Journal of Geophysical Research-Oceans最新文献

Broadened width of high chlorophyll concentration band with wavy structures, patches, and filaments are often observed along the western coastal next to the Pearl River Estuary over the northern South China Sea shelf during the transition period from winter to summer monsoon. Whereas, there is no such wide band in other seasons. By using a high-resolution numerical model, we reveal that the complex structure and wider band of high coastal chlorophyll concentration results from the smaller scale eddies (about 20–50 km in diameter) associated with buoyant plume-induced salinity front and density fronts, which are roughly along the 30 and 50 m isobaths, respectively. Two trains of eddies are formed along the fronts by the baroclinic instability triggered by frequently alternating winds over the fronts during the period of monsoon transition. The influences of these two trains of eddies are extended in the cross-shelf direction by their interactions, and they can temporally enhance the cross-shelf flow and material exchange. They serve as an efficient pathway to link the inner shelf toward the continental slope.

Heat and momentum transport processes are studied through direct numerical simulations of air-water two-phase flows with surface waves under wave-wind couplings. Three wave age cases, sea state changing from wind sea to swell, are analyzed to investigate the roles of surface waves in the turbulent transport of heat and momentum, which are examined by decomposing the statistics into the plane-averaged, wave-coherent, and turbulent-induced components. Under wind sea conditions, a dissimilarity in turbulent transfer between heat and momentum is observed in the near-surface region. This discrepancy arises from the enhanced countergradient heat transport on the leeward side, which is caused by wave-coherent structures. The surface waves induce phase-dependent variations in the temperature and flow structures, reducing the scale of temperature structure. This reduction further results in a weaker contribution of ejections and sweeps to heat transfer. In contrast, momentum transport is predominantly downgradient on the leeward side due to the large-scale flow structure. This difference in coherent structures leads to the dissimilar transport between heat and momentum. Under lower-frequency swell conditions, surface waves induce an upward momentum that enhances the vortical structures near the wave surface. The transfer efficiency of turbulent momentum and heat gradually reaches equilibrium, after which both transport processes become more analogous.

To elucidate impacts of an Indian Ocean Dipole (IOD) event on surface seawater CO₂ dynamics, we analyzed data collected along the World Ocean Circulation Experiment Hydrographic Program I08 N line (latitudes 20°S–6°N at 80°E) in December 2019, when a strong positive IOD (pIOD) event occurred. After removing the effects of anthropogenic CO₂ accumulation, we examined anomalies of the surface seawater CO₂ fugacity (fCO₂) from the climatology in relation to other marine properties. At latitudes 11°S–6°S, where horizontal advection of upwelled water off Sumatra was observed, dissolved inorganic carbon and total alkalinity, both normalized to a salinity of 35 (nTCO₂ and nTA) showed positive anomalies of +11.4 and +8.1 μmol kg⁻¹, respectively. At latitudes 5°S–5°N, where distinct low-salinity water was observed because of the pIOD, nTCO₂ and nTA showed negative anomalies of −4.0 and ‒0.5 μmol kg⁻¹, respectively. The combined effects of the nTCO₂ and nTA anomalies on fCO₂ made the observed fCO₂ anomalies small, +3.2 and −6.6 μatm for 6°S–11°S and 5°S–5°N, respectively, because the direction of the Revelle factor for TCO₂ is opposite to that for TA. We estimated that the pIOD modulated the air–sea CO₂ flux by +0.45 and −0.55 mmol m⁻² d⁻¹ on average within11°S–6°S and 5°S–5°N, respectively. The impacts of the pIOD on the CO₂ dynamics could be explained by the anomalous salinity conditions associated with upwelled water and the freshwater balance.

