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Global ocean surface currents estimated from satellite derived data based on a regular global grid are affected by the grid's shape and placement. Due to different neighbourhood relationships, the rectangular lat/lon grids lose accuracy when interpolating and fitting elevation data. Hexagonal grids have shown to be advantageous due to their isotropic, uniform neighbourhood. Considering these merits, this paper aims to estimate global ocean surface current using a global isotropic hexagonal grid from satellite remote sensing data. First, gridded satellite altimeter data and wind data with different resolutions are interpolated into the centre of the global isotropic hexagonal grid. Then, geostrophic and Ekman currents components are estimated according to the Lagerlof Ocean currents theory. Finally, the inversion results are verified. By analyzing the results, we conclude that the ocean surface currents estimated based on the global isotropic hexagonal grid have considerable accuracy, with improvement over rectangular lat/lon grids. [ABSTRACT FROM AUTHOR]

Located between the sub-Antarctic Front and the Antarctic Polar Front, the Prince Edward Islands (PEIs) provide an essential breeding ground for top predators and are an ideal location to investigate disturbances linked to climate change. This study provides the first seasonal characterisation of surface hydrography at the PEIs, using satellite and reanalysis products from 1993 to 2016. Sea surface temperature (SST) showed consistently higher values to the north, while wind, currents and eddy kinetic energy all showed higher values to the south of the islands. The highest SST (8 °C) occurred in summer and the lowest (2 °C) in winter, with a one-to two-month delay between the maximum incoming solar radiation (in December–January) and the highest SST (in February). The highest wind speed occurred in July (10.8 m s−1) and the minimum in February (7 m s−1). Geostrophic currents were four-times larger than Ekman currents, showing lower speeds between April and June (0.25–0.30 m s−1) and a peak in August (0.45 m s−1). There were no significant differences for SST and Ekman currents between the regions upstream and downstream of the PEIs. In contrast, surface total and geostrophic current velocities were weaker downstream because the islands act as a barrier to the flow. A zone of lower wind speed, coinciding with enhanced positive wind stress curl (WSC), favouring downwelling, occurred directly upstream throughout the year. Although WSC over the PEIs was negative (upwelling-favourable), no corresponding cooling was evident in SST. This seasonal characterisation provides a baseline against which interannual/decadal variability and future changes in these parameters can be assessed. [ABSTRACT FROM AUTHOR]

The mean, interannual variability, decadal variability, and longer-term trends in the across-track geostrophic velocity of the poleward-flowing boundary current along the Fiordland coast (the Fiordland Current, FC) are examined over the altimeter record (September 1992–May 2017). In the mean, the FC is southwestward, with stronger velocities in an inner current near the coast and weaker velocities in an outer current further offshore. Examination of the alongshore momentum balance suggests that the mean flow is driven by a poleward downsloping alongshore pressure gradient. At interannual time-scales, variability in the FC was correlated with the alongshore wind stress, indicating that an equatorward (poleward) wind stress drives an equatorward (poleward) velocity anomaly. At decadal time-scales, variability in the FC was correlated with the South Pacific wind stress curl, indicating that as the wind stress curl increases (decreases) the poleward flow in the FC weakens (strengthens), consistent with the response of a traditional eastern boundary current. Long-term trends show a weakening outer current and strengthening inner current. Increasing flow in the inner current may result in increased transport of subtropical water around southern New Zealand. [ABSTRACT FROM AUTHOR]

In this paper, 24 years of daily sea surface height (SSH) fields constructed by merging the measurements from two/multiple simultaneously operating altimeters are analysed to investigate mesoscale eddies. Based on efficient parallel identification and optimized tracking algorithms, we present two global daily mesoscale ocean eddy datasets () containing over 688,000 eddy trajectories that persist for at least 4 weeks as identified in the Copernicus Marine Environment Monitoring Service (CMEMS) Two-sat/All-sat datasets over the 1993–2016 period. These long-lived eddies have an average lifetime of 72 days and an average propagation speed of 5.1 k m d a y − 1 . The properties of the identified eddy contours include the amplitude, radius, area, geostrophic current speed, eddy kinetic energy (EKE), and relative vorticity, based on which a new optimized eddy tracking method is applied. The most stable properties of the eddies are included in a set of dimensionless similarity parameters. A comparison of the eddy track outputs from the 'Two-sat merged' and 'All-sat merged' data products using our algorithm (hereafter called I4Eddy) shows that the 'All-sat merged' data are more suitable for studying global eddy tracks. The eddy trajectory data are also compared with two other published global eddy trajectory products, and the results demonstrate the efficiency of our method. The accuracy of our method's eddy identification algorithm and its moderately conservative eddy tracking properties allow our algorithm to generate more continuous eddy tracks without large jumps. These elaborate datasets can be used to study the relationships between mesoscale eddies and other oceanic phenomena. [ABSTRACT FROM AUTHOR]

