Mesoscale Instability Research Articles

Strong surface winds in Storm Eunice. Part 1: storm overview and indications of sting jet activity from observations and model data

Strong surface winds in Storm <i>Eunice</i>. Part 1: storm overview and indications of sting jet activity from observations and model data

Wave, Vortex and Wave‐Vortex Dipole (Instability Wave): Three Flavors of the Intra‐Seasonal Variability of the North Equatorial Undercurrent

AbstractIntra‐seasonal variations have been frequently observed in the North Equatorial Undercurrent (NEUC) jets, yet their dynamical nature remains elusive. Based on field observations and model results, we identify three flavors of the ISVs: Wave (accounting for 10%), wave‐vortex dipole (WVD) (20%) and vortex (70%). The wave flavor refers to a Rossby wave, the WVD flavor consists of a pair of counter‐rotating vortices, and the vortex flavor refers to a sub‐thermocline monopole eddy. To our knowledge, the WVD is identified for the first time and is found characterized by a unique dynamical feature: It manifests both as a second baroclinic mode‐like Rossby wave and as a pair of dipole‐like sub‐thermocline eddies. It is further identified as a second baroclinic mode‐like Rossby wave‐initiated instability wave in an equilibrium being fueled by baroclinic conversion. These results indicate that mesoscale instability waves are an important component of subthermocline intra‐seasonal and mesoscale variations.

Seasonal and Mesoscale Variability of the Two Atlantic Water Recirculation Pathways in Fram Strait

AbstractAtlantic Water (AW), which is transported northward by the West Spitsbergen Current (WSC), partly recirculates (i.e., turns westward) in Fram Strait. This determines how much heat and salt reaches the Arctic Ocean, and how much joins the East Greenland Current on its southward path. We describe the AW recirculation's location, seasonality, and mesoscale variability by analyzing the first observations from moored instruments at five latitudes in central Fram Strait, spanning a period from August 2016 to July 2018. We observe recirculation on the prime meridian at 78°50’N and 80°10’N, respectively south and north of the Molloy Hole, and no recirculation further south at 78°10’N and further north at 80°50’N. At a fifth mooring location northwest of the Molloy Hole at 79°30’N, we observe some influence of the two recirculation pathways. The southern recirculation is observed as a continuous westward flow that carries AW throughout the year, though it may be subject to broadening and narrowing. It is affected by eddies in spring, likely due to the seasonality of mesoscale instability in the WSC. The northern recirculation is observed solely as passing eddies on the prime meridian, which are strongest during late autumn and winter, and absent during summer. This seasonality is likely affected both by the conditions set by the WSC and by the sea ice. Open ocean eddies originating from the WSC interact with the sea ice edge when they subduct below the fresher, colder water.

Meso-scale Instability Triggered by Dust Feedback in Dusty Rings: Origin and Observational Implications

Abstract High spatial resolution observations of protoplanetary disks by ALMA have revealed many substructures that are providing interesting constraints on disk physics as well as dust dynamics, both of which are essential for understanding planet formation. We carry out high-resolution, 2D global hydrodynamic simulations, including the effects of dust feedback, to study the stability of dusty rings. When the ring edges are relatively sharp and the dust surface density becomes comparable to the gas surface density, we find that dust feedback enhances the radial gradients of both the azimuthal velocity profile and the potential vorticity profile at the ring edges. This eventually leads to instabilities on meso-scales (spatial scales of several disk scale heights), causing dusty rings to be populated with many compact regions with highly concentrated dust densities. We also produce synthetic dust emission images using our simulation results and discuss the comparison between simulations and observations.

