Journal of Geophysical Research-Oceans最新文献

The Tokara Strait is a mixing hotspot due to the coexistence of complex bottom topographies and strong composite flow including both the Kuroshio and tidal currents. Although previous studies have revealed several mechanisms from the view of Kuroshio-Topography interaction, the role of tides in driving mixing is still not clear. Given that it is located at the M₂ critical latitude (29°N), parametric subharmonic instability (PSI) is expected as an important process responsible for the mixing. Here, we study PSI of the M₂ internal tides in the Tokara Strait based on a high-resolution model. Our model results indicate that intense near-inertial waves are generated via PSI, which exhibit a horizontally layered structure and have much larger vertical wavenumbers than the M₂ internal tides. Energy is transferred from the M₂ internal tides to the near-inertial waves around the generation sites, and most of the near-inertial energy is dissipated locally. The dissipation rates of near-inertial waves are comparable to those of the M₂ internal tides. Simulations with and without the Kuroshio Current revealed the suppression of PSI along the Kuroshio path, which could be attributed to two mechanisms. First, the Kuroshio Current modifies the local minimum internal wave frequency by its horizontal and vertical shear, making the condition for PSI not satisfied. Second, the Kuroshio Current advects the near-inertial waves downstream in the Okinawa Trough, which inhibits the accumulation of near-inertial energy there. However, in most of the areas outside the Kuroshio path, PSI majorly contributes to mixing in and around the Tokara Strait.

We study two sea surface temperature (SST) warming events and upper ocean stratification changes in the northern South China Sea in 2022 using data from an EM-APEX float and satellite observations. The diurnal warm layers (DWLs) and the increasing buoyancy frequency N² above the top of the thermocline can restrict the penetration depth of nighttime convection and wind-driven mixing, which prevents cooler water from mixing upward, allowing solar heating to increase the SST by more than 1°C in a few days. The stratification budget approach is used to reproduce observations below 40 m despite some uncertainties in estimating variables such as horizontal gradient. After the first SST warming event, the stratification changes in the subsurface layers constituted by an increase in N² above 70 m and a decrease below this depth can be attributed to the combined effects of turbulent diffusion and vertical advection rather than to horizontal advection or penetrative solar radiation. This ocean interior mixing is likely caused by the shear of near-inertial waves at ∼50 m, when the nighttime convection could not penetrate through the DWL's base around 20 m. The stratification budget approach fails to simulate the changes above 40 m after the second SST warming event partly due to the presence of a near-surface freshwater layer. Our observations offer insights into the effect of inertial wave-induced mixing in the ocean interior when near-surface stratified layers are present, which can lead to changes in upper ocean stratification and SST.

The bottom friction factor, $��f_e�$, estimates for computing wave bottom frictional dissipation in Gon et al. (2020, https://doi.org/10.1029/2019jc015963), titled Wave Dissipation by Bottom Friction on the Inner Shelf of a Rocky Shore, are too large by a factor of 2 due to an error in the formulations by Thornton and Guza (1983, https://doi.org/10.1029/jc088ic10p05925), titled Transformation of Wave Height Distribution. Corrected formulations reduce the prior overestimation of the bottom friction coefficient, $��f_e�$, which is more significant for rougher bottom surfaces, such as coral reefs and rocky shores, than for sandy beaches.

The Northeast U.S. continental shelf (NEUS) is a highly productive and economically important region that has undergone substantial changes in recent years. Warming exceeds the global average and several episodes of anomalously warm, sustained temperatures have had profound impacts on regional fisheries. A majority of recent research studies focused on the analysis of temperature; however, salinity can serve as a valuable tracer as well. With now more than a decade of remote-sensing sea surface salinity data, we shed new light onto salinity variability in the region with focus on the Mid-Atlantic Bight and assess its role for modulating stratification on the shelf using historical hydrographic data. Local river discharge drives decreasing salinities not only in spring and summer on the shelf but also in the Slope Sea. In spring, fresher water aids the build-up of stratification and a low salinity surface layer extends to the shelf break above the pycnocline by the beginning of summer. An observed salinification in the fall is linked to offshore forcing over the slope associated with the presence of Warm Core Rings. Coherent low-frequency salinity variability is found over the slope and shelf, highlighting that shelf conditions are significantly impacted by offshore variability. Conditions on the NEUS in 2015 were characterized by anomalously high salinities, associated with a northerly position of the Gulf Stream. A freshening between 2015 and 2021, is in agreement with increased river cumulative discharge as well as lower offshore salinities. Overall, salinity serves as a valuable additional tracer of these multi-variate processes.

Newly ventilated winter water (NVWW) is a cold, salty, nutrient-rich water mass that is critical for supporting the ecosystem of the western Arctic Ocean and for ventilating the halocline in the Canada Basin. While the formation of NVWW is well-documented on the Chukchi shelf, there remain fundamental questions regarding its formation on the western Beaufort shelf. In this study, we use hydrographic data from two late-fall cruises in 2018 and 2022 to investigate the roles of sea ice production and wind-driven upwelling in the formation of NVWW and the implications for the nutrient content of the water. For each of the shipboard transects, we apply proxies for the extent of the winter water formation and the strength of the associated upwelling, respectively. It is demonstrated that the NVWW attains higher levels of nitrate due to two factors: (a) more active formation of the water associated with enhanced sea ice production and (b) more extensive upwelling of water high in nutrients from the basin to the shelf following an easterly wind event. The latter process would be less common on the wide Chukchi shelf. These findings have significant implications for the regional primary production.