In this study, data from space-based altimeters (Jason-2 and Satellite for ARgos and AltiKa [SARAL/AltiKa]) have been used to compute alongshore geostrophic currents in the coastal regions of the Indian mainland. These derived currents are compared with high-frequency (HF) radar observations. Beyond 30–40 km away from the coastline, altimeter-derived currents match fairly well with the HF radar data. Root mean square error (RMSE) of Jason-2-derived currents ranges between 0.3 and 0.6 m s−1 while the same in the case of SARAL/AltiKa lies between 0.3 and 0.7 m s−1. Satellite-derived across-track geostrophic current components (alongshore current) were also used to study the spatiotemporal variations of the east India coastal current (EICC). The coastal trapping of the EICC, its annual and intra-seasonal peaks are clearly observed in the power spectrum of time-series of Jason-2 and SARAL/AltiKa derived currents. [ABSTRACT FROM AUTHOR]

We consider quasi-geostrophic (QG) models in two- and three-layers that are useful in theoretical studies of planetary atmospheres and oceans. In these models, the streamfunctions are given by (1+2) partial differential systems of evolution equations. A two-layer QG model, in a simplified version, is dependent exclusively on the Rossby radius of deformation. However, the f-plane QG point vortex model contains factors such as the density, thickness of each layer, the Coriolis parameter, and the constant of gravitational acceleration, and this two-layered model admits a lesser number of Lie point symmetries, as compared to the simplified model. Finally, we study a three-layer oceanography QG model of special interest, which includes asymmetric wind curl forcing or Ekman pumping, that drives double-gyre ocean circulation. In three-layers, we obtain solutions pertaining to the wind-driven doublegyre ocean flow for a range of physically relevant features, such as lateral friction and the analogue parameters of the f-plane QG model. Zero-order invariants are used to reduce the partial differential systems to ordinary differential systems. We determine conservation laws for these QG systems via multiplier methods. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the interaction between two like-signed quasi-geostrophic uniform potential vorticity internal vortices in the vicinity of a surface buoyancy anomaly filament in a three dimensional, stably stratified and rapidly rotating fluid. The surface buoyancy distribution locally modifies the pressure fields and generates a shear flow. We start the study by first considering the effects of a uniform linear horizontal shear on the binary vortex interaction. We confirm that a cooperative shear facilitates the merger of a pair of vortices while an adverse shear has the opposite effect. We next investigate the binary vortex interaction in the vicinity of the surface buoyancy filament explicitly. Here, not only the filament generates a shear flow, but it also responds dynamically to the forcing by the vortex pair. The filament destabilises and forms buoyancy billows at the surface. These billows interact with the internal vortices. In particular, a surface billow may pair with one of the internal vortices. In such cases, the like-signed internal vortex pair may separate if they are initially moderately distant from each other. [ABSTRACT FROM AUTHOR]

The Code for Unplanned Encounters at Sea [CUES 2014 Code for Unplanned Encounters at Sea (Western Pacific Naval Symposium, 22 April 2014] was finally adopted by the Western Pacific Naval Symposium in 2014. It is an important step toward improving confidence between navies. However, as a non-legally binding document with unclear geographical scope which only applies to naval vessels and aircraft, CUES 2014 is not comprehensive or potent enough, especially in the East and South China Seas where maritime disputes are unresolved and most maritime incidents involve civilian law enforcement vessels and aircraft. This article provides a geostrategic picture of the East and South China Seas, the background of the code, and analyses the practical limitations of CUES 2014 in the context of the East and South China Seas. [ABSTRACT FROM AUTHOR]

The Mascarene Plateau lies in the south-west Indian Ocean between the islands of Mauritius and the Seychelles Bank, and is characterised by a series of shallow banks separated by deep (>1 000 m), narrow channels. The plateau acts as an obstruction to the general ocean circulation in this region, separating the westward-flowing South Equatorial Current (SEC) into two branches downstream of the plateau. In this article, we present the results of a survey conducted along the entire Mascarene Plateau during the Northeast Monsoon, in October–November 2008. In addition, data from Argo floats were used to determine the origin of water masses entering this region. The plateau contains three gaps through which branches of the SEC are channelled. The northern, central and southern gaps receive 14.93 Sv, 14.41 Sv and 6.19 Sv, respectively. Although there are differences in water-mass properties to the west and east of the Mascarene Plateau due to mixing, the SEC acts as a sharp boundary between water masses of southern and northern Indian Ocean origin. Mixing occurs in the central gap between intermediate water masses (Red Sea Water [RSW] and Antarctic Intermediate Water [AAIW]) as well as in the upper waters (Subtropical Surface Water [STSW] and Indonesian Throughflow Water [ITW]). Through the northern gap, mixing occurs between Arabian Sea High-Salinity Water (ASHSW), ITW and Tropical Surface Water (TSW), while through the southern gap, mixing occurs between STSW and ITW. North Indian Deep Water (NIDW) is present in the region but the plateau appears to have no effect on it. [ABSTRACT FROM AUTHOR]

The objection of this paper is to study the asymptotical behavior of solutions for the three dimensional planetary geostrophic equations of large-scale ocean circulation with small additive noise. In this paper, we prove the existence of random attractors for the three dimensional planetary geostrophic equations of large-scale ocean circulation with small additive noise. Furthermore, we prove the random attractorof the three dimensional planetary geostrophic equations of large-scale ocean circulation with small additive noise will be close to the global attractorof the three dimensional planetary geostrophic equations of large-scale ocean circulation for anywhen the parameter of the perturbationgoes to zero. [ABSTRACT FROM PUBLISHER]