Simulation of Mesoscale Cellular Convection in Marine Stratocumulus: 2. Nondrizzling Conditions

AbstractThis is the second of two large‐eddy simulation studies on the mechanisms of mesoscale cellular organization in drizzling (open cells) and nondrizzling marine stratocumulus (closed cells). This study uses a hard nudging approach which maintains fixed horizontal‐mean temperature and humidity profiles for a well‐mixed boundary layer with a constant boundary layer depth. For the case studied, closed cells develop and broaden by 32 hr to an aspect ratio of 25. Simulations show that the closed‐cell mesoscale cellular convection is driven by positive feedback from cloud‐induced mesoscale perturbations of longwave radiative cooling. A conceptual model for closed‐cell stratocumulus as a mesoscale wavelength hydrodynamic instability in which mesoscale moist and dry anomalies spontaneously grow is presented. In simulations in which long‐wavelength sinusoidal moisture anomalies are initially imposed, these anomalies evolve into amplifying closed cells. The cell structure is visualized with a compositing approach based on sorting grid columns by their mesoscale‐smoothed total water path. A thermally direct mesoscale circulation pattern develops in the interior of the boundary layer with buoyant mesoscale updrafts, thicker cloud, and a slightly higher capping inversion in the moister columns. There is a mesoscale flow of above‐inversion air down the slightly sloping capping inversion from the moist to the dry regions, reinforced by cloud top radiative cooling. This strengthens the mesoscale anomalies by preferentially cooling and drying the already dry regions. The sloping inversion flow is not driven as efficiently if the radiative cooling is artificially horizontally homogenized, partly disrupting this positive feedback and the resulting closed‐cell development.

Idealized Study on the Effect of Bottom Topography on the Seasonality of the Stability of the Iceland–Faeroe Front

AbstractWe investigate the effects of bottom topography on the instability, eddy-driven heat flux, and overturning of a front that sits atop a ridge by varying the initial location of an idealized frontal outcrop with respect to a topographic ridge. The front is periodic in the along-ridge direction and unstable to both mixed layer and mesoscale baroclinic instabilities with both instabilities focused on the northern flank of the ridge where the front outcrops. We find agreement with the theoretical predictions for the development of mesoscale instability of the jet in the presence of sloping bottom topography, and we find the initial growth of surface mixed layer eddies is insensitive to topographic variations. However, during the finite amplitude phase of mixed layer instability, we find faster development of mesoscale eddies and thus a stronger cross-front eddy heat flux and residual circulation for the position of the jet where we found the faster growth of mesoscale baroclinic instability. Over an advective time scale that represents the transit time of a water parcel along the Iceland–Faeroe Ridge (IFR), the resulting eddy heat flux is greatest in the cases where the frontal jet experiences the most destabilizing bottom topography of the three cases tested, with values comparable to the heat flux associated with the mean flow. Therefore, eddy dynamics over the IFR frontal region are important contributors to the heat exchanges between the North Atlantic and Nordic Seas, with the bottom topography playing a key role in determining the largest heat fluxes, whether the initial growth is dominated by mixed layer eddies or mesoscale eddies.

Lagrangian and Eulerian characterization of two counter‐rotating submesoscale eddies in a western boundary current

AbstractIn recent decades, high‐spatial resolution ocean radar and satellite imagery measurements have revealed a complex tangle of submesoscale filaments and eddies, in the surface velocity, temperature, and chlorophyll a fields. We use a suite of high‐resolution data to characterize two counter‐rotating, short‐lived eddies formed at the front between the warm East Australian Current (EAC) and temperate coastal waters (30°S, Eastern Australia). In this region, submesoscale filaments and short‐lived eddies are dynamically generated and decay at time scales of hours to days. Dominant cyclonic filaments of O(1) Rossby number formed along frontal jets and eddy boundaries, generating localized ageostrophic circulations at the submesoscale. Measurements of over‐ocean wind direction and surface currents from high‐frequency radars reveal the influence of the short‐term, small‐scale wind forcing on the surface circulation, enhancement of the horizontal shear, frontal jet destabilization, and the generation and decay of the cyclonic eddy. By contrast, the anticyclonic eddy formation was most likely associated with EAC mesoscale instability and anticyclonic vorticity. Lagrangian tracks show that surface particles can be temporarily trapped in the eddies and frontal convergent zones, limiting their transport. Mixing between EAC‐derived and coastal waters was increased along the frontal regions, and particles starting at the divergent regions around the eddies experienced significant dispersion at submesoscales. The cyclonic cold‐core eddy entrained high chlorophyll a shelf waters on its convergent side, suggesting spiral eddy cyclogenesis.

