Frontal Circulation Research Articles

Rapid Downwelling of Tracer Particles Across the Boundary Layer and Into the Pycnocline at Submesoscale Ocean Fronts

AbstractA neutrally buoyant float deployed in an atmospherically driven turbulent ocean boundary layer on the dense side of a submesoscale front was repeatedly carried across the boundary layer by the turbulence and then trapped beneath the slumping front. Lagrangian particles in a large‐eddy simulation of a similar baroclinically unstable front forced by surface cooling move along convergent surface filaments toward filament junctions. They are also caught by convective plumes that downwell them at speeds similar to those of the float. Subsequently, some are trapped in the pycnocline by frontal slumping due to ageostrophic secondary frontal circulations. In both observations and simulations, boundary layer turbulence and frontal circulations work together to trap and subduct particles from the mixed layer. The small‐scale boundary layer motions move them vertically within the boundary layer and larger, submesoscale frontal circulations move them laterally out of the boundary layer and under the slumping fronts.

Kinetic Energy Exchanges between a Two-Dimensional Front and Internal Waves

Abstract Fronts and near-inertial waves (NIWs) are energetic motions in the upper ocean that have been shown to interact and provide a route for kinetic energy (KE) dissipation of balanced oceanic flows. In this paper, we study these KE exchanges using an idealized model consisting of a two-dimensional geostrophically balanced front undergoing strain-induced semigeostrophic frontogenesis and internal wave (IW) vertical modes. The front–IW KE exchanges are quantified separately during two frontogenetic stages: an exponential sharpening stage that is characterized by a low Rossby number and is driven by the imposed strain (i.e., mesoscale frontogenesis), followed by a superexponential sharpening stage that is characterized by an Rossby number and is driven by the convergence of the secondary circulation (i.e., submesoscale frontogenesis). It is demonstrated that high-frequency IWs quickly escape the frontal zone and are very efficient at extracting KE from the imposed geostrophic strain field through the deformation shear production (DSP). Part of the extracted KE is then converted to wave potential energy. On the contrary, NIWs remain locked to the frontal zone and readily exchange energy with the ageostrophic frontal circulation. During the exponential stage, NIWs extract KE from the geostrophic strain through DSP and transfer it to the frontal secondary circulation via the ageostrophic shear production (AGSP) mechanism. During the superexponential stage, a newly identified mechanism, convergence production (CP), plays an important role in the NIW KE budget. The CP transfers KE from the convergent ageostrophic secondary circulation to the NIWs and largely cancels out the KE loss due to the AGSP. This CP may explain previous findings of KE transfer enhancement from balanced motions to IWs in frontal regions of realistic ocean models. We provide analytical estimates for the aforementioned energy exchange mechanisms that match well the numerical results. This highlights that the strength of the exchanges strongly depends on the frontal Rossby and Richardson numbers. Significance Statement Fronts with large horizontal density and velocity gradients are ubiquitous in the upper ocean. They are generated by a process known as frontogenesis, which is often initialized by straining motions of mesoscale balanced circulations. Here we examine the energy exchanges between fronts and internal waves in an idealized configuration, aiming to elucidate the mechanisms that can drain energy from oceanic balanced circulations. We identify a new mechanism for energy transfers from the frontal circulation to near-inertial internal waves called convergence production. This mechanism is especially effective during the later stages of frontogenesis when the convergent ageostrophic secondary circulation that develops is strong.