This paper presents an assessment of SARAL/AltiKa satellite altimeter for the monitoring of a tropical western boundary current in the south-western Pacific Ocean: the East Caledonian Current. We compare surface geostrophic current estimates obtained from two versions of AltiKa along-track sea level height (AVISO 1 Hz and PEACHI 40 Hz) with two kinds of dedicated in situ datasets harvested along the satellite ground tracks: one deep-ocean current-meter mooring deployed in the core of the boundary current and five glider transects. It is concluded that the AltiKa-derived current successfully captures the velocity of the boundary current, with a standard error of 11 cm/s with respect to the in situ data. It also appears important to reference AltiKa sea level anomaly to the latest mean dynamic topography available in our area. Doing so, Ka-band altimetry provides a satisfactory representation of the western boundary current. Thereby, it usefully contributes to observing its variability in such a remote and under-observed ocean region. However, the rather long repeat period of SARAL (35 days) in comparison to the high frequency variability seen in the flow velocity of the boundary current calls for a combined use of SARAL with the other satellite altimetry missions. [ABSTRACT FROM PUBLISHER]

The impact of GOCE Satellite Gravity Gradiometer data on gravity field models was tested. All models were constructed with the same Laser Geodynamics Satellite (LAGEOS) and Gravity Recovery and Climate Experiment (GRACE) data, which were combined with one or two of the diagonal gravity gradient components for the entire GOCE mission (November 2009 to October 2013). The Stokes coefficients were estimated by solving large normal equation (NE) systems (i.e., the direct numerical approach). The models were evaluated through comparisons with the European Space Agency's (ESA) gravity field model DIR-R5, by GPS/Leveling, GOCE orbit determination, and geostrophic current evaluations. Among the single gradient models, only the model constructed with the vertical ZZ gradients gave good results that were in agreement with the formal errors. The model based only on XX gradients is the least accurate. The orbit results for all models are very close and confirm this finding. All models constructed with two diagonal gradient components are more accurate than the ZZ-only model due to doubling the amount of data and having two complementary observation directions. This translates also to a slower increase of model errors with spatial resolution. The different evaluation methods cannot unambiguously identify the most accurate two-component model. They do not always agree, emphasizing the importance of evaluating models using many different methods. The XZ gravity gradient gives a small positive contribution to model accuracy. [ABSTRACT FROM AUTHOR]

This study focuses on the interaction between mid depth vortices and surface jets and fronts in a three-layer quasi-geostrophic model. Such vortices may be regarded as an idealisation of meddies, eddies of Mediterranean Water in the Northeastern Atlantic Ocean, interacting with the Azores j t and front. Successively, a single vortex, a vortex doublet and a vortex pair (in the middle layer) are studied. When a single vortex is considered, the jet has a critical effect of its motion, temporarily slowing down its zonal drift and accelerating it meridionally as the vortex crosses the front. On the contrary, if the vortex does not cross the front, it can drift fairly rapidly along it. The merger of a vortex doublet (two like-signed vortices) below a surface jet is possible whatever the relative position of this doublet with respect to the jet axis. Nevertheless, doublets initially located below the front, will undergo stronger shear and merger efficiency will be diminished. The merged vortex will be circled at the surface by a large meander of the jet. Finally, eastward jet-dipole interaction experiments are performed with various orientations of the vortex dipoles. Eastward propagating dipoles below the jet follow it without deformation. Southeastward drifting dipoles finally join the previous evolution. Southward and southwestward directed dipoles cross the surface jet southeastward. The presence of meanders initially on the jet does not prevent its crossing by a single vortex. Characteristics of the surface jet meanders are also described for a possible remote detection of this process. [ABSTRACT FROM PUBLISHER]

The merger of two identical surface temperature vortices is studied in the surface quasi-geostrophic model. The motivation for this study is the observation of the merger of submesoscale vortices in the ocean. Firstly, the interaction between two point vortices, in the absence or in the presence of an external deformation field, is investigated. The rotation rate of the vortices, their stationary positions and the stability of these positions are determined. Then, a numerical model provides the steady states of two finite-area, constant-temperature, vortices. Such states are less deformed than their counterparts in two-dimensional incompressible flows. Finally, numerical simulations of the nonlinear surface quasi-geostrophic equations are used to investigate the finite-time evolution of initially identical and symmetric, constant temperature vortices. The critical merger distance is obtained and the deformation of the vortices before or after merger is determined. The addition of external deformation is shown to favor or to oppose merger depending on the orientation of the vortex pair with respect to the strain axes. An explanation for this observation is proposed. Conclusions are drawn towards an application of this study to oceanic vortices. [ABSTRACT FROM AUTHOR]

