Journal of Geophysical Research-Oceans最新文献

As one kind of submesoscale instabilities, symmetric instability (SI) with spatiotemporal scales of O (100) m–O (1) km and O (1) hour exerts significant effects on vertical material transports and forward energy cascade in the ocean. The potential vorticity (PV) is an important conservative parameter controlling quasi-geostrophic flows, whose budget can be modulated by SI. However, due to the small spatial scale of SI which is hardly resolved by most current observations and regional models, how SI affects the PV budget and how big the effect is remain unclear. In this work, the effect of SI on the PV budget in the surface mixed layer (SML) of the Kuroshio Extension region is quantitatively analyzed based on high-resolution simulations by applying an existing SI parameterization scheme. Compared with the case without SI effects, negative PV is found to be eliminated in the SML in the SI-parameterized case. The negative-PV likelihood in the SI-parameterized case is decreased by up to 12% due to SI. Analysis of the PV budget indicates that SI contributes to the PV budget mainly by modulating the friction term. The friction term tends to generate negative PV but its magnitude is decreased by 35% due to SI. Apart from the frictional term, both advection and non-adiabatic terms are also found to be modulated by SI. This work sheds light on the contribution of SI in the PV budget in the ocean mixed layer and suggests a significant role of SI in quasi-geostrophic PV dynamics.

Deep convection occurs periodically in the Gulf of Lion, in the northwestern Mediterranean Sea, driven by the seasonal atmospheric change and Mistral winds. To determine the variability and drivers of both forcings, multiple 1 year ocean simulations were run, spanning from 1993 to 2013. Two sets of simulations were performed: a control and seasonal set, the first forced by unfiltered atmospheric forcing and the other by filtered forcing. The filtered forcing was bandpass filtered, retaining the seasonal and intraday aspects but removing the high frequency phenomena. Comparing the two sets allows for distinguishing the effects of the high frequency component of the Mistral on the ocean response. During the preconditioning phase, the seasonal forcing was found to be the main destratifying process, removing on average 46% of the stratification needed for deep convection to occur, versus the 28% removed by the Mistral. Despite this, each forcing triggered deep convection in roughly half of the deep-convection events. Sensible and latent heat fluxes were found to be the main drivers of the seasonal forcing during deep-convection years, removing 0.17 and 0.43 m²s⁻² of stratification, respectively. They were themselves driven by increased wind speeds, believed to be the low frequency signal of the Mistral, as more Mistral events occur during deep-convection winters (34% vs. 29% of the preconditioning period days). An evolving seasonal forcing in a changing climate may have significant effects on the future deep convection cycle of the western Mediterranean Sea.

Sediment dynamics are fundamental to understanding coastal resiliency to climate change in the coming decades. Tropical cyclones can radically alter shallow sediment properties; however, the uncertain and destructive nature of tropical cyclones make understanding and predicting their impacts on sediments challenging. Here, grain size sampling in conjunction with continuous hydrodynamic data provided an unprecedented perspective of the impacts of two tropical cyclones, including Hurricane Sally (2020), in which the inner core of the storm passed directly over the field sites, on shallow coastal sediments in Alabama (USA). Sampling directly before and after Sally as well as out to ∼7 months after the second storm event, Hurricane Zeta, showed that the changes in sediments following storm events exhibited notable site-to-site variability. This variability during the first storm event was consistent with low sand supply and flow interactions driven by local bathymetry that led to sand transport and deposition at some previously-muddy sites, near-surface mud loss at some sandy sites, or little change at others. Post-Sally impacts to grain size were well preserved 8 months after the storm, despite passage of Zeta as well as seasonal winds and riverine inputs during winter and spring. Overall, high temporal-resolution sampling over a relatively large area (<500 km²) revealed relatively small-scale spatial variability (on the order of 5–10 km) of hurricane impacts to sediment structure. These observations demonstrate a critical limitation for accurately predicting changes to coastal sediment dynamics in the face of a changing climate and its impact on tropical cyclones.