Frontal lobe circulation in patients of schizophrenia with and without cannabis dependence: A comparative study

Abstract Background: The symptom profile of schizophrenia is related to the regional cerebral circulations as advocated in various studies. Cannabis dependence in schizophrenia is more common than in the general population. Previous studies comparing the frontal lobe function in schizophrenia with and without comorbid cannabis dependence have shown conflicting results. In this study, we compared the blood flow in the frontal lobes of patients of schizophrenia with and without comorbid cannabis dependence. Aim of the Study: The aim was to compare the blood flow in the anterior cerebral artery (ACA) and middle cerebral artery (MCA), in patients of schizophrenia with and without cannabis dependence. Materials and Methodology: This was a hospital-based cross-sectional study. After fulfilling the inclusion and exclusion criteria 28 patients of schizophrenia with cannabis dependence and 28 patients of schizophrenia without cannabis dependence were selected for the study. The blood flow was assessed using Trans-cranial Doppler (TCD) in the ACA and MCA of the left and right sides in both the study groups through the temporal window. Results: A statistically significant difference (P = 0.049) was found in the pulsatility index of the left ACA where it was lower in schizophrenia patients with cannabis dependence (0.97 ± 0.24) in comparison to schizophrenia patients without co-morbid cannabis dependence (1.11 ± 0.28). However, after using Bonferroni correction for multiple comparisons (P < 0.05/12 i.e., P < 0.004), this comparative statistics too lost its significance. No significant difference was found in the mean flow velocity and resistance index of frontal lobe circulation between the two groups. Conclusion: Chronic cannabis use is having some sparing effect on vascular resistance in the frontal circulation of schizophrenia patients. However, this effect is not significant enough to alter the pattern of frontal circulation.

Exploration of the downward transport mechanisms of biomass burning emissions from Indochina at the low boundary layer in East Asia

Springtime is the biomass burning (BB) season in Indochina. BB emissions are transported to East Asia and Taiwan at elevations of between 2000 and 4000 m and then transported downward to the near-surface layer when weather conditions are favorable. In this study, a numerical model, the Weather Research Forecast with Chemistry (WRF–Chem) model with tracer simulation was employed to explore the downward transport mechanisms and evaluate their potential effects on the planetary boundary layer (PBL) and near-surface layer during the springtime (Feb.–April) in 2018. Ambient backscatter was measured through LiDAR, aerosol optical depth was measured by MODIS and Himawari-8 satellites and by ground observations from the stations of AERONET. The data were applied to identify the transport mechanisms and evaluate the performance of the model. We summarized the downward transport mechanisms surrounding Taiwan, including the thermal effects of mountain downslope circulations (MDC), frontal secondary circulation (FSC) downward transport, and the mixing of MDC and FSC (MMF). Over Taiwan, the contributions of BB emissions for the MDC to the near-surface layer (<500 m), PBL (<1500 m), and free atmosphere (1500–6000 m) were 0.4%, 4.7%, and >95%, respectively, during springtime in 2018. For the FSC (MMF) mechanisms, the BB contributions to the near-surface layer, PBL, and free atmosphere were 2.5% (3.3%), 15.7% (17.1%), and 84.3% (82.9%), respectively. MMF occurred the most frequently among these three mechanisms and had the largest effect on the near-surface layer and PBL. The downward contributions of BB in different regions, including Taiwan, southern China, and the East China Sea (ECS), were also evaluated. Southern China had the highest value (11.0% [2.1%]) in the PBL (near-surface layer), followed by Taiwan and the ECS (8.2% [1.2%] and 4.5% [0.4%]). Overall, this research fills a gap in our understanding of the mechanisms by which BB emissions are transported to the surface and how they affect air quality in East Asia in general and Taiwan in particular.

Dobutamine-induced alternations in cerebral blood flow of healthy adults: a 3D pseudocontinuous arterial spin labeling study

