Journal of Geophysical Research-Oceans最新文献

The transfer of energy from the upper to deep oceans is a well-known and challenging subject in physical oceanography. This research investigates the intricate relationship between surface and deep ocean currents. Utilizing nearly 2 years of observations from the Kuroshio Extension System Study (KESS), our study unveils the deep-ocean kinetic energy response to the Kuroshio Extension (KE) variability. We introduce the Kuroshio Extension Jet Path Index (KEJPI), which identifies two distinct modes of the KE jet on an intra-seasonal timescale. Our findings reveal a strong correlation (0.73) between KEJPI and the mean kinetic energy of deep-ocean geostrophic circulation, suggesting that the KE jet's large amplitude meanders have a significant impact on deep-ocean kinetic energy. Singular Value Decomposition (SVD) analysis further unveils a co-evolving spatial pattern between upper and lower ocean kinetic energy. We investigate the dynamic vertical coupling (DVC) mechanism by examining the coherent variation among the sea surface height (SSH), 15°C isotherm (Z15), deep pressure anomaly, and abyssal flow. The KE jet migration can induce net divergence and convergence within the water column, which in turn generates deep-ocean quasi-geostrophic currents. These currents show a marked increase in kinetic energy, reaching levels three times higher than the background. This DVC-driven kinetic energy can further cascade into near-inertial and high-frequency internal waves, contributing to abyssal mixing. Our study underscores the role of large current system instabilities in transferring energy to the deep ocean and facilitating deep mixing processes.

Human-induced dissolved organic nitrogen (DON) input is impacting coastal ecosystems globally, and the shift from diatoms to dinoflagellates may be associated with increasing DON concentrations and changing DON compositions. Based on field microcosm experiments, changes in three-dimensional fluorescence component and extracellular leucine aminopeptidase activity were used to reveal the effects of DON on diatom and dinoflagellate growth. Additionally, a Nutrients–bi-Phytoplankton–Detritus biogeochemical model was employed to elucidate the kinetic mechanism of DON on the shift from diatoms to dinoflagellates in the Bohai Sea (BS). Our results revealed that shifting diatom–dinoflagellate dominance was associated with the hydrophobic component of terrestrial DON. Dinoflagellates (i.e., Karenia mikimotoi) efficiently assimilated humic-like substances in hydrophobic DON; whereas, diatoms (i.e., Chaetoceros spp.) efficiently utilized the protein-like components in hydrophilic DON. The reason for this was the higher extracellular leucine aminopeptidase activity of dinoflagellates compared to that of diatoms, which enabled them to degrade humic-like substances and protein-like components more effectively. The modeling study clarified that the DON composition, particularly the proportion of hydrophobic DON, regulated the shift from diatoms to dinoflagellates. Our study provides insight into the mechanisms underlying phytoplankton regime shifts in the BS and valuable guidance for decreasing eutrophication by controlling terrestrial DON inputs and compositions.

The Irminger Current (IC) brings relatively warm and saline waters northward in the North Atlantic subpolar gyre, contributing to the upper limb of the Atlantic Meridional Overturning Circulation. The IC is a two-core current with surface-intensified velocities. The eastern core, closest to the Reykjanes Ridge, is warmer and more saline than the western core. To investigate the source waters of the two IC cores, using a 1/10° ocean model, we track Lagrangian particles released in the IC at OSNAP East (∼59.5°N) in the upper 1,000 m backward in time for one model year. Over a 1-year time scale, nearly all particles are sourced from nearby regions of the Irminger Sea and Iceland Basin. Those seeded in the western IC core mostly originate from the Irminger Sea (83%), while those in its eastern core mostly originate from the Iceland Basin (69%). Iceland Basin water feeding the IC predominantly crosses the Reykjanes Ridge near 57°N and 59°N. Generally, particles from the Irminger Sea are colder and fresher than particles from the Iceland Basin. The fraction of waters from the Iceland Basin and the Irminger Sea varies from month to month. So, to explain monthly variations of the two IC cores at the OSNAP East line, changes in hydrographic properties in both basins as well as their contributions must be considered. Based on this model study, we interpret the Irminger Sea circulation as a basin-wide recirculation with an increasing contribution of Iceland Basin waters toward the ridge which is subject to monthly variations.