Seasonal Cycle of Mesoscale Instability of the West Spitsbergen Current

AbstractThe West Spitsbergen Current (WSC) is a topographically steered boundary current that transports warm Atlantic Water northward in Fram Strait. The 16 yr (1997–2012) current and temperature–salinity measurements from moorings in the WSC at 78°50′N reveal the dynamics of mesoscale variability in the WSC and the central Fram Strait. A strong seasonality of the fluctuations and the proposed driving mechanisms is described. In winter, water is advected in the WSC that has been subjected to strong atmospheric cooling in the Nordic Seas, and as a result the stratification in the top 250 m is weak. The current is also stronger than in summer and has a greater vertical shear. This results in an e-folding growth period for baroclinic instabilities of about half a day in winter, indicating that the current has the ability to rapidly grow unstable and form eddies. In summer, the WSC is significantly less unstable with an e-folding growth period of 2 days. Observations of the eddy kinetic energy (EKE) show a peak in the boundary current in January–February when it is most unstable. Eddies are then likely advected westward, and the EKE peak is observed 1–2 months later in the central Fram Strait. Conversely, the EKE in the WSC as well as in the central Fram Strait is reduced by a factor of more than 3 in late summer. Parameterizations for the expected EKE resulting from baroclinic instability can account for the observed EKE values. Hence, mesoscale instability can generate the observed variability, and high-frequency wind forcing is not required to explain the observed EKE.

Wind waves in the coastal zone of the southern crimea: Assessment of simulation quality based on in situ measurements

The study verifies the Black Sea wave model using field data obtained from the Katsiveli research platform. The WAM and mesoscale MM5 and WRF atmospheric models, which are used to calculate the wind field for the wave model, were recently adjusted to the Black Sea region at the Marine Hydrophysical Institute. The results of the work are presented as characteristics of the simulation quality used in world practice in other regions. The scatter index for a significant wave height is 70% in summer and 50% in winter. The values of the scatter index of wave parameters and wind speed appear to be at the same level as in semi-enclosed seas on the northern side of the Mediterranean Sea. It is shown that atmospheric simulation correctly reproduces the interaction between synoptic processes and the mountain range extending alongshore. Error sources in wave simulation are discussed. The most significant drawback is the possibility of mesoscale instability in the atmospheric model without assimilation of observation data within the computational domain.

An algorithm to improve the detection of ocean fronts from whiskbroom scanner images

High-resolution satellite imagery is a valuable data source to analyse ocean submesoscale dynamics (i.e., with spatial scales of the order of 1–10 km) and investigate their impact on turbulent mixing, energetics of mesoscale vortices, instability processes or phytoplankton blooms. However, data acquired by satellite sensors often suffer from instrumental noise that degrades image quality and therefore compromises the detection of ocean fronts as well as the estimation of its physical characteristics. A well-known artefact in data characteristic of whiskbroom scanners is stripe noise. In this article, we propose an algorithm that improves the detection of ocean fronts by removing the impact of striping on the observed gradient field. We use level 2 sea surface temperature and chlorophyll-a products derived from NASA’s Moderate Resolution Imaging Spectroradiometer to illustrate the algorithm performance.

Contribution of vortex Rossby wave to spiral rainband formation in tropical cyclones

The contribution of the vortex Rossby wave (VRW) to the spiral rainband in the tropical cyclones (TCs) is studied in the framework of a barotropic non-divergent TC-like vortex model. The spectral function expanding method is used to analyze the disturbance evolution of a defined basic state vortex. The results show that the numerical solution of the model is a superposition of the continuous spectrum component (non-normal modes) and the discrete spectrum component (normal modes). Only the eyewall and the rainbands in the inner core-region in a TC are related to the VRW normal modes, whereas the continuous spectrum wave components play an important role in the formation of secondary-, principal-, and distant- rainbands, especially the outer rainband, through an indirect way. The continuous spectrum can promote the development of the TC circulation for the occurrence of a mesoscale instability. The convection under a favorable moisture condition will trigger the inertial-gravitational wave to cause the formation of unstable spiral bandliked-disturbances outside of the eyewall. The complicated interaction between the basic state-vortex and the VRW disturbances will cause a positive feedback between the TC circulation and the rainband.