BackgroundIt is unclear whether dobutamine, commonly used clinically in echocardiography and short-term congestive heart failure treatment for promoting increased myocardial contractility, affects brain microcirculatory behavior. Cerebral microcirculation plays an important role in ensuring adequate oxygen transport. Therefore, we investigated the effects of dobutamine on cerebral hemodynamics.MethodsForty-eight healthy volunteers without cardiovascular or cerebrovascular disease underwent MRI to obtain cerebral blood flow (CBF) maps using 3D pseudocontinuous arterial spin labeling before and during the dobutamine stress test. Additionally, cerebrovascular morphology was obtained based on 3D-time-off-light (3D-TOF) magnetic resonance angiography (MRA). Electrocardiogram, heart rate (HR), respiration rate (RR), blood pressure, and blood oxygen were simultaneously recorded before and during dobutamine injection and during recovery (not during MRI). The anatomic features of the circle of Willis and the basilar artery (BA) diameter were assessed on MRA images by two radiologists with extensive neuroimaging experience. Binary logistic regression was used to test for the independent determinants of CBF changes.ResultsHR, RR, systolic (SBP), and diastolic blood pressure (DBP) significantly increased after dobutamine infusion. Blood oxygen levels remained similar. Compared to the CBF in the resting state, the CBF values exhibited significantly lower CBF levels in both grey matter and white matter. Furthermore, compared with the CBF in the resting state, that in the stress state was decreased in the anterior circulation, mainly in the frontal lobe (voxel level P < 0.001, pixel level P < 0.05). Logistic regression showed that body mass index (BMI; odds ratio [OR] 5.80, 95% confidence interval [CI] 1.60–21.01, P = 0.008], resting SBP (OR 0.64, 95% CI 0.45–0.92, P = 0.014), and BA diameter (OR 11.04, 95% CI 1.05–116.53, P = 0.046) were significantly associated with frontal lobe CBF changes.ConclusionsDobutamine-induced stress significantly decreased CBF in the frontal lobe anterior circulation. Individuals with a high BMI and low SBP during the dobutamine stress test are more likely to have a stress-induced CBF decrease. Thus, attention should be paid to blood pressure, BMI, and cerebrovascular morphology of patients undergoing dobutamine stress echocardiography or those receiving intensive care or anesthesia.

Climatological Characteristics of Cold Fronts and Their Impacts on Air Quality in Cold Seasons Over the Eastern China

AbstractEastern China has been frequently suffering from air pollution problems in the past decades. Besides the intensive emissions, weather‐system evolution is generally regarded as an important factor influencing air quality, among which cold fronts are of great importance in regulating the synoptic‐scale variations of regional pollution. Despite numerous case studies conducted, the long‐term climatological behaviors of cold front depicting their overall impacts on air quality remain unrevealed. In this study, an objective front identification approach has been performed to construct a 42‐year climatology of cold‐front activities in eastern China. It is found that the cold fronts and pollution events usually coincide in cold seasons, and have great impacts on air quality in metropolitan regions. A significant accumulation (cleansing) effect on surface air pollutants in the Beijing‐Tianjin‐Hebei (BTH) region before (after) the passage of the front has been observed in autumn and winter, with mean fine particles (PM2.5) concentrations varying by 40–80 μg/m3 during the processes. The cold fronts that go through the Yangtze River Delta region in winter also lead to the decrease of the PM2.5 concentration after the passages of about 15 μg/m3. Major influencing factors include the changes in meteorological conditions and regional transport driven by frontal circulation, which are favorable for pollutants dispersion and advection. On the interannual scale, the cleansing effect of the cold fronts dominates in BTH regions. The statistical relationship between cold fronts and regional air quality highlighted would help understand the impact of both past and future climate on regional pollution from a synoptic perspective.

Spring Extreme Precipitation Days in North China and Their Reliance on Atmospheric Circulation Patterns during 1979–2019

