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Oceanographic processes play a key role in influencing the structure of the marine planktonic ecosystems. Taking advantage of the quasi-simultaneous collection of a large ichthyoplanktonic dataset in different regions of the Central Mediterranean Sea (Italian/Maltese, Tunisian and Libyan waters), this study aimed at the identification of the main environmental drivers that control the structure of the larval fish assemblages. Spatial distribution and taxa composition were related to physical forcings (geostrophic currents and wind stress) and environmental conditions (bottom depth, temperature, salinity, chlorophyll-a concentration). ANOSIM and SIMPER identified contribution of fish taxa to the average Bray-Curtis dissimilarity among regions. In Italian and Libyan waters, two assemblages (neritic and oceanic) were identified, while a mixed assemblage characterized only some stations. Two neritic and one oceanic assemblages were discriminated in Tunisian waters. Random Forest classification model highlighted the essential role of the bathymetry, while Lagrangian simulations evidenced the action of the hydrodynamics in mixing neritic and oceanic assemblages in the Italian/Maltese and partially in Libyan waters. These findings highlighted the importance of the multidisciplinary approach and shed light on the potential value of the ichthyoplanktonic surveys for the assessment of the state of the marine ecosystem and the conservation of the fishery resources. [ABSTRACT FROM AUTHOR]

Six years of surface velocities have been calculated over the global ocean by combining altimeter derived geostrophic velocities and Sea Surface Temperature (SST) data. The method is based on the inversion for the velocity of the heat conservation equation using the altimeter velocities as background. By accurately prescribing the error both on the background velocities and on the forcing term (heat fluxes), the altimeter background velocities are successfully improved in areas characterized by strong SST gradients and remain unchanged in low SST gradient regions, where by construction no additional information may be brought by the SST field. This allows us to provide for the first time a global field of surface velocities from the merging of altimetry and SST. Both the spatial and temporal resolution of the altimeter derived velocities is enhanced by the SST information. Validation is performed through comparison to in-situ drifting buoy velocities. Major improvements (10–20% globally, up to 35% locally) are obtained on the meridional component of the velocity and in the equatorial band. Due to the large uncertainty on the heat fluxes and the spatio-temporal resolution of the input datasets, the high frequency ageostrophic components of the circulation is hardly resolved by the method and, outside the equatorial band, improvements are mainly on the geostrophic component. It is shown that the level of accuracy obtained by combining altimeter velocities based on a two satellite configuration and microwave SST data is equivalent or higher to the one from a four altimeter constellation in western boundary currents. This opens the perspective for a systematic use of the method to improve the altimeter derived surface velocities over the 1993–2001 period, for which only 2 altimeters have been flying simultaneously. Finally, over the recent years, the use of higher resolution SST products based on both microwave and infrared measurements leads to further enhancement of the accuracy of the altimeter velocities on both the zonal and the meridional component in strong SST gradients areas. [ABSTRACT FROM AUTHOR]

Schemes of surface flows near northwestern Kamchatka are shown, compiled from in situ observation data, numerical-model-derived data, satellite altimetry measurement data, and results calculated by the dynamic method. The flow characteristics obtained by various methods are compared. No statistically significant linear dependence of the velocities of absolute and geostrophic currents were detected. A linear dependence was observed between the directions of geostrophic flows calculated by the standard dynamic method and from satellite altimetry data, as well as between directions measured by the Argonaut MD and the calculated model. We have estimated the fraction of the ageostrophic component in currents. According to geostrophic current calculations by different methods, it ranged from 86 to 93%. A significant limitation of this data is the difficulty of their interpretation. They may not always give a perfect representation of stable water circulation in the studied area under. [ABSTRACT FROM AUTHOR]

The meridional geostrophic transport (MGT) in the interior tropical North Pacific Ocean is estimated based on global ocean heat and salt content data. The decadal variations of the zonally and vertically integrated MGT in the tropical North Pacific Ocean are found to precede the Pacific decadal oscillation (PDO) by 1–3 years. The dynamics of the MGT are analyzed based on Sverdrup theory. It is found that the total meridional transport variability (MGT plus Ekman) is dominated by the MGT variability having positive correlations with the PDO index. The Sverdrup transports differ from the total meridional transport significantly and have insignificant correlations with PDO index, suggesting that the MGT variability is not controlled by the Sverdrup dynamics. In comparison, the simulated meridional transport variability in the models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and the Ocean General Circulation Model for the Earth Simulator are dominated by the Sverdrup transports, having insignificant correlations with the simulated PDO indices. The comparison suggests that the non-Sverdrup component in the MGT is important for the predictability of PDO and that significant deficiencies exist in these models in simulating a realistic structure of the tropical ocean gyre variability and predicting the decadal climate variations associated with it. [ABSTRACT FROM AUTHOR]

