Journal of Geophysical Research-Oceans最新文献

South-west Australia has been identified as a global hotspot for the occurrence of meteotsunamis. In this study, a numerical hydrodynamic model (Regional Ocean Modelling System) was configured to investigate the generation of meteotsunamis through propagating thunderstorms. A range of simulations were performed using realistic and synthetic atmospheric forcing to establish the sensitivity of meteotsunami wave heights and waveforms along different parts of the coast to variations in the propagation speed and bandwidths of propagating pressure jumps associated with the thunderstorms. When a pressure jump propagated from the north and north-west quadrants with a speed (U) of 8–15 ms⁻¹, both the Proudman and Greenspan resonances were possible mechanisms for the generation of meteotsunamis. However, the response changed for different bandwidths of the propagating pressure jump and resulted in different meteotsunami waveforms at the coast. When U > 15 ms⁻¹, long waves were amplified initially through shoaling at the shelf slope, with Proudman resonance enhancing the wave heights at corresponding resonant depths on the shelf, and then propagated as free waves on the continental shelf. The waves were further amplified at the coast through refraction and shoaling effects and resulted in an elevation wave at the coast. Numerical simulations also indicated that edge waves can also be excited near the coast when the incoming free wave wavelengths were equal to or half the edge wave wavelength. The study provides observational and numerical evidence to suggest that the bandwidth of propagating air pressure jumps plays a major role in meteotsunami generation and their waveforms.

In January 2022, the strongest Arctic cyclone on record resulted in a record weekly loss in sea ice cover in the Barents-Kara-Laptev seas. While ECMWF operational forecasts skillfully predicted the cyclone, the loss in sea ice was poorly predicted. We explore the ocean's response to the cyclone using observations from an Argo float that was profiling in the region, and investigate model biases in simulating the observed sea ice loss in a fully coupled GCM. The observations showed changes over the whole ocean column in the Barents Sea after the passage of the storm, cooling and mixing with enough implied heat release to melt roughly 1 m of sea ice. We replicate the observed cyclone in the GCM by nudging the model's winds to observations above the boundary layer. In these simulations, the associated loss of sea ice is only about 10%–15% of the observed loss, and the ocean exhibits very small changes in response to the cyclone. With the use of a simple 1-D ice-ocean model, we find that the overly strong ocean stratification in the GCM may be a significant source of model bias in its simulated response to the cyclone. However, even initialized with observed stratification profiles, the 1-D model also underestimated mixing and sea ice melt relative to the observations.

The Antarctic marginal ice zone, the regularly wave-affected outer band of the sea ice covered Southern Ocean, typically contains an unconsolidated ice cover comprised of smaller, thinner floes than the inner ice pack. Thus, it is a highly dynamic region and susceptible to rapid expansion and contraction, making it a focal area for understanding and predicting the response of Antarctic sea ice to a changing climate. This novel study uses unsupervised statistical clustering of sea ice data simulated by a global sea ice model (standalone CICE6 combined with a wave propagation module and prescribed ocean) to address the outstanding challenge of separating the marginal ice zone from the inner ice pack in sea ice data sets. The method identifies a marginal ice zone with the desired characteristics and floe size is shown to be the key variable in the classification. Simulated marginal ice zone widths are similar to those derived from satellite observations of wave penetration distances, but contrast with those using the standard 15%–80% areal sea ice concentration proxy, particularly during austral winter. The simulated marginal ice zone is found to undergo a seasonal transition due to new ice formation in winter, increased drift in spring, and increased rates of wave-induced breakup and melting in summer. The understanding gained from the study motivates incorporation of wave and floe-scale processes in sea ice models, and the methods are available for application to outputs from high-resolution and coupled sea ice–ocean–wave models for more detailed studies of the marginal ice zone (in both hemispheres).