Abstract The interannual variation of springtime extreme precipitation (SEP) days in North China (NC) and their reliance on atmospheric circulation patterns are studied by using the continuous daily record of 396 rain gauges and the fifth generation of the European Centre for Medium-Range Weather Forecasts atmospheric reanalysis during 1979–2019. The SEP days are defined as the days when at least 10% of rain gauges in NC record daily precipitation no less than 10.5 mm. Results show that the number of SEP days shows large interannual variability but no significant trend in the study period. Using the objective classification method of the obliquely rotated principal analysis in T-mode, we classify the atmospheric circulation into five different patterns based on the geopotential height at 700 hPa. Three circulation patterns all have fronts and are associated with strong southerly wind, leading to 88% of SEP days in NC. The strong southerly wind may provide moisture and dynamic forcing for the frontal precipitation. The interannual variation of SEP days is related with the number of the three above-mentioned dominant circulation patterns. Further analysis shows that the West Pacific pattern could be one of the possible climate variability modes related to SEP days. This study reveals that the daily circulation pattern may be the linkage between SEP days and climate variability modes in NC. Significance Statement Springtime extreme precipitation (SEP) has important social and economic impacts on North China (NC), but few studies have examined the variation of SEP in NC. We find that the number of SEP days has no significant trend but shows large interannual variability during 1979–2019. The interannual variation of SEP days is mainly determined by the total days of three frontal circulation patterns associated with strong southerly wind. Based on the daily circulation pattern, the West Pacific pattern is found to be one of the possible climate variability modes related to SEP days. These findings may shed light on the understanding of SEP in midlatitudes around the world.

The modification of turbulent thermal wind balance by non-traditional effects

The meridional component of the earth's rotation is often neglected in geophysical contexts. This is referred to as the ‘traditional approximation’ and is justified by the typically small vertical velocity and aspect ratio of such problems. Ocean fronts are regions of strong horizontal buoyancy gradient and are associated with strong vertical transport of tracers and nutrients. Given these comparatively large vertical velocities, non-traditional rotation may play a role in governing frontal dynamics. Here the effects of non-traditional rotation on a front in turbulent thermal wind balance are considered using an asymptotic approach. Solutions are presented for a general horizontal buoyancy profile and examined in the simple case of a straight front. Non-traditional effects are found to depend strongly on the direction of the front and may lead to the generation of jets and the modification of the frontal circulation and vertical transport.

Antarctic Polar Front migrations in the Kerguelen Plateau region, Southern Ocean, over the past 360 kyrs

In the Southern Ocean (SO), climate-driven latitudinal migrations of the Antarctic Circumpolar Current (ACC) frontal system impact large-scale ocean circulation and primary productivity. Latitudinal migrations may not have been identical in all SO basins due to the presence or absence of regional bathymetric obstacles. The Antarctic Polar Front (APF), defined by the 3–5 °C surface temperature range and the 2 °C subsurface temperature minimum at 200 m, is particularly important for nutrient redistribution and biodiversity, influencing the soft tissue carbon pump in the modern SO. However, previous assessments of its migrations in the past, mostly based on a single metric or indirect observations, were not always robust. Here, we combine a new proxy for subsurface temperature (sub-ST) reconstructions based on radiolarian assemblages (sub-STrad), with relative abundance variations of key radiolarian species, and sea-surface temperatures (SST) reconstructions, based on diatom assemblages (SSTdiat), to refine estimations of the past mean APF locations in the Kerguelen Plateau (KP) region. Data from three sediment cores on a south (55°S) to north (47°S) transect are used to trace the mean APF locations for three climate states, glacials, peak-interglacials and mild-interglacials. Our results suggest that the APF, presently located south of Kerguelen Islands, shifted by 6–7 degrees of latitude and was located north of the KP during all glacial periods of the last 360 kyrs. This suggests that the ACC major flow interacted less with the bottom topography relative to its modern counterpart, probably resulting in less mixing of the water column over and in the lee of the KP. We propose that this process participated in the isolation of Antarctic surface waters (AASW) and in the reduction of macro-nutrient supply, thus resulting in lower regional productivity. During the warmer-than-present early interglacial periods, the APF probably migrated south by ~5 degrees of latitude relative to its modern position, to pass through the Fawn Trough. Contrary to glacial periods, the APF was forced in an “S” shape while the ACC main flow was constrained against the northern tip of the KP. In this configuration, a stronger interaction between the ACC, its associated fronts, and topography is expected, resulting in more mixing of the water column over and east of the KP. Congruently, siliceous productivity was probably restrained to latitudes south of the Fawn Trough. • New approach to estimate Southern Ocean fronts migrations in the past. • Comparison of sea-surface and subsurface temperature records over the last 360 kyrs. • APF positioned north of the Kerguelen Plateau during glacials. • APF located in the Fawn Trough during early warm interglacials. • Potential impact of ACC and associated fronts shifts on water column mixing.