A model for the Zambezi River plume, the largest on the Indian Ocean coast of Africa, is presented and the results of experiments with different discharges and wind forcings are analysed. Although the river plays an important role in the southern African economy through power generation on large dams, artisanal fisheries, and frequent flooding events that impact greatly on local populations, the plume has not been well studied. Observations during the period 2004–2007, when the winds were mainly easterly or south-easterly, indicated that the plume waters can extend both downstream (equatorwards) and upstream (polewards) of the Zambezi Delta with a recirculating bulge near the river mouth. The model is constructed using the Regional Ocean Modeling System (ROMS), with a 40-km long, 3-km wide river discharging into a rectangular coastal ocean with a linearly sloping bottom. When the model is forced only by a constant river discharge of 1 000 m3s−1(typical of observed discharge amounts in summer), the Kelvin and Froude numbers for the resulting plume imply a ‘large-scale’ buoyant discharge with a coastal current that is close to being in geostrophic balance with the across-shore pressure gradient and a recirculating ageostrophic bulge near the mouth. The distributions of the bulge and plume waters are found to be relatively insensitive to the discharge amount. Under constant wind forcing, the plume distribution changes dramatically. Northerly and easterly winds produce the largest changes with the latter able to deflect the plume up to 180° due to Ekman drift. When sea breeze-like winds are imposed, accumulation of water in the bulge occurs with substantial spreading upstream. Stronger sea breezes lead to less downstream spreading of the plume than gentle winds. When the winds are mainly across-shore, Ekman drift dominates, but the dynamics become almost geostrophic when the winds are roughly aligned to the coast. These experiments suggest that the Zambezi River plume is sensitive to the winds on diurnal to synoptic time-scales. [ABSTRACT FROM AUTHOR]

We investigate quasi-geostrophic waves in a rotating spherical shell permeated by an imposed magnetic field. A projection of the momentum equation onto a well chosen class of velocity fields results in a quasi-geostrophic reduced model where, in contrast with previous such models, the evolution of the velocity is not prescribed by an equation for the axial vorticity. We consider fieldsthat may be longitudinally dependent. Increasing the angular rotation frequency, we find that the non-axisymmetric Alfvén waves that are present at low rotation morph into inertial waves, torsional Alfvén waves and low frequency magnetostrophic waves that satisfy Taylor’s constraint (i.e. vanishing acceleration of the geostrophic cylinders by the magnetic forces). [ABSTRACT FROM PUBLISHER]

The evolution of localised jets and periodic nonlinear waves in rotating shallow water magnetohydrodynamics (rotating SWMHD) and standard rotating shallow water model (RSW) is compared within the framework of translationally-invariant 1.5-dimensional configurations, which are traditionally used in geophysical fluid dynamics for studying geostrophic adjustment and frontogenesis. Such configurations also allow for exact nonlinear wave solutions in both models. A theory of the magneto-geostrophic adjustment, i.e. adjustment of an arbitrary initial configuration to a state of magneto-geostrophic equilibrium in RSWMHD, is developed and confirmed by numerical simulations with a finite-volume well-balanced code. The code is resolving all kinds of waves in the model and corresponding weak solutions equally well. It is benchmarked by reproducing exact solutions – steady essentially nonlinear Alfvèn and mixed magneto-inertia-gravity waves – and used to demonstrate robustness of these solutions with respect to localised along-wave perturbations. It is also shown how the results on adjustment can be extended to the fully 2-dimensional case. [ABSTRACT FROM PUBLISHER]

The aim of this study was to understand the variability in the fronts and water masses, and the effect of melt water on the concentration of chlorophyll (Chla) in the Indian Ocean sector of the Southern Ocean using hydrographic data collected during the austral summer (February 2010 and 2011). The Southern Subtropical Front (SSTF) and Northern Sub Antarctic Front (SAF1) were found to be further south at 57°30′E than at 47–48°E. This southward shift of the fronts was consistent with the southward meandering (c. 2°) of the Antarctic Circumpolar Current (ACC) core from the western section to the eastern section, which could have been caused by the bottom topography. The intrusion of water masses also differed between the western and eastern transects of the study region as a result of the meandering of the ACC core. Fresh water layer thickness relative to the winter water in 2011 was more compared to that during 2010. This could have been due to the larger amount of sea ice that was present in the winter of 2010, which subsequently melted, resulting in the advection of melt water from the south and west of the study region. In situ observations and satellite data detected a high Chlaconcentration (c. 0.38 mg m−3) south of the Northern Polar Front (PF1) in 2011, which was caused by this melt water. [ABSTRACT FROM AUTHOR]

The quality of brightness temperatures (TBs) provided by the Soil Moisture and Ocean Salinity (SMOS) satellite is assessed and validated by comparing them with simulatedTBs.TBsimulations are computed using the default transfer model implemented by the European Space Agency (ESA) level 2 ocean salinity processor, with auxiliary data taken from The European Centre for Medium Range Weather Forecasts (ECMWF). The ascending SMOS data of the western Pacific Ocean region (0 N–30 N, 120 E–150 E) in July and August 2012 are analysed, and biases of several kelvin are observed between the averaged SMOSTBand simulatedTB, which strongly affect the accuracy of sea surface salinity (SSS) retrieval. Two methods are proposed in this article to deal with the biases. The first is that the biases are corrected, and the full range of SMOS multiangularTBis used for retrieval. The second is that the biases are not corrected, and onlyTBat angles of incidence in the range of 35–55 are used. Then all measurements of a given Stokes parameter in a pixel are regressed with respect to angles of incidence using a second-order polynomial fit in order to reduce noise in theTB. Finally, a least squares iterative process is used to retrieve SSS using the fittedTB. The accuracy of the retrieved SSS using the above two methods is estimated by comparing them with the Array for Real-Time Geostrophic Oceanography SSS and the ESA level 2 SSS. [ABSTRACT FROM AUTHOR]

