Journal of Geophysical Research-Oceans最新文献

Transformation of light to dense waters by atmospheric cooling is key to the Atlantic Meridional Overturning Circulation in the Subpolar Gyre. Convection in the center of the Irminger Gyre contributes to the formation of the densest waters east of Greenland. We present a 19-year (2002–2020) weekly time series of hydrography and convection in the central Irminger Sea based on (bi-)daily mooring profiles supplemented with Argo profiles. A 70-year annual time series of shipboard hydrography shows that this mooring period is representative of longer-term variability. The depth of convection varies strongly from winter to winter (288–1,500 dbar), with a mean March mixed layer depth (MLD) of 470 dbar and a mean maximum density reached of 27.70 ± 0.05 kg m⁻³. The densification of the water column by local convection directly impacts the sea surface height in the center of the Irminger Gyre and thus large-scale circulation patterns. Both the observations and a Price-Weller-Pinkel mixed layer model analysis show that the main cause of interannual variability in MLD is the strength of the winter atmospheric surface forcing. Its role is three times as important as that of the strength of the maximum stratification in the preceding summer. Strong stratification as a result of a fresh surface anomaly similar to the one observed in 2010 can weaken convection by approximately 170 m on average, but changes in surface forcing will need to be taken into account as well when considering the evolution of Irminger Sea convection under climate change.

Based on the novel energetic analysis tools, namely multiscale window transform and multiscale energy and vorticity analysis, this study investigates the seasonal variability of eddy kinetic energy (EKE) in the greater Agulhas Current system (GACS), which is divided into three subdomains based on the horizontal structures of background flows: the Mozambique Channel (MZC) subdomain, the northern Agulhas Current (NAC) subdomain, and the Agulhas retroflection (ARF) subdomain. Results show that the seasonality of EKE is spatially inhomogeneous. In the MZC subdomain, the EKE is strongest in spring and weakest in autumn, whereas in the NAC and ARF subdomains, the EKE level is highest in summer and lowest in winter. In all the three subdomains, the seasonal cycle of the barotropic instability of the mean flow corresponds well with that of the EKE. In contrast, the nonlocal transportation that mainly works on the redistribution of EKE is out of phase with the seasonality of EKE. Regarding the local wind forcing and baroclinic instability, they both have weak impacts on the EKE evolution, contributing power only about 10% of the barotropic instability. Moreover, neither of their seasonal cycles is consistent with the seasonality of EKE. Therefore, it is the barotropic instability of the mean flow that controls the seasonal variability of the EKE in the GACS.

The distribution of helium isotopes in the upper kilometer of the water column along the GP15 section in the central Pacific reflects the large-scale patterns of upwelling hydrothermal ³He in the tropics and sub-polar gyre, tracing two important pathways whereby bottom water exits from the deep Pacific. Heavy noble gas saturation anomalies, particularly in the upper two hundred meters of the water column, are more strongly increased by seasonal radiative heating, while lighter noble gas saturation anomalies are increased more by air injection processes. A similar, seasonally persistent radiative heating feature was observed in the Equatorial Undercurrent, and appears to be replicated in climate system model simulations. The origin of this feature, however, remains a mystery. A heuristic component model explains the noble gas saturation anomaly distributions, separating the influences of air injection, barometric pressure and radiative heating/cooling. Results show cohesive spatial patterns consistent with where water masses originate, their circulation, and gas exchange dynamics in relation to their formation regions. Using this model, we diagnose the distribution of “non-atmospheric” ⁴He in shallow waters, which parallels the helium isotope anomaly and silica distributions.

The Kuroshio intrusion from the Luzon Strait significantly affects ecosystems in the South China Sea (SCS), especially during the Northeast Monsoon, a time when field observations are notably sparse and where vertical mixing induced by strong winds can obscure the effects of the Kuroshio intrusion. In this study, we address these gaps by reanalyzing data from 20 cruises (5,067 samples) in the SCS between 2004 and 2015. We also carried out two dedicated field cruises during the Northeast and the Southwest Monsoon in 2018. Field observations from both cruises revealed a consistent unimodal relationship between total chlorophyll a (Chla) concentrations in the upper 50 m of the water column and the index of the Kuroshio intrusion. Specifically, a strong Kuroshio intrusion during the Northeast Monsoon significantly enhanced Chla concentrations in the northern SCS. This enhanced Chla concentration during the Northeast Monsoon was primarily driven by increases of Synechococcus and nanophytoplankton that contrasted with the dominance of Prochlorococcus during the Southeast Monsoon. Long-term remote sensing data corroborated these findings and demonstrated a consistent pattern wherein intrusion by the Kuroshio led to elevated Chla concentrations, particularly during the Northeast Monsoon. There was a significant positive correlation between the intensity of the Kuroshio intrusion and the magnitude of the Chla increase. Furthermore, these findings suggested a concerning possibility: weakening of the Kuroshio intrusion intensity over time might diminish future biogeochemical effects on SCS ecosystems. Continued monitoring and research will be crucial to understanding and responding to these changes.

