristics and drivers of OAH events. These simulations reveal the emergence of hotspots in which seasonal declines in oxygen and pH are accompanied by localized short-term extremes in OAH. While OAH hotspots show substantial seasonal variability, significant intra-seasonal fluctuations occur, reflecting the interaction between low- and high-frequency forcings that shape OAH events. The mechanisms behind the seasonal decreases in pH and oxygen vary along the USWC. While remineralization remains the dominant force causing these declines throughout the coast, physical transport partially offsets these effects in Southern and Central California, but contributes to seasonal oxygen loss and acidification on the Northern Coast. Critically, the seasonal decline is not sufficient to predict the occurrence and duration of OAH extremes. Locally enhanced biogeochemical rates, including shallow benthic remineralization and rapid wind-driven transport, shape the spatial and temporal patterns of coastal OAH.

Starting in 2012, the eastern subpolar North Atlantic experienced the strongest surface freshening in the past 120 years. It is yet unknown whether this salinity anomaly propagated downward into the water column and affected the properties of the boundary currents of the subpolar gyre, which could slow down the overturning. Here, we investigate the imprint of this salinity anomaly on the warm and saline Irminger Current (IC) in the decade thereafter. Using daily mooring data from the IC covering the period 2014–2022 combined with hydrographic sections across the adjacent basins from 1990, the evolving signal of the salinity anomaly over the water column and its imprint on the transport variability is studied. We find that due to the salinity anomaly, the northward freshwater transport of the IC increased by 10 mSv in summer 2016 compared to summer 2015. In 2018, the salinity anomaly covered the water column down to 1,500 m depth. Hydrographic sections across the basin showed that this recent freshening signal spread across the Irminger Sea. Overall, the freshwater transport of the IC increased by a factor of three between 2014–2015 and 2021–2022. The associated density decrease over the upper 1,500 m of the water column resulted in an increase in the northward transport of waters lighter than σ₀ = 27.55 kg m⁻³ from 1.7 to 4.2 Sv. This change in northward IC transport by density class may impact the characteristics of the overturning in the Northeastern Atlantic, its strength and the density at which it peaks.

Model projections suggest that the continental shelf in the southern Weddell Sea may experience a shift from today's near-freezing temperature to a much warmer state, where warm water floods the shelf and basal melt rates beneath the Filchner Ronne Ice Shelf increase dramatically. Today, the Filchner Trough serves as a conduit for the southward flow of Warm Deep Water (WDW) during summer and, thus, requires continuous monitoring of its hydrographic conditions. An extensive network of moorings was installed at key sites along the inflow pathway from 2017 to 2021, to expand on existing mooring records starting in 2014. The moorings complemented with under-ice profiling floats reveal two inflow pathways, where WDW enters along the eastern flank of the Filchner Trough as well as through a smaller trough east of there. Within the observed period, 2017 and 2018 feature anomalously warm inflows. The inflow is regulated by the heaving of isopycnals over the continental slope, and the southward propagation toward Filchner Ice Shelf is two times faster during these warm years. Furthermore, the warm years coincide with low summer sea ice concentration, which enhances surface stratification through increased freshwater input and modifies sea ice-ocean stresses that both act to lift the warm water layer and increase the temperatures on the continental shelf. Finally, the recent record low sea ice conditions around the Antarctic emphasize the importance of our findings and raise concerns regarding a potentially increasing presence of WDW on the southern Weddell Sea shelf.