The fully compressible semi-geostrophic system is widely used in the modelling of large-scale atmospheric flows. In this paper, we prove rigorously the existence of weak Lagrangian solutions of this system, formulated in the original physical coordinates. In addition, we provide an alternative proof of the earlier result on the existence of weak solutions of this system expressed in the so-called geostrophic, or dual, coordinates. The proofs are based on the optimal transport formulation of the problem and on recent general results concerning transport problems posed in the Wasserstein space of probability measures. [ABSTRACT FROM AUTHOR]

In this paper, we derive and study approximate balance models for nearly geostrophic shallow water flow where the Coriolis parameter is permitted to vary across the domain as long as it remains nondegenerate. This situation includes, for example, the β-plane approximation to the shallow water equations at mid-latitudes. Our approach is based on changing configuration space coordinates in the underlying variational principle in such a way that consistent asymptotics in the transformed Lagrangian leads to a degenerate Lagrangian structure. In this paper, we restrict our attention to first-order models. We show that the resulting models can be formulated in terms of an advected potential vorticity with a nonlinear vorticity inversion relation. We study the associated solvability conditions and identify a subfamily of models for which these conditions are satisfied without additional restrictions on the data. Finally, we provide the link between our framework and the theory of constrained Hamiltonian systems. [ABSTRACT FROM AUTHOR]

The physical and biological environments of the northeast South China Sea (SCS) were investigated using 11 year satellite and reanalysis data, including ocean chlorophyll-a (Chl-a) concentrations, sea surface wind (SSW) values, sea surface temperatures (SSTs), sea surface height anomalies (SSHAs), etc. The findings reveal that ocean Chl-a concentrations west of the Luzon Strait have the most significant annual cycles in the SCS. The dominant forcing mechanisms are monsoon winds and mesoscale eddies studied by multiple regression analysis. In the offshore regions, strong winds directly caused the enhancements by local vertical mixing and entrainment. As in the near-shore regions, the alongshore winds indirectly caused the enhancements by inducing coastal upwelling. Although SST is highly correlated with Chl-a concentration, SST cooling is difficult to observe in the bloom region. It is considered a consequence of monsoon and eddy forcings. Other mechanisms, such as local Ekman pumping by the wind stress curl and the geostrophic potential vorticity, have little effect on Chl-a seasonal variations. [ABSTRACT FROM AUTHOR]

The surface geostrophic currents (SGC) can be derived via the principle of geostrophy from the dynamic height of the ocean, or the deviation of the true, variable sea surface height with respect to the Earth's static geoid, both of which can be measured by geodetic means. Here we calculate the Mean Dynamic Topography (MDT) by subtracting the geoid height determined by the GOCE satellite mission from the Mean Sea Surface Topography (MSST) derived from multi-satellite ocean altimetry (T/P, Jason 1/2, ERS-1/2, GEOSAT). Results for SGC are compared with those obtained from a GRACE-based mean geoid, as well as with the mean circulation patterns from measurements done byin situdrifter buoys and from simulations of the ECCO Ocean General Circulation Model. We found GOCE-based geoid solution clearly leads to significant improvements in the spatial resolution of SGC globally except in the Equatorial band where special filtering may be needed, with current velocities and spatial patterns closest to thein situmeasurements of currents, compared with the GRACE-based results or ECCO model simulations that give significantly weaker Values with lower spatial resolution. [ABSTRACT FROM AUTHOR]

We examine the equilibrium form, properties, stability and nonlinear evolution of steadily-rotating simply-connected vortex patches in the single-layer quasi-geostrophic model of geophysical fluid dynamics. This model, valid for rotating shallow-water flow in the limit of small Rossby and Froude numbers, has an intrinsic length scale L D called the “Rossby deformation length” relating the strength of the stratification to that of the background rotation. Here, we generate steadily-rotating vortex equilibria for a wide range of γ = L/L D , where L is the typical horizontal length scale of the vortex. We vary both γ (over the range 0.02 ≤ γ ≤ 10) and the vortex aspect ratio λ (over the range 0 < λ < 1). We find two modes of instability arising at sufficiently small aspect ratio λ < λ c (γ): an asymmetric (dominantly wave 3) mode at small γ (or large L D ) and a symmetric (dominantly wave 4) mode at large γ (or small L D ). At marginal stability, the asymmetric mode dominates for γ ≲ 3, while the symmetric mode dominates for γ ≳ 3. The nonlinear evolution of weakly-perturbed unstable equilibria results in major structural changes, in most cases producing two dominant vortex patches and thin, quasi-passive filaments. Overall, the nonlinear evolution can be classified into three principal types: (1) vacillations for a limited range of aspect ratios λ when 5 ≤ γ ≤ 6, (2) filamentation and a single-dominant vortex for γ ≲ 1, and (3) vortex splitting – asymmetric for 1 ≲ γ ≲ 4 and symmetric for γ ≳ 4. [ABSTRACT FROM AUTHOR]