Mesoscale Folding, Instability, and Disruption of Laminar Flow in Metal Surfaces

Using in situ imaging, we report surface fold formation and fluidlike flow instabilities in sliding of annealed copper. We demonstrate using simulations that folding is principally driven by grain-induced plastic instability. The phenomenon shows remarkable similarities with Kelvin-Helmholtz-type flow instabilities in fluids. While such instabilities have been conjectured to exist in sliding interfaces at the nanoscale, we find vortices and folding in metals at the mesoscale. The occurrence of folds impacts many applications, including surface generation processes and tribology.

Numerical simulation of a torrential rain event in the northeast of Huaihe Basin. Part II: Instability conditions and the mechanism of intensification and maintenance

Based on the simulation displayed in Part I of this study, the intensification and maintenance, the relationship between deep moist mesoscale convective system (DMMCS) and instability, convective vorticity vector (CVV) are analyzed in the present paper. Results show that: (1) middle-low level convective instability is the precondition of the occurrence of DMMCS. The convergence and merger enhancement of convection cells, as well as the convective instability energy transporting from the left-front of typhoon play an essential role in the re-establishment and enhancement of convective instability. (2) Baroclinic instability and conditional symmetric instability appear not only in the middle-low level, but also are distinct in the middle-upper level of DMMCS. (3) In DMMCS, there is an alternative distribution of inertial instability column and inertial stability column. In the west and south, there are negative CVV columns, which is favorable for the burst of deep moist convection. (4) The strong slantwise convection induced by inertial instability, baroclinic instability, and conditional symmetric instability enhance the upper-level southerly component. Due to the appearance of the compensated downdraft at the low level of south side of DMMCS, the low level southerly intensified, and the enhancement of upper- and low-level cores is in favor of the development of DMMCS, which will be beneficial to the reinforcement and maintenance of inertial instability, baroclinic instability, and conditional symmetric instability. It is a positive feedback process. (5) There is a downshear circulation to the east of rainfall cell. Shallow convections near this cell absorb the vapor and instability energy coming from the south. In the meanwhile, the mesoscale convergence line and meso-β-scale vortex organize and intensify convective cells. In DMMCS, there is an alternative distribution of convergence and divergence columns, and the couple between strong divergence and vorticity columns. They are both conducive to the development of DMMCS, and the instability will be intensified and maintained for its development in depth.

Targeted coastal circulation phenomena in diagnostic analyses and forecasts

Oceanography is moving toward the construction of operational observing systems in coastal regions. The essential system design scheme is widely distributed measurements assimilated in computational simulation models, from which a variety of analysis products and forecasts are extracted and publicly disseminated. The detailed specifications for this system must be tested against the relevant oceanic phenomena. This essay surveys coastal physical phenomena in two categories. The more familiar ones are external tides, storm surges, river plumes, coastal topographic waves, upwelling and alongshore boundary currents, mesoscale instability eddies, and topographic contour currents and standing meanders. The phenomena with more recent attention are internal tides, surface fronts, submesoscale vortices, wakes, and littoral currents. Some illustrations are drawn from model simulations made without the aid of data assimilation.

Adhesion induced mesoscale instability patterns in thin PDMS-metal bilayers

We show that the surface of a thin elastomer-metal (aluminum) hybrid bilayer becomes spontaneously patterned when brought in adhesive contact with a rigid surface. The self-organized surface patterns show three distinct morphological phases-columns, labyrinths, and holes-depending on the area of contact. The characteristic wavelength of these patterns is found to be 2.94+/-0.20 times the total film thickness, independent of the morphological phase and the surface properties of the contacting surface. Interestingly, the metal films 60-120 nm thick showed the same scaling, but the bilayers with thicker metal films were completely stable. This observation demonstrates for the first time a "hard" transition to the instability as the elastic stiffness of the film is varied. We also report a protocol for alignment of the instability patterns and for transferring the metal patterns to another surface.