The cyclonic eddy uplifts nutrient-rich seawater into the euphotic zone, typically directly enhancing phytoplankton abundance and primary production. However, its impact on heterotrophic prokaryotic production (HPP) remains unclear due to the complex interplay of multiple indirect factors governing this process. Here, we conducted a comprehensive investigation of the distribution of picophytoplankton and heterotrophic prokaryotes, prokaryotic community structure, and HPP within a cyclonic eddy in the western North Pacific subtropical gyre. The results indicated that despite the higher abundance of picophytoplankton accompanied by nutrient upwelling at the eddy core compared to the edge, higher levels of HPP were observed at the eddy edge between 100 and 200 m, consistent with the distribution of the low nucleic acid content (LNA) prokaryotes. The significant positive correlation between HPP and the proportion of LNA group in total heterotrophic prokaryotes suggested a primary contribution from the LNA group over the high nucleic acid content (HNA) group. SAR11, a typical member of the LNA group, may primarily contribute to the elevated HPP observed at the eddy edge. The changes in temperature, nutrients, and light intensity induced by the cyclonic eddy may significantly influence the distribution and activity of HNA and LNA groups, potentially exerting a greater impact on HPP compared to phytoplankton-related factors. These findings contribute to understanding the underlying mechanisms of HPP responses to cyclonic eddies in the oligotrophic open ocean.

Subsurface eddies, characterized by their cores located within or below the pycnocline, can transport materials over long distances in the ocean's interior. Observations of these eddies are sparse, limiting our understanding of their regional distribution and detailed horizontal structures, particularly in high-latitude areas. The Bering Sea, situated in the subarctic region, is among the world's most productive areas and significantly influences the Arctic Ocean's state, thereby impacting climate change. In this study, we utilize ultrahigh resolution (approximately 10 m) data to investigate the distribution and characteristics of subsurface eddies in the Aleutian Basin, Bering Sea. We detected 44 subsurface eddies in 13 survey transects and analyzed their morphological and hydrographic characteristics, spatial distribution, propagation, and transport. The results show that the average core radius of the subsurface eddies is about 11.62 km and they exhibit complex structures in both the core and flank regions. The dichothermal layer cold-core eddies are prevalent in the deep-water region of the Bering Sea, contributing approximately 1.76 Sv poleward and westward transport in the subsurface layer. This is the first three-dimensional depiction of subsurface eddies in the Bering Sea, revealing that the prevalence of subsurface eddies in the Bering Sea may have been negligent, with significant implications for the hydrographic and biogeochemical properties of both the Bering Sea and the Arctic Ocean. More detailed comprehensive and long-term observations should be made to assess the global impact of subsurface eddies in the future.

A topography change in the continental plain plays an important role in nutrient replenishment mechanisms in oligotrophic oceans. Effects of the topography on the nutrient distribution in the western South China Sea (WSCS) have been overlooked since most studies have focused on the dipole-induced upwelling and downwelling processes of nutrients. We hypothesize that the seamount topography in the northwestern side of the WSCS contributes to the upward distribution of nutrients. We conducted a cruise to investigate the vertical distribution of nutrients in a large area where there is a gradient in the topography: shallow in the north to deep in the south. Our results showed that the depth contours of nutrients, temperature, and salinity shoaled upward from deep to shallow with their isolines being parallel to the bottom depth. The depth of mixed layer, pycnocline, nutricline, and deep chlorophyll maximum showed the similar topographic effect. In the deep water column of 4,308 m deep, integrated NO₃⁻ and PO₄^3– over 0–200 m were 879.60 and 81.78 mmol m⁻², but increased to 2010.17 and 143.79 mmol m⁻² in the shallow water column of 930 m deep, respectively. The increased supply of nutrients enhanced 0–200 m integrated chlorophyll from 21.71 mg m⁻² in the deep water column to 51.51 mg m⁻² in the shallow water column. These results demonstrate that topographic elevations such as seamounts induce deep-to-shallow shoaling and upwelling that lead to enhanced nutrients and biological production in the euphotic zone of oligotrophic oceans.