Composite Vertical-Motion Patterns near North American Polar–Subtropical Jet Superposition Events

AbstractA polar–subtropical jet superposition is preceded by the development of a polar cyclonic potential vorticity (PV) anomaly at high latitudes and a tropical anticyclonic PV anomaly at subtropical latitudes. A confluent large-scale flow pattern can lead to the juxtaposition of these respective PV anomalies at middle latitudes, resulting in the addition of the nondivergent circulations induced by each PV anomaly and an increase in upper-tropospheric wind speeds at the location of jet superposition. Once these PV anomalies become juxtaposed, vertical motion within the near-jet environment facilitates the advection and diabatic redistribution of tropopause-level PV, and the subsequent formation of the steep, single-step tropopause structure that characterizes a jet superposition. Given the importance of vertical motion during the formation of jet superpositions, this study adopts a quasigeostrophic (QG) diagnostic approach to quantify the production of vertical motion during three types of jet superposition events: polar dominant, eastern subtropical dominant, and western subtropical dominant. The diagnosis reveals that the geostrophic wind induced by polar cyclonic QGPV anomalies is predominantly responsible for QG vertical motion in the vicinity of jet superpositions. The QG vertical motion diagnosed from the along-isotherm component of the Q vector, which represents the vertical motion associated with synoptic-scale waves, is dominant within the near-jet environment. The QG vertical motion diagnosed from the across-isotherm component of the Q vector, which represents the vertical motion associated with frontal circulations in the vicinity of the jet, is subordinate within the near-jet environment, but is relatively more important during eastern subtropical dominant events compared to polar dominant and western subtropical dominant events.

Examining tidal impacts on seasonal circulation and hydrography variability over the eastern Canadian shelf using a coupled circulation-ice regional model

A coupled circulation and sea ice model is used to examine the tidal impacts on the seasonal variability of circulation, hydrography and sea ice over the eastern Canadian shelf (ECS). The model performance is assessed using in-situ and satellite remote sensing observations. The tidal impacts have significant spatial variability, which are relatively small over the Labrador and Newfoundland Shelves, moderate over the Scotian Shelf (ScS), and significant over the St. Lawrence River Estuary (SLRE), northwestern Gulf of St. Lawrence (GSL), southwestern ScS, Gulf of Maine (GoM), Bay of Fundy (BoF), and the northern flank of Georges Bank (GeB). The tidal impacts on the seasonal mean circulation are greater in winter than in summer in the SLRE, while greater in summer than in winter over several other areas in the GSL, the GoM-BoF and the southwest ScS. The tidal impacts on temperature and salinity are the most significant near fronts, where both tidal mixing and frontal circulation play important roles. The tidal residual circulation, especially that due to tidal rectification in the GSL and GoM-BoF, spreads the large tidal impacts generated near fronts into broader areas. The changes in circulation and stratification also account for the reduced sea ice concentrations in the GSL.

Cognitive and emotional impairment in patients with chronic cerebrovascular insufficiency

Cerebrovascular disease is one of the most important problems of clinical neurology, which is a significant cause of cognitive impairment and depression. Chronic forms of cerebrovascular insufficiency are most often associated with cerebral microangiopathy or a disease of small vessels, which is characterized by the presence of lacunae, microinfarction and microbleeding, leukoaraiosis and dilated perivascular spaces. One of the main and most common clinical symptoms is vascular cognitive impairment. They are characterized by extremely variable cognitive deficits, highly dependent on the location and severity of vascular damage. However, clinical manifestations associated with impaired frontal circulation are usually observed. Recent international criteria for vascular cognitive impairment presented by the VASCOG (International Society for Vascular Behavioral and Cognitive Disorders) working group emphasize the particular importance of impaired information processing speed, attention, and/or frontal regulatory functions, often in combination with personality or emotional disorders. The frequency of vascular depression in chronic cerebrovascular insufficiency reaches 70%. The addition of affective disorders significantly aggravates the existing cognitive deficit, increases the risk of dementia. This is due to the fact that depression itself leads to the development of cognitive impairment, causes multiple functional disorders of cerebral microcirculation and a number of other mechanisms. The presence of cerebral microangiopathy underlying this pathology makes the use of drugs with vascular effects in the treatment of these patients. The use of preparations with multimodal action, in particular the combined drug and, is most preferable.

