Journal of Geophysical Research-Oceans最新文献

Enhancing typhoon forecasts hinges on a deeper understanding of the upper-ocean’s response and feedback mechanisms to typhoons. Presently, our knowledge of typhoon-ocean interactions is largely derived from low-resolution numerical simulations (often >10 km) and limited observations, which inadequately capture submesoscale processes (SPs) in the ocean. Connecting extreme typhoons to upper-ocean SPs remains a challenge. This study reveals the formation of a distinctive vortex filament pool (VFP) in high-resolution (∼1.2 km) numerical experiments. The experiments show that under specific conditions, typhoons can generate this visually striking phenomenon, displaying SP dynamics and kinematics typical of the upper ocean, with Rossby numbers and the nondimensional strain and divergence rates exceeding 2. The VFP formation is mainly driven by strain-induced frontogenesis linked to the flow generated by Typhoon Nangka after a major turn. Initially, the pool consists of many near-parallel filaments, but processes such as merging, stretching, and destabilization subsequently occur lead to numerous small vortices with a mean radius of ∼13 km. While the high-resolution numerical experiments highlight phenomena requiring observational validation, they suggest the presence of natural processes previously undetectable with low-resolution models and limited observations. This study underscores the need for enhanced observations and numerical models to better understand refined ocean dynamical processes.

The Antarctic Bottom Water (AABW) exported from the Weddell Sea has experienced warming and contraction in the past 30 yrs. Superposed on this decadal trend is substantial annual and interannual variability in the volume and properties of Weddell-sourced AABW. Several mechanisms have been suggested to explain these variations, many of which highlight a role of wind stress, but the comparative importance and possible simultaneity of the different mechanisms remains unclear. Using data from two mooring sites within the Weddell Sea, we find a rapid intensification of the abyssal boundary current carrying AABW through Orkney Passage (OP), the most direct export pathway of AABW from the Weddell Sea, in response to periods of strong zonal wind stress and anomalous wind stress curl along the South Scotia Ridge upstream of OP. This acceleration is concomitant with a 40% reduction in northward AABW transport in late 2015. The changes in transport follow anomalous wind forcing by approximately 3 months, with the short timescale indicative of a barotropic response in the flow through OP. The bottom boundary layer over the OP's sloping topography is found to have a key role in regulating export on monthly to interannual timescales. Increased boundary current velocity leading up to the passage forms a thickened bottom boundary layer, resulting in reduced AABW thickness and density, and thus restricting northward transport of AABW through the passage. Whilst other processes are likely to dominate on longer (decadal) periods, the dynamics identified here can explain significant variability on timescales up to interannual.

The settling velocity (w_s) in estuarine environments can impact whether a region is eroding or accreting sediment on the bed, yet determining this rate can be an indirect process requiring a number of assumptions. Accurate determination of w_s is especially needed for numerical models to reproduce observed sediment concentrations at the appropriate timescale. We collected information on suspended sediment flocculation at a channel site (13 m deep) and a shallows site (4 m deep) within South San Francisco Estuary, alongside timeseries of flow, wave statistics, turbulent shear, and bottle samples analyzed for both w_s and particle size. Using the measurements of floc size and settling velocity, we performed a sensitivity analysis on the unknown parameters in the general explicit formula for settling velocity. The collected particle size distribution data show that multiple classes of flocs are present; these are characterized as flocculi, microflocs, and macroflocs. We show that w_s of flocculi is closest to w_s for the full distribution. The determined parameter values lead to near-bed mass-weighted settling velocities (standard deviation) of 1.18 (0.55) and 0.22 (0.15) mm/s at the channel and shallows sites, respectively. Modeling efforts can use this work to help select an appropriate sediment model and parameter values.