The formation of dense Brine-enriched Shelf Water (BSW) in Storfjorden is analyzed during Winter 2016–2017 from mooring observations, a polynya model nudged to satellite observations, and an original BSW production model. The ice season was two months shorter than average, yet 44.2 $��{\text{km}}^3�$ of sea ice were formed, in line with estimates for the period preceding the atlantification of the Barents Sea in the mid-2000s: A thinner, more fragile ice may favor polynya openings and frazil ice production. A saline specimen of BSW was produced in large volumes, corresponding to an annual mean transport of 0.042 Sv, larger than previous estimates. The important production is due to the preconditioning of the polynya with a more saline source water, exceeding the pre-2005 values by 0.37. The BSW overflow was observed on the West Spitsbergen shelf slope from hydrographic sections down to 750 m, thus entering the Norwegian Sea Deep Water layer. Its core temperature was about $��1�°�$C warmer than the pre-2005 values owing to the entrainment of a warmer water in Storfjordrenna, suggesting that a part of the excess surface heat of the Barents Sea could be exported into the deep ocean. Overall our results suggest that dense water formation in the Storfjorden polynya may not, at least for now, be hampered by the atlantification of the Barents Sea, and perhaps even temporarily favored by the more saline source water. Anomalous atmospheric warming during the Winter-Spring may however disrupt the production, as was observed 1 year before.

In the marine cryosphere, seasonal sea ice dynamics affect the behavior of gaseous mercury, yet the mechanism remains poorly understood. By carrying out an outdoor sea ice mesocosm study, we examine primarily the abiotic factors influencing mercury dynamics and show distinct behaviors of gaseous mercury across the sea ice-seawater interface over the full growth-melt cycle. The distribution of gaseous mercury in sea ice is influenced by entrapment of gaseous mercury from different sources into sea ice of different textures, transport schemes within sea ice, and in situ cryo-processes that affect mercury speciation. In the growing sea ice sections where solar radiation penetrated, production of gaseous mercury was observed, supporting the occurrence of in-ice cryo-photoreduction of divalent mercury. In under-ice seawater, concentrations of dissolved gaseous mercury decreased gradually during ice growth and increased rapidly to pre-freezing levels as ice started to melt, suggesting that the atmosphere-sea ice-seawater exchange pathway of gaseous mercury could be re-established through melting first-year sea ice. Our results from this unique mesocosm study provide new insights on the dynamics of gaseous mercury in and around sea ice that are primarily driven by abiotic processes, assisting model parameterizations for mercury cycling in polar regions.

Shelf water is influenced by atmospheric forcing, river outflows, and the open ocean. Studying its variability is crucial for understanding anthropogenic impacts on coastal oceans and their transport to the open ocean. In the Middle Atlantic Bight (MAB), the interaction of the Gulf Stream with shelf/slope circulation leads to some of the complex exchanges between the shelf and open ocean along the U.S. East Coast. This study employs a Lagrangian particle tracking approach, grounded in a high-resolution, data-assimilative ocean reanalysis, to examine the export pathways of surface shelf water in the MAB. We analyzed over 700 daily images of simulated particle distributions using image clustering techniques. This revealed three distinct export patterns: abrupt entrainment to the Gulf Stream, gradual entrainment, and southern transport. Each pattern was observed roughly equally during the study period from January 2017 to December 2018. The observed export patterns are closely linked to the coastal circulation dynamics near Cape Hatteras. Understanding the timing and duration of these patterns is vital for assessing water quality and predicting the settlement of species that spawn in the region. Our study further underscores the influence of tropical cyclones, including Hurricanes Jose, Maria, and Chris, on these export patterns. These extreme weather events lead to significant shifts in coastal circulation near Cape Hatteras.

