Journal of Geophysical Research-Oceans最新文献

Sudden peaks in south-westward wind strength (storms) have been observed to drive pulses of enhanced southward currents on the continental shelf east of the Filchner Trough. However, the link between wind and southward flow is not persistent, and it is uncertain which conditions favor the wind-driven pulses that typically bring modified Warm Deep Water southward toward the front of the Filchner Ice Shelf. We run a set of experiments in an idealized numerical model setup and find that storms induce a net southward “warm” (Θ > −1.5°C) volume transport in the trough region throughout the year. This is mainly explained by enhanced barotropic circulation on the shelf. The greatest storm-driven increase in southward heat transport occurs during summer and fall, with an exceptionally large increase in November and December due to storm-enhanced circulation on the shelf and seasonally varying heat content availability south of the shelf break. Analysis of ERA5-data shows that the number of storm days (wind speed >10 m s⁻¹) per year in the region co-vary with SAM. The positive trend in SAM can hence be expected to further enhance the importance of storm-driven southward heat transport toward the Filchner Ice Shelf cavity, which may have consequences for the basal melting.

This study presents remotely generated internal tides that propagate into shallow regions and are modulated by background flows using numerical simulations and field observation data. Numerical results indicate that strongly enhanced semi-diurnal (∼M₂) internal tide energy is generated over a shallow ridge, the Izu-Ogasawara Ridge. The generated internal tides propagates toward the Kuroshio upstream and shoal toward shallow regions near Cape Shiono-Misaki. The intensified internal tide energy flux toward the cape is explained by two mechanisms: (a) intensified internal tide generation over the ridge due to the interaction between tides and the Kuroshio and (b) wave energy convergence along the Kuroshio due to wave refractions. A 13-year field data set obtained from the cape was used to investigate shoaling internal tides influenced by the Kuroshio. The observed results reveal a significant positive correlation between the kinetic energy of semi-diurnal internal tides and the background flows caused by the Kuroshio, which evidently supports the intensified internal tides attributed to the interaction between background flows and tides, as proposed by recent studies. The intensity of shoaling internal tides is also largely influenced by the path of the Kuroshio and seasonal effects. The magnitude of shoaling internal tides is clearly weaken as the Kuroshio meander occurs. Shoaling internal tides modulated by the Kuroshio can provide new insights into energy transport and mixing processes in coastal oceans.

Global mean sea level has been rising at a rate of 3.25 ± 0.4 mm yr⁻¹ over 1993–2018. Yet several regions are increasing at a much faster rate, such as the Beaufort Gyre in the Arctic Ocean at a rate of 9.3 ± 7.0 mm yr⁻¹ over 2003–2014. At interannual to decadal time scales, the Beaufort Gyre sea level is controlled by salinity changes due to sea ice melt and wind-driven lateral Ekman convergence–divergence of freshwater. This study uses recent Greenland discharge and river runoff estimates to isolate and quantify the sea level response to freshwater fluxes variability over the period 1980–2018. It relies on sensitivity experiments based on a global ocean model including sea-ice and icebergs. These sensitivity experiments only differ by the freshwater fluxes temporal variability of Greenland and global rivers which are either seasonal climatologies or fully time varying, revealing the individual and combined impact of these freshwater sources fluctuations. Fully varying Greenland discharge and river runoff produce an opposite impact on sea level trends over 2005–2018 in the Beaufort Gyre region, the former driving an increase, while the latter, a decrease. Their combined impact leads to a fairly weak sea level trend. The sea level response is primarily driven by salinity variations in the upper 300 m, which are mainly caused by salinity advection involving complex compensations between passive, active, and nonlinear advection. This study shows that including the temporal variability of freshwater fluxes in forced global ocean models results in a better representation of regional sea level change.