The Southern Ocean is a high-nutrient, low-chlorophyll (HNLC) region characterized by incomplete nitrate (NO₃⁻) consumption by phytoplankton in surface waters. During this incomplete consumption, phytoplankton preferentially assimilate the ¹⁴N- versus the ¹⁵N-bearing form of NO₃⁻, quantified as the NO₃⁻ assimilation isotope effect (¹⁵ε). Previous summertime estimates of the ¹⁵ε from HNLC regions range from 4 to 11‰. While culture work has shown that the ¹⁵ε varies among phytoplankton species, as well as with light and iron stress, we lack a systematic understanding of how and why the ¹⁵ε varies in the field. Here we estimate the ¹⁵ε from water-column profile and surface-water samples collected in the Indian sector of the Southern Ocean—the first leg of the Antarctic Circumnavigation Expedition (December 2016–January 2017) and the Crossroads transect (April 2016). Consistent with prior work in the mid-to-late summer Southern Ocean, we estimate a higher ¹⁵ε (8.9 ± 0.6‰) for the northern Subantarctic Zone and a lower ¹⁵ε (5.4 ± 0.9‰) at and south of the Subantarctic Front. We interpret our data in the context of coincident measurements of phytoplankton community composition and estimates of iron and light stress. Similar to prior work, we find a significant, negative relationship between the ¹⁵ε and the average mixed-layer photosynthetically active radiation flux of 30–100 μmol m⁻² s⁻¹, while above 100 μmol m⁻² s⁻¹, ¹⁵ε increases again. In addition, while we observe no robust relationship of the ¹⁵ε to iron availability or phytoplankton community, mixed-layer nitrification over the Kerguelen Plateau appears to strongly influence its magnitude.

The bottom boundary layer (BBL) dynamics play an important role in regulating the energy, momentum balance, and circulation in the shallow shelf areas. Unlike previous studies that disconnected BBL with background variable shelf circulation, we investigate the dynamic connection between the wind-driven shelf circulation and BBL dynamics, and show the spatial characteristics of BBL dynamics in response to three-dimensional (3D) heterogeneous transport over the highly variable shelf topography in the Northern South China Sea. Our process-oriented modeling study demonstrates that the mixing dynamics and upslope buoyancy transport over varying shelf topography alter the spatial variability of BBL dynamics. Driven by southwesterly upwelling-favorable winds, the along-shelf current generated a frictional upslope Ekman transport. The along-isobath pressure gradient force ($��P�G�F^{x^{\ast }}�$) formed by the flow-topography interactions over the meandering shelf induces the geostrophic cross-isobath transport. The downwave (upwave) $��P�G�F^{x^{\ast }}�$ enhances (offset) the frictional upslope transport over the east (west) of the shelf that has a concaving (uniform) bottom topography. Over the eastern shelf with concave isobaths, the intensified $��P�G�F^{x^{\ast }}�$ and upslope cross-isobath dense water transport strengthen stratification and weaken the effect of bottom stress-induced mixing, limiting the development of the BBL. The antithesis occurs over the western shelf, where a small bottom stress controls the BBL. River discharge and the tidal current modulate the alongshore current, upslope transport, bottom stress intensity, and BBL development. We model the trajectory of seabed

The subpolar North Atlantic plays an outsized role in the atmosphere-to-ocean carbon sink. The central Irminger Sea is home to well-documented deep winter convection and high phytoplankton production, which drive strong seasonal and interannual variability in regional carbon cycling. We use observational data from moored carbonate chemistry system sensors and annual turn-around cruise samples at the Ocean Observatories Initiative's Irminger Sea Array to construct a near-continuous time series of mixed layer total dissolved inorganic carbon (DIC), pCO₂, and total alkalinity from summer 2015 to summer 2022. We use these carbonate chemistry system time series to deconvolve the physical and biological drivers of surface ocean carbon cycling in this region on seasonal, annual, and interannual time scales. We find high annual net community production within the seasonally varying mixed layer, averaging 9.8 ± 1.6 mol m⁻² yr⁻¹ with high interannual variability (range of 6.0–13.9 mol m⁻² yr⁻¹). The highest daily net community production rates occur during the late winter and early spring, prior to the observed high chlorophyll concentrations associated with the spring phytoplankton bloom. As a result, the winter and early spring play a much larger role in biological carbon export from the mixed layer than traditionally thought.

Ice shelves in the Amundsen Sea have experienced the most rapid melting in the Antarctic and are expected to accelerate throughout this century under climate warming. In this study, a high-resolution ocean-sea ice-ice shelf model is employed to conduct sensitivity experiments to explore the effects of increasing melt rates of the Amundsen Sea ice shelves on hydrography, sea ice and ice shelf in the Ross Sea. The results indicate that the substantial inflow of meltwater significantly freshens the Ross Sea, inhibiting the formation and export of the Dense Shelf Water, the volume of which is reduced by 19%–33% on the shelf. Temperatures in the eastern and western Ross Sea exhibit different responses to the enhanced meltwater input. The freshening of the western Ross Sea weakens the mesoscale eddy activities which are efficient in carrying warm Circumpolar Deep Water onto the shelf, leading to an average temperature decrease of 0.02–0.08°C. Conversely, in the eastern Ross Sea, there is a marked warming outside the shelf under increased meltwater, which results from zonal pressure gradients associated with the meltwater distributions. This consequently causes stronger on-shelf heat transport and shows warming of 0.12–0.22°C on the eastern Ross Sea shelf. The narrow shelf in the eastern Ross Sea allows the warmer water to reach the Ross Ice Shelf (RIS) front, resulting in an about 6%–9% increase in the RIS melting. These results suggest a possible mechanism for acceleration in the RIS melting in the future that is associated with enhanced meltwater inflow.