The effect of regional warming on the East Asian summer monsoon

East Asian summer monsoon (EASM) precipitation has changed significantly due to regional warming. In this study, effect of regional warming on the EASM summer precipitation is investigated during 1979–2018 with a particular emphasis on the frontogenesis. The frontogenetic function shows that change in circulation and temperature distribution associated with regional warming is fairly consistent with precipitation change. Specifically, changes in temperature gradient and deformation, both in terms of its magnitude and direction with respect to thermal gradient, result in cross-frontal circulation change with respect to its position and magnitude; this, in turn, is closely linked with EASM precipitation change. Due to warming, frontal circulation has weakened except in the mature stage, when the front is over the Kuroshio Current. From a slightly southward shifted position, the northward migration of the front is more precipitous than in the past reaching its northern-most latitude earlier by ~ 2 weeks. On the other hand, the southward migration has become slightly sluggish. In association with the frontogenesis, large-scale precipitation is more strongly affected north of ~ 30° N. South of ~ 25° N, precipitation change is due largely to convective precipitation.

On the physical mechanisms of the two-way coupling between a surface wave field and a circulation consisting of a roll and streak

The governing equations of a surface wave field and a coexisting roll–streak circulation typical of Langmuir circulations or submesoscale frontal circulations are derived to better describe their two-way interactions. The gradients and vertical velocities of the roll–streak circulation induce wave refraction, amplitude modulation and higher-order waves. These changes then produce wave–wave nonlinear forces and divergence of the wave-induced mass transport, both of which in turn affect the circulation. To accurately represent these processes, both a wave theory and a wave-averaged theory are developed without relying on any extrapolation, any spatiotemporal mapping or an approximation that treats the wave-induced mass divergence as being concentrated at the surface. This wave theory finds seven types of current-induced higher-order wave motions. It also determines the wave dynamics such as the governing equation of the wave action density valid in the presence of the complex circulation. The evolution of the wave action density is clearly affected by the upwelling or downwelling. The new wave-averaged theory presents the governing equations of the wave-averaged circulation which satisfies the wave-averaged mass conservation. This circulation is different from the circulation considered to satisfy the mass conservation in the Craik–Leibovich theory, and the difference becomes critical when the wave field evolves due to refraction. In this case, compared to the Craik–Leibovich theory, long waves are more important and also the rolls are more weakly forced.

Enhanced Radiation of Near-Inertial Energy by Frontal Vertical Circulations

AbstractNear-inertial waves (NIWs) radiate energy out of the mixed layer when they develop small lateral scales. Refraction of these waves by gradients in planetary and vertical vorticity has traditionally been invoked to explain this phenomenon. Here, a new mechanism for the enhancement of NIW radiation is described involving the interaction of NIWs with vertical circulations at fronts undergoing frontogenesis. Frontal vertical circulations drive a Doppler shift that is proportional to the wave’s vertical wavenumber m and that changes sign across a front, inducing large lateral differences in wave phase within a few inertial periods. Theory predicts that the process should generate a vertical energy flux that varies inversely with m in contrast to the m−3 dependence expected from refraction. As a consequence, high-mode NIWs are much more effective at radiating energy when fronts and their vertical circulation are present. Numerical simulations initialized with fronts, an array of eddies that drive frontogenesis, and NIWs of various modes are used to test the theory. In the simulations, the interaction of the NIWs with the frontal vertical circulations generates wave beams that radiate down from the fronts. The resultant downward energy flux varies with m following the theoretical scaling laws. In the beams, the Eulerian frequency is inertial within a few percent, yet the waves’ potential and kinetic energies are comparable, thus indicating a superinertial intrinsic frequency. The downshift in Eulerian frequency from the intrinsic frequency is due to horizontal advection of the waves by the eddies.