Coastal ocean temperatures can respond to different atmospheric and oceanic processes at local spatial scales or through remote teleconnections. This study focused on subsurface ocean temperatures (subT) at 10 m depth in Maxwell Bay, northern Antarctic Peninsula from February 2017 and January 2022. It investigated extreme warming events during austral summers and their interaction with atmospheric and oceanic conditions regionally and locally. The analysis identified active and extreme Marine Heat Waves (MHWs) in March 2017 and January-February 2020 associated with a significantly negative Southern Annular Mode index observed 3–4 months before the temperature increase. In March 2017, temperatures exceeded the climatological mean by over 1°C. This anomaly was linked to a strengthened Amundsen Sea Low and a blocking anticyclone moving between the Scotia Sea and the South-West Atlantic Ocean that deflected westerly winds and facilitated the anomalous transport of warmer northern air masses to the AP. In January-February 2020, the highest recorded subT was observed (2.97°C), although air-sea heat fluxes did not show a similar pattern. In February 2020, one of the most intense atmospheric heatwaves ever recorded in West Antarctica was observed. This heatwave corresponded with maximum subT and positive sea surface temperature anomalies extending throughout the western region of the Southern Ocean, related to an extremely negative Southern Annular Mode. This study provides valuable insights into the impact of strong MHWs, a phenomenon that has been less documented in Antarctic coastal regions.

Atmospheric fronts embedded in extratropical cyclones are high-impact weather phenomena, contributing significantly to mid-latitude winter precipitation. The three vital characteristics of the atmospheric fronts, high wind speeds, abrupt change in wind direction, and rapid translation, force the induced surface waves to be misaligned with winds exclusively behind the cold fronts. The effects of the misaligned waves under atmospheric cold fronts on air-sea fluxes remain undocumented. Using the multi-year in situ near-surface observations and direct covariance flux measurements from the Pioneer Array off the coast of New England, we find that the majority of the passing cold fronts generate misaligned waves behind the cold front. Once generated, the waves remain misaligned, on average, for about 8 hr. The parameterized effect of misaligned waves in a fully coupled model significantly increases the roughness length (185%), drag coefficient (19%), and air-sea momentum flux (11%). The increased surface drag reduces the wind speeds in the surface layer. The upward turbulent heat flux is weakly decreased by the misaligned waves because of the decrease in temperature and humidity scaling parameters being greater than the increase in friction velocity. The misaligned wave effect is not accurately represented in a commonly used wave-based bulk flux algorithm. Yet, considering this effect in the current formulation improves the overall accuracy of parameterized momentum flux estimates. The results imply that better representing a directional wind-wave coupling in the bulk formula of the numerical models may help improve the air-sea interaction simulations under the passing atmospheric fronts in the mid-latitudes.

The Labrador Shelf is integral to the North Atlantic Ocean's climate system, exerting a significant influence on both regional and global scales. This study examines continuous, high-temporal resolution hydrographic profiles collected by two Argo floats located on the Labrador Shelf. Our focus centers on the subsurface changes. Among observed seasonal variations, the most pronounced and consistent change is a marked temperature increase during autumn in the Cold Intermediate Layer, which is also confirmed in an eddy resolving global reanalysis data set. Contrary to previous studies that attributed this warming to the autumnal deepening of the mixed layer, our analysis indicates that the warming extends to underneath the deepest mixed layer. Thus, mixed layer development cannot account for the observed warming in the deeper layer. On the other hand, analysis of velocity fields from reanalysis data set reveals active onshore intrusions at several locations, with a section at 58°N emerging as the northernmost hotspot. Budget analysis further indicates that the dominant factor driving autumn warming at Section 58°N is cross-isobath advection that is associated with the intrusion of slope waters onto the shelf. Subsequently, the positive temperature anomaly at Section 58°N and other enhanced intrusion locations are transported downstream through along-isobath currents, resulting in lagged yet intensified warming at lower latitudes. Our findings underscore the essential role of cross-isobath intrusion, in combination with along-isobath movements in governing seasonal temperature variability in the deep layer of the Labrador Shelf. Incorporating this mechanism is crucial for accurately hindcasting and forecasting bottom environmental conditions in the region.