Understanding the impact of the ocean barrier layer (BL) on regional ocean dynamics requires the knowledge of BL variability and the factors influencing it. Herein, using SODA reanalysis data sets, we systematically investigate the seasonal variations and formation mechanisms of BLs in the North Indian Ocean (NIO). Our results show that BLs are mainly found in the southeastern Arabian Sea (SEAS), Bay of Bengal (BoB), and the Eastern Equatorial Indian Ocean (EEIO), with the thickest BLs occurring in SEAS and BoB in January, and in EEIO in November. Seasonal BL variability is primarily driven by isothermal layer (IL) modulation. Further budget analysis reveals that the factors affecting seasonality of BL variability through modulating IL differ across the three regions. In the SEAS, BL variability is primarily modulated by ocean vertical processes, notably oceanic planetary waves and Ekman pumping. In the BoB, vertical dynamics, including Ekman pumping and oceanic planetary waves, are the primary determinant of BL variability, while horizontal advection affects the spatial extent of BL. In the EEIO, equatorial upwelling and oceanic planetary waves significantly affect BL variability. These results contribute to a better understanding of regional ocean dynamics and may improve the accuracy of ocean state estimates and representation of BL processes using ocean model simulations.

Salinity is an essential variable for characterizing and understanding the state of the ocean and its role in the climate system. Gridded ocean salinity products, heavily reliant on Argo float measurements since the early 2000s, are widely used in oceanographic and climate research. However, a concerning issue of instrument drift leading to spurious salinity increases (“salty drift”) has been identified in a significant number of Argo floats since 2015. This study investigates the potential consequences of this “salty drift” issue on various gridded salinity products. We compare a suite of these products and evaluate their consistency, particularly from 2015 to 2019. Our analysis reveals two major issues with the gridded salinity products after 2015: a sudden increase in global mean salinity and elevated inconsistencies between gridded salinity products. In 2015–2019, the North Indian and North Atlantic Oceans emerged as regions displaying particularly large disagreements between gridded products compared to the prior period, 2010–2014. These findings highlight the substantial impact of the “salty drift” on the reliability of the gridded salinity products. They also underline the critical need for the oceanography community to address these issues to ensure the validity of future ocean and climate studies that utilize gridded salinity products.

Saltwater intrusion occurs frequently in the Yangtze Estuary during winter, when the river discharges are low along with strong wind and waves. However, the influence of wind and waves on saltwater intrusion in the Yangtze Estuary remains unclear. This study uses a coupled wind-wave-current numerical model based on Delft3D to investigate the impacts of wind and waves on saltwater intrusion in the Yangtze Estuary. The results show that the strong northerly wind alone enhances saltwater intrusion in the estuary by inducing a counterclockwise circulation and reducing the stratification. However, with the combined effect from wind and waves, it is found that stratification is reduced in the outer North Channel, but enhanced in the inner North Channel, which results in an increase of salt transport in the estuary by approximately 40%. The results highlight the fact that saltwater intrusion in the Yangtze Estuary could be significantly underestimated without considering waves.

The annual maximum (AM) method, which subsamples time series to retain the maximum event per year, and the peak-over-threshold (POT) method, which extracts values exceeding a threshold to define extremes, have long histories in determining flood event frequency. In practice, extreme value distributions applied to AM and POT events often assume that the data comes from the same statistical population. Locations across the world, like the United States (U.S.) Atlantic coastline, however, experience high coastal water levels driven by various individual processes and storms with different driving mechanisms during different seasons. This research investigates when extreme total water levels (TWLs) occur during the year along the U.S. Atlantic coast and whether individual components, like waves, tides, and storm surge, contributing to TWLs vary across regions and during the year. From 1980 to 2020, extreme TWLs occurred during the extratropical and tropical seasons, with the relative proportion of extreme TWLs occurring during the extratropical season increasing northward. Still water levels drive spatial variability in extreme TWL magnitude, while wave climate drives differences in extreme TWL magnitude between extratropical and tropical seasons. Month-to-month variability in the composition of extreme TWLs varies more than spatial variability, highlighting the importance of understanding the components driving extremes at different times of the year. Variations across storm seasons in the processes contributing to extreme TWLs may have implications for how large-scale changes to the climate impact hazards along open sandy coastlines and influence the robustness of extrapolating rare events from models fit to a single population.