Recent work has revealed the presence of an offshore near-surface plume of dissolved trace elements in the South Atlantic Ocean (SAO). Dissolved Fe (dFe) supply from the Congo plume is equivalent to ∼40% of the annual atmospheric dFe supply to the SAO. However this plume is not captured by biogeochemical models, raising questions about its exact sources. To help understand the potential source mechanisms, we use particle tracking experiments to investigate elemental distributions. Results suggest that elevated concentrations of some elements in the Congo plume are primarily sourced from river discharge and wet atmospheric deposition with minimal influence from shelf sediments. River discharge is the main source in shelf regions and some off-shelf regions, whereas atmospheric deposition dominates the area to the southwest of the Congo River outflow. A quantitative analysis along 3$��°�$S specifically for dFe suggests a decrease in the contribution of river discharge from 90% to 30% moving off-shelf, with a corresponding increase in the contribution of atmospheric deposition. Within the shelf zone, atmospheric deposition accounts for roughly 20%–40% and could be a major source of dFe around the river mouth. Integration of data from cruise GA08 reinforces the finding that wet deposition augments the concentrations of dFe, manganese (dMn), and cobalt (dCo) at distances over 1,000 km from the river mouth. Given present-day patterns of nitrate, Fe, and Co limitation for primary producers in the SAO, changing rainfall patterns may have long-term implications for both regional elemental budgets and ecologically dependent processes sensitive to trace element ratios.

The northeastern South China sea (NESCS) is characterized by highly dynamic currents regulated by monsoonal variability. To understand the responses of copepods to local currents, two cruises were conducted in the NESCS during the 2015/16 El Niño event in summer and winter. In the shelf waters off the NESCS, coastal upwelling (CU) occurs in summer and the southward China Coastal Current (CCC) happens in winter. The intrusion of Kuroshio Current (KC), which extends from the basin to the slope-shelf, was observed with in situ observations and remote sensing in both summer and winter. The individual-size-based community structure of copepods varied with seasons, areas, and depths, and was characterized by an increase in large-sized taxa from the shelf to basin, and from upper to deep water. This size-based distribution pattern is more pronounced during winter. The species-based copepod community structure was altered by increased KC intrusion, which was associated with high species diversity, and relatively low abundance and carbon biomass, and greater network stability of copepod community in winter than in summer through the network invulnerability test. The CU in summer and the CCC in winter shaped the copepod community structure different from the KC, shown by a low species number, and a high abundance and biomass of copepods. Temora turbinata, Calanus sinicus and Lucicutia flavicornis were effective indicators for the occurrence of the CU, CCC, and KC, respectively, in the NESCS. This study highlights the importance of quantifying the responses of copepods to the ocean currents at a species level.

Biogenic volatile organic compounds (BVOCs) such as dimethyl sulfide (DMS), methanethiol (MeSH), isoprene, and monoterpenes partly control the Earth's radiation budget. As such, measuring coral-reef BVOC emissions are important for understanding the impact of future climate and shading interventions that are being developed as part of the Reef Restoration and Adaptation Program to protect the Australian Great Barrier Reef (GBR). Here we report high temporal resolution BVOC concentrations and fluxes from the central GBR in the austral summer of 2021–2022, including the first flux estimates of MeSH, isoprene, and monoterpenes from reef systems, using a proton-transfer-reaction quadrupole mass spectrometer. Median fluxes of DMS, MeSH, isoprene, and monoterpenes across the two voyages were 4.26 ± 2.53, 0.94 ± 0.64, 0.05 ± 0.03, and 0.03 ± 0.03 μmol m⁻² d⁻¹, respectively, indicating that the central GBR is a net source of climatically relevant BVOCs in summer. Dissolved concentrations of BVOCs were positively correlated with sea surface temperature and proximity to the coast, which was most likely due to a coastward gradient for chl-a and nutrient concentrations. Predictably, wind speed and dissolved BVOC concentrations were the main drivers of BVOC fluxes. This study adds to our understanding of ocean-atmosphere exchange processes in coral reef systems and will help improve model predictions of the local climate of the GBR under future climate change scenarios.