MODIS chlorophyll variability in the northern Adriatic Sea and relationship with forcing parameters

The variability of the MODIS chlorophyll concentration in the northern Adriatic Sea is described between July 2002 and January 2005 and related to the Po River discharge rates and local wind forcing. After interpolation, the MODIS data were decomposed into EOF modes. Significant variability (∼61%) is explained by the first three modes in which mode 1 includes a strong seasonal cycle with enhanced signal in winter for the open sea waters and a second maximum in April–May, and mode 2 separates the coastal and river‐influenced waters, from the open sea area. The temporal amplitude of the modes is significantly correlated with the Po River runoff. The interpolated MODIS data were also correlated with the Po River discharge rates, resulting in significant positive correlation north and south of the delta and off the Croatian Islands. When correlated with wind speed, the MODIS chlorophyll shows positive correlation in the area off Istria, off Croatia and at the edge of the coastal layer north of Ancona. Using wind regimes, similar patterns are found for the Bora‐wind like winds. In contrast, for Sirocco‐like winds, the correlation is significantly positive in the vicinity and north of the Po River delta. The regions off Ancona and Istria have poor correlation with the Po and wind because the MODIS chlorophyll variability is associated with mesoscale instability features not directly related to these forcing parameters. Poor (and even slightly negative) correlations are found off the Po delta. In most areas, correlations reach maximum values after 1–2 days.

Mesoscale subduction at the Almeria–Oran front: Part 2. Biophysical interactions

The Almeria–Oran front forms where surface waters of Atlantic and Mediterranean origin meet at the eastern end of the Alboran Sea. A multidisciplinary field experiment on RRS Discovery in December 1996, in the second observational phase of the EU-funded Observations and Modelling of Eddy scale Geostrophic and Ageostrophic motions (OMEGA) project, observed the biological impact of mesoscale frontal instability of the Almeria–Oran frontal jet. It is concluded that periodic vertical velocities of ∼20 m/day, associated with the propagation of wave-like meanders along the front, have a significant effect on the vertical distribution of zooplankton across the front despite their ability to migrate at greater speeds. Observations of a layer of fluorescence coincident with subducted surface waters indicated that phytoplankton were drawn down and along isopycnals, by cross-front ageostrophic motion, to depths of 200 m. From the study of sound-scattering layers (SSL) identified in acoustic backscatter data, a layer of zooplankton was found coincident with the drawn-down phytoplankton. This layer persisted during and despite diel vertical migration. High-resolution optical plankton counter (OPC) data showed smaller zooplankton, which did not undertake diel vertical migration, remained concentrated near the surface in the fast-flowing frontal jet.

Mesoscale Instability and Upwelling. Part 1: Observations at the North Atlantic Intergyre Front

Abstract Mesoscale instabilities have been found with a wavelength of about 85 km growing along the meandering jet of the North Atlantic Intergyre Front at about 52°N, 33°W. They show the signature of baroclinic instabilities and are typified by vertical motions of up to 5 m d−1, which have been diagnosed using the ω equation. The effect of the earth's curvature on motion on these scales is shown to be negligible.

Mesoscale Instability and Upwelling. Part 2: Testing the Diagnostics of Vertical Motion with a Three-Dimensional Ocean Front Model

Abstract A three-dimensional primitive equation ocean front model is employed to simulate mesoscale instabilities and then to investigate the validity (and the contingent differences) of three alternative methods used for the diagnosis of mesoscale vertical motion from combined density and horizontal velocity mappings: 1) the isopycnic advection equation, 2) the vorticity advection equation, and 3) the ω equation. The model is integrated for 60 days during which period meanders form along the initially unidirectional jet, first preferably at a wavelength of just under 100 km and later also at smaller horizontal scales; these growing instabilities are associated with vertical velocities of up to some tens of meters per day. The horizontal distribution, pattern of vertical velocity can reasonably be estimated by use of method 3 and also method 2, but method 1 may yield a pattern having the wrong phase relationship.

An Observation of Banded Convective Development in the Presence of Weak Symmetric Stability

Satellite imagery dramatically portrays a mesoscale organization of deep convection over the south central United States on 5 June 1986. Free convection was expected over the region. The rapid development and organization of the convection simultaneously across a broad area suggests the presence of a mesoscale instability. Analysis of satellite and conventional data suggests that a layer of weak symmetric stability modified the atmosphere's response to free convective instability, contributing to the highly organized banded structure observed.