Numerical predictions of nearshore waves and shoreline runup are usually initialized on the inner shelf, seaward of the surfzone, with sea-swell (SS) waves from local wave buoys or regional wave models. Lower frequency infragravity (IG) waves are not reliably measured by buoys or included in regional models. Here, co-located pressure and velocity observations are used to characterize IG waves in 10–15 m depth in southern California. Shoreward propagating IG waves are often dominated by free waves, with the boundwave energy fraction <30% for moderate and low energy incident SS waves. Only 5% of records, with energetic long swell, show primarily bound waves. The shoreline slope of concave beaches increases by ∼3 between spring high and low tides, and free seaward and shoreward IG energy in 10–15 m vary tidally. The observed linear dependency of free IG energy on SS energy and period is consistent with Ardhuin et al. (2014, https://doi.org/10.1016/j.ocemod.2014.02.006)'s parameterization (R² = 0.71). Including the tide level as a proxy for beach slope and modifying the SS frequency dependency increases R² to 0.91. The ratio of free seaward to shoreward propagating IG energy suggests between 50 and 100% of the energy radiated seaward in depths of 10–15 m is trapped offshore and redirected shoreward. Free (random phase) and bound (phase-coupled) IG waves are combined to initialize the SWASH numerical model. SWASH predicted runup is only weakly influenced by waves at the offshore boundary. Nonlinear IG generation and dissipation in the shoaling and surfzone overwhelm the effects of shoreward propagating waves observed at the offshore boundary.

The frequency of riverine floods is predicted to increase in East Asia. However, the response of coastal hypoxia (<63 μmol L⁻¹) to floods has not been well understood. In the summer of 2020, characterized by one of the most significant Changjiang water fluxes in three decades, we conducted a cruise during the flood period on the East China Sea inner shelf. Our observations revealed severe bottom hypoxia with a maximum spatial coverage of ∼11,600 km² and a minimum dissolved oxygen concentration (DO) of 21 μmol L⁻¹. In the surface layer, the relationships between salinity and nitrate, dissolved inorganic carbon (DIC) indicated significant organic matter production, validated by a high-Chlorophyll-a (Chl a) patch (>5 μg L⁻¹). Furthermore, the significant relationship between apparent oxygen utilization and DIC of deep waters reveals that the organic matter decomposition primarily drove the hypoxia during the flood period. Episodic wind events also influenced bottom DO and DIC, by transporting surface waters to the deep. Multiple-years data set shows that the average Changjiang nitrate flux during flood years is about 1.4 times that during non-flood years. The flood waters mix with estuarine waters, forming the high-nutrient plume waters, which expanded farther offshore during the flood period. While high turbidity remained confined to the inner estuary. Consequently, the high-Chl a area significantly expanded, which significantly exacerbated the hypoxia.

Fram Strait is one of the main gateways for fresh water leaving the Arctic Ocean toward the deep-water formation regions of the North Atlantic. Monitoring transport through Fram Strait is important to quantify the impact of Arctic amplification on the hydrography in lower latitudes. We update existing time series from the moorings in the western Fram Strait and investigate the monthly and interannual variability of the liquid freshwater transport (FWT, reference salinity 34.9), volume transport and freshwater content between 2003 and 2020. We examine composites and correlations of sea-level pressure (SLP) reanalysis, and remote-sensing dynamic-ocean topography (DOT) in the Arctic Ocean. We identify two remote forcing mechanisms of FWT: (a) North Pole convergence freshens the region north of Fram Strait 13–24 months before high FWT events. (b) Beaufort Gyre weakening allows spreading of fresh water to the margins of the Arctic Basin zero to 9 months before high FWT events. In addition a third mechanism occurs locally, (b) Fram Strait northerly winds confine freshwater to the Greenland shelf and drive stronger southward FWT. Additionally, we find a decreasing trend in the total volume transport, concurrent with weakening northerly winds and reducing north-south DOT gradient across the strait. We also examined correlations between the Fram Strait time series and the Arctic Oscillation and Arctic Ocean Oscillation. Both are found to correlate positively with the total volume transport, while the Arctic Oscillation correlates negatively with FWT with 1-year lag.

Eastern boundary upwelling systems (EBUSs) are among the most productive regions in the ocean because deep, nutrient-rich waters are brought up to the surface. Previous studies have identified winds, mesoscale eddies and offshore nutrient distributions as key influences on the net primary production in EBUSs. However uncertainties remain regarding their roles in setting cross-shore primary productivity and ecosystem diversity. Here, we use a quasi-two-dimensional (2D) model that combines ocean circulation with a spectrum of planktonic sizes to investigate the impact of winds, eddies, and offshore nutrient distributions in shaping EBUS ecosystems. A key finding is that variations in the strength of the wind stress and the nutrient concentration in the upwelled waters control the distribution and characteristics of the planktonic ecosystem. Specifically, a strengthening of the wind stress maximum, driving upwelling, increases the average planktonic size in the coastal upwelling zone, whereas the planktonic ecosystem is relatively insensitive to variations in the wind stress curl. Likewise, a deepening nutricline shifts the location of phytoplankton blooms shore-ward, shoals the deep chlorophyll maximum offshore, and supports larger phytoplankton across the entire domain. Additionally, increased eddy stirring of nutrients suppresses coastal primary productivity via “eddy quenching,” whereas increased eddy restratification has relatively little impact on the coastal nutrient supply. These findings identify the wind stress maximum, isopycnal eddy diffusion, and nutricline depth as particularly influential on the coastal ecosystem, suggesting that variations in these quantities could help explain the observed differences between EBUSs, and influence the responses of EBUS ecosystems to climate shifts.