Phytoplankton communities in the open ocean are high-dimensional, sparse, and spatiotemporally heterogeneous. The advent of automated imaging systems has enabled high-resolution observation of these communities, but the amounts of data and their statistical properties make analysis with traditional approaches challenging. Spatiotemporal topic models offer an unsupervised and interpretable approach to dimensionality reduction of sparse, high-dimensional categorical data. Here we use topic modeling to analyze neural-network-classified phytoplankton imagery taken in and around a retentive eddy during the 2021 North Atlantic EXport Processes in the Ocean from Remote Sensing (EXPORTS) field campaign. We investigate the role physical-biological interactions play in altering plankton community composition within the eddy. Analysis of a water mass mixing framework suggests that storm-driven surface advection and stirring were major drivers of the progression of the eddy plankton community away from a diatom bloom over the course of the cruise.

The Western Antarctic Peninsula (WAP) has been experiencing rapid regional warming since at least the 1950s, however, the impacts of this warming at the local scale are variable and nuanced. Previous studies that have linked sea-ice variability to biogeochemical cycles and food web dynamics often combine local-scale biogeochemical data with coarse-resolution regional satellite sea-ice data, which may not adequately capture local sea-ice conditions. In this study, we analyzed local-scale in situ sea-ice observations collected as part of a 28-year record (1992–2020) from the Palmer Long-Term Ecological Research site at Anvers Island, mid-WAP, in conjunction with isotopically-derived sea-ice meltwater (SIM) fractions and satellite-derived sea-ice motion and concentration, to quantify the variability and long-term trends in local sea-ice behavior. In situ sea ice observations at Palmer Station displayed higher variability than satellite observations and showed no significant declines over this time, despite region-wide declines identified in prior studies. Higher spring SIM fractions were attributed to strong northward sea-ice motion throughout the winter. Applying these local-scale sea-ice insights to similarly scaled stratification and chlorophyll-a measurements, we found that a longer-lasting, more consistent sea-ice pack led to greater water column stratification following the spring sea-ice retreat. Greater sea-ice persistence and stronger stratification led to larger peaks in chlorophyll-a, though sea-ice metrics did not explain the positive temporal trends in either stratification strength or chlorophyll-a. Through this study, we identify how local sea-ice observations and meltwater data can enhance satellite data to build an understanding of the intricate connections between ice, water column dynamics, and phytoplankton.

To understand mechanisms of salt intrusion in estuaries, we develop a semi-analytical model, that explicitly accounts for salt transport by both exchange flow and tidal flow. This model, after calibration, successfully hindcasts hydrodynamics and salinity dynamics in three estuaries that have strongly different characteristics. We find, from analyzing the model results for these three estuaries, that salt transport processes by exchange flow and tidal flow interact through the subtidal stratification. Transport by exchange flow creates stratification, thereby generating a phase shift of tidal salinity with respect to the tidal flow, which is important for the magnitude of the tidal salt transport. Conversely, the strength of tidal currents determines the vertical mixing that breaks down stratification. A new analytical formulation is presented for the component of the salt transport driven by the depth-averaged tidal flow. This salt transport is larger than the component associated with the vertical shear of the tidal current. Finally, a method that yields analytical equations that quantify the importance of different contributions to the salt transport using only primary information is developed using approximate solutions for the subtidal stratification. This method performs well for the estuaries considered.

We combined long-term observational data from 1999 to 2021 with numerical simulations to study the seasonal changes in the currents along the continental shelf in the Northern South China Sea (NSCS) during winter and how these changes relate to the El Niño-Southern Oscillation (ENSO). Our results indicate that during El Niño events, the eastward movement of the warm pool in the tropical Pacific Ocean not only leads to colder sea surface temperatures and a stronger Kuroshio current intrusion but also significantly affects the formation of a high-pressure system in the subtropics over the NSCS. In El Niño years, an anomalous anticyclonic wind stress curl in the South China Sea weakens the northeast winter monsoon, which in turn weakens the cyclonic shelf circulation. The first principal component from the multivariate empirical orthogonal function decomposition, which accounts for 49.02% of the total variance, shows a significant correlation of 0.64 with the Niño 3.4 index, indicating the circulation's sensitivity to tropical climate changes. Our analysis of the winter shelf circulation, based on the along-isobath depth-integrated vorticity equation, reveals that the exchange of water across the isobaths over the shelf is mainly controlled by the nonlinear advection of relative vorticity, with wind stress curl and bottom stress curl playing a less significant role in regulating the structure of these exchanges. The combined effect of baroclinic forces and topography likely governs the dynamics over the slope.