The airflow separation from the water surface strongly impacts the coupling between wind and waves. To visualize the airflow and quantify the potential airflow separation's effect on the momentum transfer from wind to the wave, we conducted colocated sampling of air pressure, airflow, and water elevation under a range of wind and wave conditions in the laboratory. The experiments were conducted in the SUrge-STructure-Atmosphere INteraction (SUSTAIN) facility at the University of Miami. Three background wave conditions were subjected to wind forcing ($��U_{10}�\sim �$ 5–16 m $��s^{�-�1�}�$) to investigate the effects of wave amplitude, frequency, and wind forcing on the airflow regime and momentum transfer. We found the wind forcing shifts the phase of the wave-induced pressure fluctuations, enhancing the momentum transfer. An increase in wave amplitude or frequency also enhances the aerodynamic sheltering and the momentum transfer. The airflow-derived pressure based on the nonseparated sheltering (NSS) hypothesis accounts for more than 90% of the momentum transfer until potential airflow separation. We found that the potential airflow separation over the waves' leeward face, which is not accounted for by the NSS hypothesis, leads to an underestimate in momentum transport of more than 30% than the actual value of momentum transport obtained from pressure measurements. Our wave growth rate is consistent with the revisited Miles theory that incorporates the wave-induced Reynolds stress. The results of this work help explain the wave growth mechanism and inform the development of wind input functions for numerical wave models that account for airflow separation.

Mesoscale anticyclonic eddies (ACEs) act as an important physical disturbance for marine biogeochemical cycle, but our knowledge of the dynamics of critical new nitrogen (N) sources in such environments remains ambiguous. Here, we report concurrent data on two major sources of new N, that is, the N₂ fixation rate (via ¹⁵N₂ bubble release method) and vertical diffusive nitrate flux (F_diff-$��{\text{NO}}_3^{-}�$), for the euphotic zone (EZ) in the northern South China Sea ACEs during summer 2020. Depth-integrated N₂ fixation rates (INF) were moderately elevated (∼30%) in the center of the ACEs compared with those in the outside stations, suggesting that ACEs generally provide a more favorable environment for N₂ fixation. In contrast, the upward F_diff-$��{\text{NO}}_3^{-}�$ into the EZ were greatly lowered by an order of magnitude in the ACE center (center: 26.0 ± 8.2 μmol N m⁻² d⁻¹; outside: 124.0 ± 127.6 μmol N m⁻² d⁻¹), thus making N₂ fixation a much more significant contributor to new production under ACE influence. Such contribution is further demonstrated in the nutrient depleted layer where substantial carbon export may be taking place. Interestingly, a significant positive correlation for the ratio of the INF to the upward F_diff-$��{\text{NO}}_3^{-}�$ versus sea level anomaly was observed. ACE will likely leave an imprint on the isotopic composition of exported N, implying that there is possibly a need to take mesoscale forcings into account when interpreting isotopic signals from sinking particles. These findings will improve our understanding of N₂ fixation dynamics in response to mesoscale ACEs in tropical/subtropical oceanic regions, also helping better constrain biogeochemical models.

The hydrodynamics of river mouths are the result of a complex interaction between river flow, tidal conditions, and outlet geometry. This complex interaction of factors shapes the jet that flows onto the continental shelf and influences the dynamics of these areas. This work analyses the influence of idealized but realistic outlet geometries under steady and unsteady hydrodynamic conditions on the jet structure. The analysis is carried out using a numerical model which is validated by comparison with the classical jet theory for highly simplified conditions where this theory applies. The results show that both the outlet geometry and the transient hydrodynamic conditions have a significant influence on the jet structure and evolution along the nearshore. For constant river discharge and water level conditions, the results indicate that the nearshore profile plays a key role in determining the expansion or contraction of the jet. The momentum balance shows that the jet behavior is related to the momentum transport and the barotropic terms. In cases where the river discharge and tidal conditions are transient, the jet alternates between a structure with two velocity maxima at the edges or a single peak in the center during the tidal cycle depending on the phase lag between the tidal conditions and the river hydrograph. These two different jet structures play an important role in the morphodynamic evolution of the river mouths and bar development, favoring in some cases the formation of lateral levees parallel to the channel walls.