For Williamstown tide gauge, at the northern-most point of Port Phillip Bay (PPB), Melbourne, Victoria, tide registers from 1872 to 1966 and marigrams from 1950 to 1966, were digitized to extend sea-level records back almost 100 years. Despite some vertical datum issues in the early part of the record, the data set is suitable for extreme sea-level trend analysis after removal of the annual mean sea level. The newly digitized data was combined with the digital record to produce a combined record from 1872 to 2020. Analysis of this record revealed known problems of siltation of the tide gauge stilling well and associated reduction in tidal range at times during 1880–1895 and 1910–1940. A positive trend in tidal amplitude of 0.41 ± 0.01 mm yr⁻¹ was found over 1966–2020, likely due to reduced hydraulic friction at the narrow entrance to PPB. Extreme sea-level trends were examined over 1872–2020 for storm tides (the combination of storm surge and tide) after removal of the annual mean, and residuals after subtraction of the predicted tides. A non-stationary Gumbel distribution with a time-varying location parameter revealed statistically significant declining trends in the residuals of −0.73 ± 0.02 mm yr⁻¹, consistent with the observed poleward movement of storm surge-producing mid-latitude weather systems. For storm tides a smaller declining trend of −0.40 ± 0.01 mm yr⁻¹ was found. These trends are approximately an order of magnitude smaller than the current positive rates of mean sea level rise, meaning that storm tide hazard will continue to increase in the future. This information is relevant for future adaptation planning.

A 25-year (1996–2020) hindcast from a coupled physical-biogeochemical model is evaluated with nutrients, phytoplankton and zooplankton field data and is analyzed to identify mechanisms controlling seasonal and interannual variability of the northern Gulf of Alaska (NGA) planktonic food web. Characterized by a mosaic of processes, the NGA is a biologically complex and productive marine ecosystem. Empirical Orthogonal Function (EOF) analysis combining abiotic and biotic variables averaged over the continental shelf reveals that light intensity is a main driver for nanophytoplankton variability during spring, and that nitrate availability is a main driver for diatoms during spring and for both phytoplankton during summer. Zooplankton variability is a combination of carry-over effects from the previous year and bottom-up controls from the current year, with copepods and euphausiids responding to diatoms and microzooplankton responding to nanophytoplankton. The results also demonstrate the effect of nitrate availability and phytoplankton community structure on changes in biomass and energy transfers across the planktonic food web over the entire growing season. In particular, the biomass of large copepods and euphausiids increases more significantly during years of higher relative diatom abundance, as opposed to years with higher nitrate availability. Large microzooplankton was identified as the planktonic group most sensitive to perturbations, presumably due to its central position in the food web. By quantifying the combined variability of several key planktonic functional groups over a 25-year period, this work lays the foundation for an improved understanding of the long-term impacts of climate change on the NGA shelf.

Storm surges events are investigated using the ECHAR method, which identifies and quantifies the different dynamical structures of a typical storm surge event. In the North Atlantic, analysis of 65 tide gauges revealed that storm surge events display two major and two minor structures, each of them corresponding to specific ocean dynamics. The two major structures refer to a slow-time Gaussian structure, lasting around 24 days, associated with the impact of the atmospheric pressure and a fast-time Laplace structure, lasting around 1.4 days, mainly wind-driven. The absence of the Gaussian structure along the North America coasts is explained by storms of smaller spatial extension, compared to Europe. Concerning the minor structures, a negative surge of around 6 cm just after the peak surge is observed over North America only. Such a sudden drop of the sea level is explained by the turning winds during the storm event, favored by the smaller spatial extension of storms. Finally, high frequency oscillations, with amplitude typically of 3 cm and up to 25 cm, are observed at some tide gauges. These oscillations refer to tide-surge interactions and they are often maximum at a specific phase of the tide and/or enhanced because of resonant basins.