The Iceland Scotland Overflow Water (ISOW) plume supplies approximately a third of the production of North Atlantic Deep Water and is a key component of the meridional overturning circulation (MOC). The Overturning in the Subpolar North Atlantic Program (OSNAP) mooring array in the Iceland Basin has provided high-resolution observations of ISOW from 2014 to 2020. The ISOW plume forms a deep western boundary current along the eastern flank of Reykjanes Ridge, and its total transport varies by greater than a factor of two on intra-seasonal timescales. EOF analysis of moored current meter records reveal two dominant modes of velocity variance. The first mode explains roughly 20% of the variance and shows a bottom intensified structure concentrated in the rift valley that runs parallel to the ridge axis. The transport anomaly reconstructed from the first mode explains nearly 80% of the total ISOW plume transport variance. The second mode accounts for 15% of velocity variance, but only 5% of the transport variance. The geostrophically estimated transport (2.9 Sv) recovers only 70% of the total ISOW transport along the ridge flank estimated from the direct current meter observations (4.2 Sv), implying a significant ageostrophic component of ISOW mean transport and variability. Ageostrophic flow is strongly linked to the leading mode of velocity variability within the rift valley. The ISOW transport variability along the upper and middle part of the ridge is further shown to correlate with changes in the strength of deep MOC limb across the basin-wide OSNAP array.

Understanding of internal solitary wave (ISW) behavior has been limited due to sparse observations. We used high-resolution Himawari-8 satellite imagery and mooring observations to reveal the two-dimensional (x–y) propagation process of ISWs in the South China Sea as they westward propagate onto the Dongsha plateau and encounter Dongsha Atoll. The 2D depiction of wave speed distribution, derived from detected wave crest positions every 10 min, shows the wave speeds range from 3 m s⁻¹ to 1 m s⁻¹ and have a tight correspondence to the local water depth. The correlation coefficient between the wave speeds and the Dubreil–Jacotin–Long (DJL) solutions is around 0.7, with a root mean squared value of 0.26 m s⁻¹, and the representative available potential energy for this region is considered to be 130 MJ m⁻¹. However, diffusions of wave speed in the ISW's lateral direction, particularly around abrupt topography, contribute to occurrences of outliers. Pairs of incident and reflected waves are well recognized east of Dongsha Atoll. The incident wave packet is known to be classified into a-type and b-type waves. The reflected waves associated with the b-wave, identifiable as mode-1 depression ISWs, are traced back to their generation site at depths of 100–200 m. In contrast, the reflected waves of the a-wave remain elusive in shallower waters (<300 m), likely due to interference from their longer incident counterparts. The reflected wave, however, is slower and decelerates toward deeper water, deviating from the DJL prediction. These comprehensive observations can help refine models for improved accuracy.

River floods and human activities would impact land-to-sea sediment transport, which is essential for understanding the evolution of the global sediment cycle in the Anthropocene era. This study focused on investigating how river floods and a dike constructed in 2005 along a river channel influence sediment transport from the Ou River Estuary to the East China Sea. A validated three-dimensional sediment transport model based on the Finite Volume Community Ocean Model was utilized for this study. The presence of the dike obstructs alongshore currents and fish migrations, leading to negative effects on marine ecology. Therefore, selective dismantling and ecological restoration measures are deemed necessary. Three scenarios were considered in this study: no-dike, dike-constructed, and dike-partially-removed conditions, along with various types of river floods. The findings indicate that the monthly sediment flux to the sea decreased by 7.7% from 8.11 × 10⁶ to 7.49 × 10⁶ t following dike construction, while the proportion of cross-shore sediment flux to the total flux increased from 37% to 59%. The dike consistently has a greater impact than river floods in their interactions. However, partially removing the dike reduces its influence and restores sediment transport to pre-dike levels, the effectiveness of which is more pronounced with more frequent floods, larger volumes, and rising sea levels. This study provides valuable insights into the interplay between river floods and dikes on sediment transport, thereby enhancing our understanding of the repercussions of human interventions on sediment dynamics.

Understanding the drivers of iceberg calving from Antarctic ice shelves is important for future sea level rise projections. Ocean waves promote calving by imposing stresses and strains on the shelves. Previous modeling studies of ice shelf responses to ocean waves have focused on highly idealized geometries with uniform ice thickness and a flat seabed. This study leverages on a recently developed mathematical model that incorporates spatially varying geometries, combined with measured ice shelf thickness and seabed profiles, to conduct a statistical assessment of how 15 Antarctic ice shelves respond to ocean waves over a broad range of relevant wave periods, from swell to infragravity waves to very long period waves. The results show the most extreme responses at a given wave period are generated by features in the ice shelves and/or seabed geometries, depending on the wave regime. Relationships are determined between the median ice shelf response and the median shelf front thickness or the median water cavity depth. The findings provide further evidence of the role of ocean waves in large-scale calving events for certain ice shelves (particularly the Wilkins) and indicate a possible role of ocean waves in calving events for other shelves (Larsen C and Conger). Further, the relationships determined provide a method to assess the potential for increased calving as ice shelves evolve with climate change, and, hence, contribute to assessments of future sea level rise.