Observations of the Antarctic Circumpolar Current Over the Udintsev Fracture Zone, the Narrowest Choke Point in the Southern Ocean

AbstractAn up‐to‐date map of the Antarctic Circumpolar Current (ACC) fronts is constructed from the latest version of mean dynamic topography (MDT) from satellite altimetry and reveals the narrowest ACC width in the Udintsev Fracture Zone (UFZ), with the strongest concentration of the three major ACC fronts within a limited distance as short as 170 km, about 40% narrower than that at Drake Passage. At 144°W, at the entrance of the UFZ, which lies between the Pacific‐Antarctic Ridge (PAR) and its eastwardly offset segment (offset PAR segment), there is a triple confluence of the Subantarctic Front, Polar Front, and Southern ACC Front . Downstream of this longitude, the Subantarctic Front progressively meanders northward over the relatively shallow offset PAR segment before channeling through the Eltanin Fracture Zone, thus diverging from the Polar Front which proceeds through the UFZ. In situ observations from two recent cruises at 144°W confirm the satellite altimetry‐derived frontal circulation in the UFZ region and yield a baroclinic transport relative to the bottom of 113 × 106 m3/s, comparable to that through Drake Passage. The hydrographic sections show no Antarctic bottom water colder than 0.2 °C. Characteristics of major water masses are described, and the implications for their potential downstream modifications at Drake Passage are discussed in terms of the meridional overturning circulation across the ACC. Mesoscale eddy activity with periods shorter than 90 days is predominantly concentrated in the immediate downstream area of the offset PAR segment, suggesting a substantial poleward eddy heat flux there.

Aquaculture-induced boundary circulation and its impact on coastal frontal circulation

With the development of coastal suspended aquaculture industry, aquaculture facilities have extended into the open sea to depths of up to 30 m. This practice will likely affect the natural ocean circulation in such areas. For a case study in the high-density aquaculture region of the coastal Yellow Sea, an aquaculture-induced boundary circulation (ABC) is identified and its effects on the background frontal circulation and cross-shore nutrient supply are examined. The ABC is composed of a southward along-boundary current and a counterclockwise cross-boundary cell. The southward along-boundary current (∼5 cm s−1) along with the natural frontal current (∼5 cm s−1) forms a strong coastal current (∼10 cm s−1) along 20–30 m isobaths. The counterclockwise cross-boundary cell exhibits the opposite direction to the natural clockwise cross-frontal cell in the frontal area, which reduces the cross-shore nutrient supply by nearly 25%. Our results suggest that aquaculture boundaries and densities should be considered when planning high-density aquaculture activities.

Nutrient pumping by submesoscale circulations in the mauritanian upwelling system

Observations made within a cold filament in the Mauritanian upwelling system demonstrate that intense submesoscale circulations at the peripheral edges of the filament are likely responsible for anomalously high levels of observed primary productivity by resupplying nutrients to the euphotic zone. Measurements made on the shelf within the recently upwelled water reveal that primary production (PP) of 8.2 gC/m−2 day−1 was supported by nitrate concentrations (NC) of 8 mmol m−3. Towards the front that defined the edge of the filament containing the upwelled water as it was transported offshore, PP dropped to 1.6 gC m−2 day−1 whilst NC dropped to 5.5 mmol m−3. Thus, whilst the observed nutrients on the shelf accounted for 90% of new production, this value dropped to ∼60% near the filament’s front after accounting for vertical turbulent fluxes and Ekman pumping. We demonstrate that the N15 was likely to have been supplied at the front by submesoscale circulations that were directly measured as intense vertical velocities ⩾100 m day−1 by a drifting acoustic Doppler current profiler that crossed a submesoscale surface temperature front. At the same time, a recently released tracer was subducted out of the mixed layer within 24 h of release, providing direct evidence that the frontal circulations were capable of accessing the reservoir of nutrients beneath the pycnocline. The susceptibility of the filament edge to submesoscale instabilities was demonstrated by O(1) Rossby numbers at horizontal scales of 1–10 km. The frontal circulations are consistent with instabilities arising from a wind-driven nonlinear Ekman buoyancy flux generated by the persistent northerly wind stress that has a down-front component at the northern edge of the inshore section of the filament. The prevalence of submesoscale instabilities and their associated vertical circulations are proposed to be a key mechanism operating at sub-grid scales and sustaining new production throughout the upwelling system.