This study investigates the mechanisms driving the acceleration and meandering of the Agulhas Current (AC), focusing on the role of eddy-mean flow interactions. The analysis revealed that anticyclones originating from the Mozambique Channel and south of Madagascar played pivotal roles in accelerating the AC. Simultaneously, when anticyclones collide with the AC, they undergo processes of rotating and elongating into ellipses. In addition to the previously suggested barotropic instability induced by anticyclones, this study revealed that the merging of cyclones with the AC plays a role in the generation of meanders. Upstream cyclones reduce the horizontal potential vorticity gradient, facilitating eddies to traverse the current. The AC envelops these cyclones and flows in meandering patterns. The places where these meanders form are not exclusive to the Natal Bight. In addition, we further diagnose the kinetic energy conversion to reveal the interaction between eddy anisotropy (i.e., eddy deformation and orientation) and mean flow strain (i.e., stretching and shearing). The results suggest that the anisotropy of anticyclonic and cyclonic eddies prompts downscale KE transfer and the growth of meanders, establishing a positive feedback loop. Contrary to the findings of previous hypotheses, the acceleration of AC in turn leads to a decrease in the mean flow strain rate, exerting negative feedback on energy conversion and inhibiting the development of meanders. These two feedback mechanisms work together to determine the fate of AC meandering. The energetic anisotropy diagnosis holds potential applicability to other western boundary current systems.

Eleven Earth System Models (ESMs) contributing to the Coupled Model Intercomparison Project (CMIP6) were evaluated with respect to climate-related stressors impacting Arctic marine ecosystems (temperature, sea ice concentration, oxygen, ocean acidification). Stressors show regional differences and varying differences over time and space among models. Trends calculated over three consecutive 40-year time periods are highest for 2061–2100 for temperature and O₂. Differences between scenarios SSP2-4.5 and SSP5-8.5 vary among models and regions, mainly driven by sea-ice retreat and dilution effects. Differences in biogeochemical parameterizations contribute to acidification differences. Projections indicate consistent ocean acidification until 2040 and faster progression for the high-end emission scenario thereafter. For SSP5-8.5 all Arctic regions show aragonite undersaturation by 2080, and calcite undersaturation for all but two regions by 2100 for all models. Most regions can avoid calcite undersaturation in a medium emission scenario (SSP2-4.5). All variables show increases in seasonal amplitude, most prominently for temperature and oxygen. Calcium carbonate saturation state $��\left(�\Omega �\right)�$ shows little change to the seasonal range but temporal shifts in extrema. Seasonal changes in $��\Omega �$ may be underestimated due to lacking carbon cycle processes within sea ice in CMIP6 models. The analysis emphasizes regionally varying threats from multiple stressors on Arctic marine ecosystems and highlights the propagation of uncertainties from physical to biogeochemical variables. Large model differences in seasonal cycles emphasize the need for improved model constraints regarding the representation of sea-ice decline, river inflow and Atlantic and Pacific water circulation to enhance the applicability of CMIP models in multi-stressor impacts assessments.

The Atlantic Water plays a major and increasing role in the heat budget of the Arctic Ocean (Atlantification). The pathways of Atlantic Water within the Arctic Ocean, and in particular their sensitivities to large-scale atmospheric patterns such as the Arctic Oscillation, remain unclear. In this study, we used the trace gases CFC-12 and $��{\text{SF}}_6�$ to investigate the Atlantic Water pathways during different phases of the Arctic Oscillation. We calculated tracer ages for the temperature maximum of the Atlantic Water, focusing on repeated transects (1994, 2005, 2015) in the Amerasian Basin of the Arctic Ocean. During a positive phase of the Arctic Oscillation in 1994, tracer ages were low along the Chukchi shelf due to a strong coherent boundary current. In contrast, the ages were up to 10 years higher in 2015 without this coherent current during a mixed phase of the Arctic Oscillation. Further, we identified a discontinuity in the inflow between the Makarov Basin and the Canada Basin during this phase. Tracer ages were 10 years higher in the Canada Basin, suggesting a closed circulation without direct inflow in this region. Our tracer ages generally align with previously proposed circulation schemes and water ages, with major exceptions in 2015. We have shown that the tracer ages are applicable to identify decadal changes in the Atlantic Water core pathways in the central Arctic Ocean.