Bering Strait is the only ocean gateway connecting the Pacific and Arctic oceans. The ∼1 Sv northward flow of Pacific water through the strait to the Arctic Ocean has been increasing by ∼0.01 Sv/yr since 1990. Monthly dynamic ocean topography (DOT), wind, and sea-ice data at Bering Strait are analyzed in context with the long-term record of flow through the strait to investigate local drivers. Ocean transport is found to be proportional to the across-strait slope in DOT, suggesting some component of the flow is in geostrophic balance. Along-strait ocean surface stresses, which modulate the across-strait DOT slope via Ekman transport, are analyzed in the presence of a seasonally varying ice cover. It is shown that northward interior ocean flow under sea ice in winter results in southward surface stresses, and westward Ekman transport that slows the geostrophic component of the northward ocean flow. As the number of open water days local to Bering Strait increase each year, we find no trend in the annual mean surface stress, that is, the loss of sea ice is not leading to increased northward wind stress input that would enhance northward ocean flow. This analysis is consistent with the theory that changes in both the atmosphere and ocean non-local to Bering Strait are likely driving the increased transport from the Pacific into the Arctic via Bering Strait.

Antarctic sea ice extent has been persistently low since late 2016, possibly owing to changes in atmospheric and oceanic conditions. However, the relative contributions of the ocean, the atmosphere and the underlying mechanisms by which they have affected sea ice remain uncertain. To investigate possible causes for this sea-ice decrease, we establish a seasonal timeline of sea ice changes following 2016, using remote sensing observations. Anomalies in the timing of sea ice retreat and advance are examined along with their spatial and interannual relations with various indicators of seasonal sea ice and oceanic changes. They include anomalies in winter ice thickness, spring ice removal rate due to ice melt and transport, and summer sea surface temperature. We find that the ice season has shortened at an unprecedented rate and magnitude, due to earlier retreat and later advance. We attribute this shortening to a winter ice thinning, in line with ice-albedo feedback processes, with ice transport playing a smaller role. Reduced ice thickness has accelerated spring ice area removal as thinner sea ice requires less time to melt. The consequent earlier sea ice retreat has in turn increased ocean solar heat uptake in summer, ultimately delaying sea ice advance. We speculate that the observed winter sea ice thinning is consistent with previous evidence of subsurface warming of the Southern Ocean.

This study investigates the biogeochemical drivers of aragonite saturation state (Ω_Ar) in Baffin Bay, with a focus on the relatively undersampled west Greenland shelf. Our findings reveal two main depth-dependant processes controlling the spatial distribution of Ω_Ar in Baffin Bay; within the upper 200 m, lower Ω_Ar coincides with increasing fractions of Arctic-outflow waters, while below 200 m organic matter respiration decreases Ω_Ar. A temporal analysis comparing historical measurements from 1997 and 2004 with our 2019 data set reveals a significant decrease in the Ω_Ar of Arctic-outflow waters, coinciding with reduced total alkalinity (TA). However, no discernible anthropogenic ocean acidification signal is identified. Significant Arctic water fractions (20%–40%) are found to be present on the west Greenland shelf, associated with reduced TA and Ω_Ar. A numerical modeling simulation incorporating a passive tracer demonstrates that periodic changes in wind direction lead to a switch from onshore to offshore Ekman transport along the Baffin Island current, transporting Arctic waters toward the west Greenland shelf. This challenges the conventional understanding of Baffin Bay's circulation and underscores the need for further research on the region's physical oceanography. Based on salinity-TA relationships, surface waters on the west Greenland shelf have a significantly lower meteoric TA end-member compared to waters of the Baffin Island Current in western Baffin Bay. The low eastern TA freshwater end-member agrees well with recent glacial meltwater TA measurements, suggesting that glacial meltwater is the main freshwater source to surface waters on the west Greenland shelf.