Submesoscale flows (0.1–10 km) are often associated with large vertical velocities, which can have a significant impact on the transport of surface tracers, such as carbon. However, global models do not adequately account for these small-scale effects, which still require a proper parameterization. In this study, we introduced a passive tracer into the surface mixed layer (ML) of a northern Atlantic Ocean simulation based on the primitive-equation model CROCO, with a horizontal resolution of Δx = 800 m, aiming to investigate the seasonal submesoscale effects on vertical transport. Using surface vorticity and strain rate criteria, we identified regions with submesoscale fronts and quantified the associated subduction, that is the export of tracer below the ML depth. The results suggest that the tracer vertical distribution and the contribution of frontal subduction can be estimated from surface strain and vorticity. Notably, we observed significant seasonal variations. In winter, the submesoscale fronts contribute up to 40% of the total vertical advective transport of tracer below the ML, while representing only 5% of the domain. Conversely, in summer, fronts account for less than 1% of the domain and do not contribute significantly to the transport below the ML. The findings of this study contribute to a better understanding of the seasonal water subduction due to fronts in the region.

The upper ocean salinity in the eastern subpolar North Atlantic undergoes decadal fluctuations. A large fresh anomaly event occurred during 2012–2016. Using the ECCOv4r4 state estimate, we diagnose and compare mechanisms of this low salinity event with those of the 1990s fresh anomaly event. To avoid issues related to the choice of reference salinity values in the freshwater budget, we perform a salt mass content budget analysis of the eastern subpolar North Atlantic. It shows that the recent low salt content anomaly occurs due to the circulation of anomalous salinity by mean currents entering the eastern subpolar basin from its western boundary via the North Atlantic Current. This is in contrast to the early 1990s, when the dominant mechanism governing the low salt content anomaly was the transport of the mean salinity field by anomalous currents.

The interactions between the internal tide and the mesoscale circulation are studied from the internal tide energy budget perspective. To that end, the modal energy budget of the internal tide is diagnosed using a high resolution numerical simulation covering the North Atlantic. Compared to the topographic contribution, the advection of the internal tide by the low-frequency flow component and the horizontal and vertical shear are found to be significant at global scale, while the buoyancy contribution is important locally. The advection of the internal tide by the low-frequency currents is responsible for a net energy transfer from the large scale to smaller scale internal tide, without exchanges with the low-frequency flow. On the opposite, the shear of the mesoscale circulation and the buoyancy field are responsible for exchanges between the internal tide and the low-frequency flow. The importance of the shear increases in the northernmost part of the domain, and a partial compensation between the buoyancy and the shear contributions is found in some areas of the North Atlantic, such as in the Gulf Stream region. In addition, the temporal variability of these energy transfers is investigated. In contrast to topographic scattering, for which the spring-neap cycle is the dominant frequency, the energy transfer terms driven by low-frequency motions in areas of strong mesoscale activity are also modulated by variations of the low-frequency current spatial distribution.

We investigate mesoscale circulations in an oligotrophic western boundary current, the East Australian Current, during a sustained offshore plankton bloom. Using a series of high resolution hydrographic sections taken a few days apart, supplemented with in situ samples of nutrients, satellite and long-term mooring measurements, we describe a dynamic situation by which the East Australian Current's velocity core and associated front, interacts with a mesoscale eddy, migrating zonally by approximately 100 km over the course of 10 days. This interaction between the boundary current and mesoscale eddy field occurred concurrently with a sustained offshore plankton bloom. Sub-mesoscale upwelling motions on the inshore flank of the boundary current core coincides with increased nutrient and plankton concentrations in the near surface. Calculations based on the quasi-geostrophic omega equation and finite size lyapunov exponents suggest that these vertical motion arise from the interaction of the mesoscale with the East Australian Current. Frontolysis (the destruction of horizontal buoyancy gradients) leads to a thermally indirect ageostrophic secondary circulation that has the potential to supply nutrients to the near surface ocean. Using satellite data, we investigate the mesoscale conditions associated with all offshore phytoplankton blooms identified by an automated method, finding similar mesoscale patterns to those observed during the field campaign. We conclude that the interaction between the East Australian Current and mesoscale eddies is a recurrent catalyst for the complex sub-mesoscale dynamics we observed, and is likely a fundamental processes in driving offshore biological productivity in the region.