This work addresses the linear dynamics underlying the formation of density interfaces at the periphery of energetic vortices, well outside the vortex core, both in the radial and axial directions. We compute numerically the unstable modes of an anticyclonic Gaussian vortex lens in a continuously stratified rotating fluid. The most unstable mode is a slow mode, associated with a critical layer instability located at the vortex periphery. Although the most unstable disturbance has a characteristic vertical scale which is comparable to the vortex height, interestingly, the critical levels of the successively fastest growing modes are closely spaced at intervals along the axial direction that are much smaller than the vortex height. [ABSTRACT FROM PUBLISHER]

The linear baroclinic and parametric instabilities of boundary currents with piecewise-constant potential vorticity are studied in a two-layer quasi-geostrophic model. The growth rates of both the exponential modes and of the optimal perturbations are calculated for the baroclinic instability of steady coastal currents. We show that the growth rates of the exponential modes are maximal for a vertically symmetric flow. Furthermore, the vertical asymmetries induced by different layer thicknesses, the presence of a barotropic potential vorticity or bottom topography, all act to dampen the growth rates and favor growth at shorter wavelengths. It is shown that this behavior can be predicted from the conditions for vertical resonance of Rossby waves on the two potential vorticity fronts. Also, the baroclinic instability of the optimal perturbations has larger growth rates at shorter wavelengths and shorter time scales. As well, the presence of a sloping bottom of moderate amplitude favors the growth of these optimal perturbations. Finally, we compute the growth rates of parametric instability of oscillatory coastal flows. We show that subharmonic resonance is the most unstable mode of growth. In addition, a second region of parametric instability is found (for the first time) away from marginality of exponential-mode baroclinic instability. It is shown that the functional dependency of the growth rates of parametric instability, for optimal excitation, are similar to that of the optimal perturbations of baroclinic instability. To explain this a mechanism for parametric instability, involving the rapid growth of short-wave optimal perturbations, is proposed. [ABSTRACT FROM AUTHOR]

An elliptical region having a particular distribution of anomalous buoyancy or temperature at the surface of an otherwise unbounded rotating stratified fluid is shown to steadily rotate under the quasi-geostrophic approximation. The particular distribution is proportional to the vertical thickness of an ellipsoid, divided by its mean thickness, in the limit of vanishing thickness. The steady rotation of this structure or vortex is assured by the known steady rotation of any ellipsoid, and can be obtained by an appropriate limit. It is found by numerical experimentation that this vortex is stable if its minor to major aspect ratio λ exceeds 0.427, approximately. Notably, a two-dimensional elliptical vortex (having uniform vorticity) is stable for λ > 1/3. Instabilities of surface vortices are characterised by the ejection of a weak tongue of buoyancy, which subsequently rolls up into a street of weak vortices. The main vortex is thereby reduced in aspect ratio and remains robust for long times. [ABSTRACT FROM AUTHOR]

The influence of external flow field on the statistical equilibrium state of quasi-geostrophic point vortices (vortex cloud) is investigated numerically. The numerical computations are performed using the fast special-purpose computer for molecular dynamics simulations, MDGRAPE-3. The equilibrium state in otherwise quiescent fluid is axisymmetric, whose radial distribution depends on both the vertical distribution of vortices P(z) and the total energy of the vortex system E. At a certain critical energy value Ec, the number of microscopic state with a given angular momentum attains its maximum (zero-inverse temperature state), where the radial distribution is Gaussian at any vertical height. When the energy is smaller (E < Ec: positive temperature), the radial distribution becomes flatter than the Gaussian. In contrast, if the energy is higher (E > Ec: negative temperature), the radial distribution becomes sharper showing tighter concentration near the axis of symmetry. If an equilibrium vortex cloud of positive temperature is immersed in the horizontal strain field Ue = ey, Ve = ex, the vortex distribution is stretched in the y-direction, and the azimuthally averaged radial distribution becomes Gaussian-like. Similarly, when the equilibrium state of positive temperature is immersed in the vertical shear field Uτ = τz, Vτ = 0, the vortex cloud is tilted in the y-direction, and the radial distribution becomes Gaussian-like. These findings explain how the internal vorticity distributions inside interacting vortex clouds of positive temperature change to be nearly zero-inverse temperature state. [ABSTRACT FROM AUTHOR]

We formulate a dynamically consistent two-layer quasigeostrophic model of geophysical flow using the concept of background currents which are characterized by a constant potential vorticity minimizing the energy. An incident current over a delta-like isolated topographic feature generates a topographically trapped vortex in the bottom layer with a singular elliptic point, and one with a regular elliptic point in the upper layer. Such vortices are finite regions of recirculation which occur in the vicinity of isolated topographic features. The corresponding Hamiltonian equations of motion for a fluid particle are known to produce chaotic advection under the presence of the periodic incident current. When a periodic incident current is superimposed, fluid is entrained and detrained from the neighbourhood of the vortex and chaotic particle motion occurs. At a small amplitude periodic incident current we have a near-integrable case of weak chaotization in narrow stochastic layer near separatrix. At a finite amplitude periodic incident current we have the case of strong chaos, which will be investigated here. For the bottom layer of the flow, there is always a regular region (regular vortex core) around the singular elliptical point even in the case of a finite amplitude periodic incident current. Particles in this regular region have regular trajectories and will remain there permanently, whereas particles with chaotic trajectories will be emanated from the vortex by the incident current. Using the Chirikov criterion of resonance overlap, we estimate the radius of this regular vortex core and a range of the optimal frequencies, i.e. those frequencies of the incident current which provide a maximal possible stochastization of fluid particle trajectories. [ABSTRACT FROM AUTHOR]