Frontal Circulation and Submesoscale Variability during the Formation of a Southern Ocean Mesoscale Eddy

AbstractObservations made in the Scotia Sea during the May 2015 Surface Mixed Layer Evolution at Submesoscales (SMILES) research cruise captured submesoscale, O(1–10) km, variability along the periphery of a mesoscale O(10–100) km meander precisely as it separated from the Antarctic Circumpolar Current (ACC) and formed a cyclonic eddy ~120 km in diameter. The meander developed in the Scotia Sea, an eddy-rich region east of the Drake Passage where the Subantarctic and Polar Fronts converge and modifications of Subantarctic Mode Water (SAMW) occur. In situ measurements reveal a rich submesoscale structure of temperature and salinity and a loss of frontal integrity along the newly formed southern sector of the eddy. A mathematical framework is developed to estimate vertical velocity from collocated drifter and horizontal water velocity time series, under certain simplifying assumptions appropriate for the current dataset. Upwelling (downwelling) rates of O(100) m day−1 are found in the northern (southern) eddy sector. Favorable conditions for submesoscale instabilities are found in the mixed layer, particularly at the beginning of the survey in the vicinity of density fronts. Shallower mixed layer depths and increased stratification are observed later in the survey on the inner edge of the front. Evolution in temperature–salinity (T–S) space indicates modification of water mass properties in the upper 200 m over 2 days. Modifications along σθ = 27–27.2 kg m−3 have climate-related implications for mode and intermediate water transformation in the Scotia Sea on finer spatiotemporal scales than observed previously.

Idealized simulations of sea breezes over mountainous islands

High‐resolution simulations of diurnally heated airflows over mountainous islands are conducted to study the impacts of island terrain on sea‐breeze circulations. As the simulated island terrain height increases, the sea‐breeze front (SBF) propagates inland faster but its frontal circulation weakens dramatically, as does the sea‐breeze flow behind it. This sensitivity is interpreted through the frontogenesis budget in a SBF‐relative reference frame. The dominant frontogenetic term (the cross‐frontal convergence) weakens over taller islands, but this trend is offset by a similar weakening of the dominant frontolytic term (the front‐relative advection) to yield minimal sensitivity of the SBF baroclinicity to island terrain height. At first glimpse, this finding appears inconsistent with the systematic reduction in SBF circulation strength over taller islands. However, as the island height is increased, baroclinicity at the SBF becomes increasingly associated not with the front itself but with the ‘background’ baroclinicity associated with elevated heating over steeper island slopes. When this latter contribution is removed, a clear weakening of the SBF baroclinicity is recovered over the taller islands. Analysis of the cross‐frontal convergence budget indicates that this reduction in cross‐frontal convergence, and hence the SBF baroclinicity, is caused by an increased slope‐parallel buoyancy gradient. As the SBF migrates inland, this gradient preferentially accelerates the air ahead of the SBF up the slope, which weakens the convergence along the front. Finally, as shown by a simple scaling of the slope‐parallel momentum equation, the weakened sea‐breeze flow over taller islands is associated with a weaker onshore perturbation pressure gradient force, owing to the protrusion of the convective boundary layer into the stable free troposphere.