The global ocean plays an important role in the overall budgets of nitrous oxide (N₂O) and methane (CH₄), especially in continental estuaries and shelf areas. Four cruises were conducted between 2021 and 2022, covering the spring, summer, and fall seasons, to study the spatial and seasonal characteristics of N₂O and CH₄ distributions and emissions in the Taiwan Strait (TWS). The surface N₂O and CH₄ concentrations gradually decreased from the coast to the open sea, with maximum values (14.3 and 15.6 nmol L⁻¹) occurring near the Jiulong and Minjiang estuaries, respectively. The mean surface N₂O concentration (8.2 nmol L⁻¹) was highest in the spring and approximately the same in the summer as in the fall. The mean surface concentrations of CH₄ (8.8 nmol L⁻¹) were greater in summer than in spring and fall, probably because of the high freshwater input in summer. Except for several stations in fall, surface waters were oversaturated with N₂O and CH₄ relative to the atmosphere in other seasons, and the TWS was a net source of atmospheric N₂O and CH₄ in the spring, summer, and fall. In situ production is the main source of N₂O and CH₄ in the TWS, with nitrification being the dominant mechanism of N₂O production in the TWS. In contrast, physical influences (riverine inputs and water mass mixing) reshape the distributions of N₂O and CH₄. The annual emissions of N₂O and CH₄ from the TWS were estimated to be 0.9 × 10⁻³ ± 2.9 × 10⁻³ Tg yr⁻¹ and 1.6 × 10⁻³ ± 3.0 × 10⁻³ Tg yr⁻¹, respectively. Taken together, the TWS accounts for 0.025% of the surface area of the world's oceans and 0.022% ± 0.07% and 0.017% ± 0.033% of global oceanic N₂O and CH₄ emissions, respectively.

This study investigates the influence of ocean currents on wave modeling and satellite observations using in situ wave measurements from the One Ocean Expedition 2021–2023. In January 2023, six OpenMetBuoy drifters were deployed in the Agulhas Current region. Their high immersion ratio minimized wind effects, allowing them to follow the current and return to the Indian Ocean by the Agulhas retroflection, collecting data for about 2 months. Comparing surface current velocities from both the Mercator model and Globcurrent product with drifter data reveals underestimation for velocities over $��0.5��m��s^{�-�1�}�$ with Mercator showing greater variability. Significant wave height and Stokes drift parameters from MFWAM and ERA5 were also evaluated against drifters. Both models tend to overestimate Stokes drift more noticeable in ERA5, indicating sensitivity to wind seas. For significant wave height, both models agree well with drifter measurements with correlations of 0.90 for MFWAM and 0.83 for ERA5. However, ERA5's lack of surface current data combined with its coarse resolution (0.5$��°�$) lead to underestimation of wave heights exceeding 2.5 m. MFWAM products including and excluding currents exhibit root mean square errors of 0.39 and 0.45 m, respectively, when compared to drifter measurements. This confirms that neglecting currents introduces additional errors particularly in areas with sharp current gradients. Analyzing MFWAM wave spectra, including and excluding currents, reveals wave energy transfer attributed to wave-current interactions. The spatial extent of these interactions is captured by satellite altimeters, revealing wave modulations with considerable wave height variations when waves cross eddies and the current core.

The southeastern Atlantic Ocean is a crucial yet understudied region for the ocean absorption of anthropogenic carbon (C_anth). Data from the A12 (2020) and A13.5 (2010) cruises offer an opportunity to examine changes in dissolved inorganic carbon (DIC), its stable isotope (δ¹³C), and C_anth over the past decade within a limited region (1∼3°E, 32∼42°S). For the decade of 2010–2020, C_anth invasion was observed from the sea surface down to 1,200 m based on both DIC and δ¹³C data. The mean C_anth increase rate (1.08 ± 0.26 mol m⁻² yr⁻¹) during this period accelerated from 0.87 ± 0.05 mol m⁻² yr⁻¹ during the previous period (1983/84–2010). The δ¹³C-based C_anth increase closely matches the DIC-based estimation below 500 m but is 26% higher in the upper ocean. This discrepancy is likely due to δ¹³C's longer air-sea exchange timescale, seasonal variability in the upper ocean, and the chosen ratio of anthropogenically induced changes in δ¹³C and DIC. Finally, column inventory changes based on the two methods also exhibit very similar mean C_anth uptake rates. The paired DIC concentration and stable isotope dataset may enhance our ability to constrain C_anth accumulation and its controlling mechanisms in the ocean.