An analytical dispersion relation is derived for linear perturbations to a Rankine vortex governed by surface quasi-geostrophic dynamics. Such a Rankine vortex is a circular region of uniform anomalous surface temperature evolving under quasi-geostrophic dynamics with uniform interior potential vorticity. The dispersion relation is analysed in detail and compared to the more familiar dispersion relation for a perturbed Rankine vortex governed by the Euler equations. The results are successfully verified against numerical simulations of the full equations. The dispersion relation is relevant to problems including wave propagation on surface temperature fronts and the stability of vortices in quasi-geostrophic turbulence. [ABSTRACT FROM AUTHOR]

Satellite altimeter sea surface height anomaly (SSHA) data accuracy and applications are rapidly maturing over the open ocean, but more attention is required where continental shelves and marginal seas are concerned. Refined radar range measurement correction approaches are needed to enhance the quality of coastal altimeter SSHA data and to enhance the potential impact that altimeter data can play in improved ocean circulation modeling and prediction. This study provides a regional assessment of satellite altimeter SSHA data from the Jason-2, Jason-1, and TOPEX/Poseidon data sets collected over the Northwest Atlantic coastal region of the United States. The validation approach follows recent studies in numerous shelf regions to address changes from standard or default data quality flagging scheme that can enhance satellite altimeter SSHA data recovery, optimal choice of de-aliasing corrections in deriving subtidal sea level time series, and validation of altimeter data using in situ tide gauge and current measurement data sets. Spatial analyses show the impact of data filtering and corrections from the coastline out onto the slope. Revised handling of the data provides a 5–10% increase in availability (from about 70% up to 80% for waters beyond 50 km from shore). The RMS differences between tide gauge and surface layer current measurements at the 60 day time scale are 3–4 cm and 6 cm/s, respectively, and of magnitude consistent with several recent coastal altimeter data validation efforts. [ABSTRACT FROM PUBLISHER]

The determination of long-term trends in the behavior of the Gulf Stream is critical to understanding the meridional heat transfer cycle and associated global climate change. In this study, in situ current measurements from cross-stream transects of an Acoustic Doppler Current Profiler (ADCP) were used in conjunction with satellite altimetry data to obtain accurate estimates of Gulf Stream surface geostrophic current within the South Atlantic Bight (SAB) between 1992 and 2004. Analysis of time series data reveals that the magnitude of the geostrophic flow varies seasonally and that the seasonally averaged velocity was greatest following the intense 1997-1998 El Nino. Because of the relatively small fluctuations in the Gulf Stream's path, our analysis provides a method for accurate, long-term measurements of Gulf Stream currents for this region of the SAB. [ABSTRACT FROM AUTHOR]

Geostrophic currents were calculated at the surface and subsurface levels in the Indian Ocean using satellite altimetry data from TOPEX/POSEIDON and Jason-1, a geoid obtained from the GRACE mission, and historical hydrographic profiles from the World Ocean Atlas 2005. The results of this method were validated with currents measured using Acoustic Doppler Current Profilers moored along the equator at 77°E, 83°E, and 93°E. The measured and computed currents compared reasonably well with similar features present in both. The presence of the seasonal Wyrtki Jets and Equatorial Undercurrent and their variability from 2000-2001 was observed in the estimated currents. [ABSTRACT FROM AUTHOR]

Two mean dynamic topography (MDT) fields are determined in the Fram Strait between Svalbard and Greenland. New airborne gravity anomalies, older data, and two different mean sea surface (MSS) fields are combined using the least squares collocation (LSC) technique. The results are compared to an oceanographic MDT model and two synthetic MDT fields. The same main currents are seen in all fields. Additionally, smaller scale features are revealed in the new MDT fields. Geostrophic surface currents derived from the MDT models are compared to moorings and Lagrangian drifters. The agreement is desultory. The oceanographic data are an inadequate basis of comparison due to data gaps. Nevertheless, it is the only one available. [ABSTRACT FROM AUTHOR]

The solution for the bottom Ekman layer has a somewhat counter intuitive character, which seems to violate the maximum principle: at a certain level the velocity within the Ekman layer is higher than the velocity in the geostrophic layer above. I explain this character by looking at an analogous problem in an inertial frame of reference and show that it is the result of observing the flow from a rotating frame of reference (i.e. within a system that is not in steady state). The flow in the bottom Ekman layer is a superposition of the flow that results from the force exerted on the fluid by the rotating Earth and of the flow that results from the pressure-gradient term. Therefore, at a certain level the speed is higher than the speed of the geostrophic layer above which results from the pressure gradient alone. [ABSTRACT FROM AUTHOR]