This study represents the first use of the artificial radionuclides ¹²⁹I and ²³⁶U, released into the ocean mainly from Nuclear Reprocessing Plants, as a dual tracer in the vicinity of Iceland with novel estimation of ocean circulatory pathways and mixing in the region. Iceland lies at the gateway to the Arctic where warm, saline Atlantic waters interact with waters of Arctic origin in ways that have critical consequences for the strength and stability of the Atlantic Meridional Overturning Circulation. Many of these interactions are not yet fully understood, such as how Atlantic water circulates around the Arctic Ocean and Nordic Seas and the composition and fate of the major overflows of the Greenland-Scotland Ridge. Using new and previous measurements of ¹²⁹I and ²³⁶U in seawater, we present a new method of appraising water mass provenance and mixing in the form of the ¹²⁹I–²³⁶U dual mixing plot. With this method, we estimate that at least half the Atlantic-origin water entering the Arctic Ocean circulates around the Canada Basin before exiting at Fram Strait and that this outflow is increased by about 40% by mixing with Return Atlantic Water “short-circuiting” the Arctic Ocean at Fram Strait. We present tracer-based evidence that water carried by the East Greenland Current has an unbroken pathway to the Faroe-Shetland Channel and that Iceland-Scotland Overflow Water (ISOW) entrains 60% Labrador Sea Water during transit past southeast Iceland. We present an unambiguous way to differentiate ISOW from DSOW after they partially merge in the Irminger Sea.

The layered structure and seasonal variation of the water exchanges in the Sulawesi Sea areas, as well as the corresponding dynamics, are investigated using HYCOM data. A net inward water mass transport is identified in the upper and intermediate layers in the Sulawesi Sea and it is compensated by the net outward transport in the deep layer. As an important source of this net inward transport, the southward flow through the Sibutu Passage (denoted hereafter as Sibutu Flow) reaches 5.06 Sv in winter, accounting for ∼43% of the net inflow. However, it decreases substantially to 1.79 Sv in summer, accounting for only 12% of the inflow. Further analysis reveals that this Sibutu Flow modulates the seasonal variation of the corresponding outward flow at the Makassar Strait, the major exit of the water mass in the Sulawesi Sea. In winter, the enhancement of the Sibutu Flow associated with the greater intrusion of Kuroshio water into the Luzon Strait produces a stronger eastward pressure gradient force, suppressing the surface intrusion of the Mindanao Current. Conversely, this pressure gradient force weakens and shifts westward in summer, permitting a larger water intrusion from the eastern section. Given that the outward Makassar Flow is the main passage of the Indonesian Throughflow (ITF), our study highlights the essential role of the Sibutu Flow in modulating seasonal variabilities of the ITF via the Makassar Strait and, thus, the transfer of water masses and heat content between the Pacific and Indian Oceans.

Using the wide-swath sea surface height (SSH) data from the 1-day repeat calibration/validation phase of the Surface Water and Ocean Topography (SWOT) mission, we explored fine-scale evolution of nonlinear internal solitary waves (ISWs) in the Banda/Molucca Seas in the Indonesian Archipelagos. Generated in the Ombai Strait and Lifamatola Passage through semidiurnal tide-topography interaction, respectively, ISWs in the Banda/Molucca Seas have a SSH amplitude of 10–20 cm and exhibit multiple wave packets with the leading wave crest followed by a series of rank-ordered secondary wave crests. Due to nonlinearity, the ISWs are observed to propagate northward at a speed faster than the regional mode-1 internal gravity waves and their amplitudes are modulated fortnightly following the regional spring-neap tidal cycle. On longer timescale, the observed ISW amplitudes are controlled by seasonal upper ocean stratification changes with a weaker stratification favoring a larger amplitude. By converting the SWOT-measured SSH data to the interior ocean pressure signals, we quantified the depth-integrated energy flux associated with the northward-propagating ISWs to be 5 and 2 kW m⁻¹ in the central Banda and Molucca Seas, respectively. Comparisons with past studies indicate that the ISWs contribute to close to 100% and 40% of the tidally-induced, northward energy fluxes, respectively, across the Banda and Molucca Seas.