Utilizing an eigenfunction decomposition, we study the growth and spectra of energy in the vortical (geostrophic) and wave (ageostrophic) modes of a three-dimensional (3D) rotating stratified fluid as a function of ε = f/N, where f is the Coriolis parameter and N is the Brunt-Vaisala frequency. Throughout, we employ a random large-scale forcing in a unit aspect ratio domain and set these parameters such that the Froude and Rossby numbers are roughly comparable and much less than unity. Working in regimes characterized by moderate Burger numbers, i.e. Bu = 1/ε2 < 1 or Bu ≥ 1, our results indicate profound change in the character of vortical and wave mode interactions with respect to Bu = 1. Indeed, previous analytical work concerning the qualitatively different nature of these interactions has been in limiting conditions of rotation or stratification domination (i.e. when Bu << 1 or Bu >> 1, respectively). As with the reference state of ε = 1, for ε < 1 the wave mode energy saturates quite quickly and the ensuing forward cascade continues to act as an efficient means of dissipating ageostrophic energy. Further, these saturated spectra steepen as ε decreases: we see a shift from k-1 to k-5/3 scaling for kf < k < kd (where kf and kd are the forcing and dissipation scales, respectively). On the other hand, when ε > 1 the wave mode energy never saturates and comes to dominate the total energy in the system. In fact, in a sense the wave modes behave in an asymmetric manner about ε = 1. With regard to the vortical modes, for ε ≤ 1, the signatures of 3D quasigeostrophy are clearly evident. Specifically, we see a k-3 scaling for kf < k < kd and, in accord with an inverse transfer of energy, the vortical mode energy never saturates but rather increases for all k < kf. In contrast, for ε > 1 and increasing, the vortical modes contain a progressively smaller fraction of the total energy indicating that the 3D quasigeostrophic subsystem, though always present, plays an energetically smaller role in the overall dynamics. Combining the vortical and wave modes, the total energy for k > kf and ε ≤ 1 shows a transition as k increases wherein the vortical modes contain a large portion of the energy at large scales, while the wave modes dominate at smaller scales. There is no such transition when ε > 1 and the wave modes dominate the total energy for all k > kf. [ABSTRACT FROM AUTHOR]

We have investigated sea level and surface currents features over the Newfoundland Shelf and Slope using the tandem TOPEX/Poseidon (T/P) and Jason altimetry data (2002-2003). The consistency and error characteristics of T/P and Jason measurements are examined not only in terms of sea level and cross-track current anomalies but also with respect to current anomalies at crossovers and the Labrador Current transport. Nominal absolute currents are constructed by adding the altimetric geostrophic current anomalies to a climatological-mean model circulation field. The comparison of the sea level and cross-track current anomalies from January to July 2002 shows overall good agreement between T/P and Jason, with correlation coefficients different from zero at the 95% confidence level at almost all locations for sea level and at most locations for currents. Errors are estimated to be 2.5 cm for sea level and 10 cm/s for cross-track current anomalies. Analyses of the current variability at crossovers indicate approximate agreement of T/P and Jason measurements, except for the Northeastern Newfoundland Shelf and Slope probably due to the ice presence during the period. Model-altimetry combined absolute currents are used to estimate near-surface transport associated with the shelf-edge Labrador Current, showing good correlation between T/P and Jason estimates and strong seasonal changes. The cross-track geostrophic current anomalies from September 2002 to December 2003 are used to calculate the root-mean-square (rms) current variability at crossovers and to derive the shelf-edge Labrador Current. The interleaved T/P and Jason observations can better capture the spatial distribution of shelf and slope circulation variability. [ABSTRACT FROM AUTHOR]

The plume rich with chlorophyll-a concentration (>0.3 mg/l) observed on 22 March 2003 along the 16° N in the western Bay of Bengal off the Krishna-Godavari river delta was studied. Relatively high sea surface temperature (>30° C) observed in the plume area indicated their origin in the coastal waters. The bloomy plume was found spreading 400 to 500 km offshore in the form of an offshore jet extending as far as 86° E in the Bay of Bengal. An offshore flow was observed with geostrophic velocity exceeding 50 cm/s with a cyclonic eddy on its north around 17° N, 82° E and an anti-cyclonic eddy around 14° N, 83° E to its south. The hydrographic data of the area were studied with the observations made by GTS data buoy indicated coastal upwelling. Similar plumes were also observed during the years 2004 and 2005. [ABSTRACT FROM AUTHOR]

Geostrophic dynamics in the horizontally differential super-rotation of Venus are examined using f A (the Coriolis parameter defined by the angular velocity of a basic flow in an inertial frame) and Γ (the differential rotation parameter defined by the latitudinal gradient of the angular velocity) under the conditions that vertical shear of the basic field is not considered and the intrinsic phase velocity has a magnitude comparable to that of an eddy horizontal flow. The geostrophic and quasi-geostrophic approximations are valid in the regions of weakly differential and rigid-body super-rotations; however, they are invalid in the regions of strongly differential super-rotation even when the Rossby number R O is sufficiently smaller than unity for synoptic eddies. In a general circulation model of a Venus-like atmosphere, the horizontal divergence that results from the strong differential cannot be ignored over a wide range within latitudes ±60° and below 60&ThickSpace;km elevation because of large Γ/ f A (≥1/2). [ABSTRACT FROM AUTHOR]