Abstract: Observations, numerical simulations, and theoretical scaling arguments suggest that in mid‐latitudes, away from the high‐latitude source regions and the equator, the meridional transport of abyssal water masses along a continental slope correspond to geostrophic flows that are gravity or density driven and topographically steered. These dynamics are examined using a nonlinear reduced‐gravity geostrophic model that describes grounded abyssal meridional flow over sloping topography that crosses the planetary vorticity gradient. It is shown that this model possesses a noncanonical Hamiltonian formulation. General nonlinear steady solutions to the model can be obtained for arbitrary bottom topography. These solutions correspond to nonparallel shear flows that flow across the planetary vorticity gradient. If the in‐flow current along the poleward boundary is strictly equatorward, then no shock can form in the solution in the mid‐latitude domain. It is also shown that the steady solutions satisfy the first‐order necessary conditions for an extremal to a suitably constrained potential energy functional. Sufficient conditions for the definiteness of the second variation of the constrained energy functional are examined. The theory is illustrated with a nonlinear steady solution corresponding to an abyssal flow with upslope and down slope groundings in the height field. [ABSTRACT FROM AUTHOR]

Several fast, narrow boundary currents flow along the eastern margin of New Zealand forming part of the western boundary current system of the South Pacific. In this study, using over 20 years of satellite altimeter observations and in situ data, we investigate the mean and variability of the current transports at seasonal, interannual and decadal time scales. We relate the means and fluctuations of the currents to the Southern Oscillation Index and to potential drivers of the circulation, including local and basin-scale winds. We also investigate the barotropic and baroclinic responses of the currents to basin-scale wind forcing using Rossby wave models. We find the currents are highly variable and there are no discernible trends in the transports since 1993. There is little coherence between transports along the boundary, suggesting a range of mechanisms drive the variability of the currents individually rather than the system as a whole. There is little correspondence between transports in any of the boundary currents and the El Niño/Southern Oscillation. We also find that the transports are not well described by the arrival of linear Rossby waves. At decadal time scales, only transports in the Subantarctic Front show a close correspondence with the variability of the wind stress curl averaged over the South Pacific. Seasonality is significant in two of the currents, likely driven by seasonality in local winds. In contrast with the larger East Australian Current, which forms part of the same South Pacific western boundary current system, we see collectively little evidence of a link between trends and variability in the New Zealand boundary currents and large-scale or local wind forcing. [ABSTRACT FROM AUTHOR]

Since the 1960s, the Instituto del Mar del Perú (IMARPE) collected tens of thousands of in-situ temperature and salinity profiles in the Northern Humboldt Current System (NHCS). In this study, we blend this unique database with the historical in-situ profiles available from the World Ocean Database for the period 1960–2014 and apply a four-dimensional interpolation scheme to construct a seasonal climatology of temperature and salinity of the NHCS. The resulting interpolated temperature and salinity fields are gridded at a high spatial resolution (0.1° × 0.1° in latitude/longitude) between the surface and 1000 m depth, providing a detailed view of the hydrology and geostrophic circulation of this region. In particular, the mean distribution and characteristics of the main water-masses in the upper ocean of the NHCS are described, as well as their seasonal variations between austral summer and winter. The coastal upwelling region is well documented due to the increased data density along 3 highlighted cross-shore vertical sections off Paita (∼5°S), Chimbote (∼9°S) and San Juan (∼16.5°S). The large and long-term database also allowed us, through a composite analysis, to investigate the impact of the eastern Pacific El Niño and La Niña events on the NHCS hydrology. On average, during these periods, large temperature (±3–4 °C) and salinity (±0.1–0.2) anomalies are observed, impacting the water column of the coastal ocean off Peru down to 100–200 m depth. At 100 km from the coast, these anomalies are associated with a maximum deepening (shoaling, respectively) of the thermocline of 60 m (25 m) during composite El Niño (La Niña) events. At interdecadal scale, a similar approach reveals sea-surface temperature variations of ±0.5°C, associated with a deepening (shoaling) of the thermocline of 5–10 m during warm (cold) periods. [ABSTRACT FROM AUTHOR]

This study solves two-dimensional (cross-shelf and depth directions) steady-state nonlinear primitive equations to infer the cross-shelf circulation induced by the Kuroshio shear stress in the East China Sea (ECS). The Kuroshio velocity data are estimated from hydrographic observations at the PN section in the ECS. Nonlinear momentum equations are solved using an iterative approach in a terrain-following coordinate system, which helps to adequately take into account the boundary conditions over complex topography. The vertical shear stress of the Kuroshio is shown to induce two offshore transport pathways over the continental shelf, which are related to the structure of the interior geostrophic current and bottom Ekman transport, respectively. As a result of the vertical shear stress, an upwelling is induced above the bottom Ekman layer on the continental slope. The horizontal shear stress of the Kuroshio has the effect of inducing onshore transport at the flow core. The advection terms in the primitive equations are found to amplify the cross-shelf velocity solved from the linear equations. This study reveals that the Kuroshio has a substantial effect on the cross-shelf circulation and that it might drive multiple transport pathways. [ABSTRACT FROM AUTHOR]

Numerical ocean circulation forecasts diverge from reality due to non-deterministic processes. Through data assimilation, ocean forecasting systems routinely make corrections to scalar ocean variables of temperature and salinity derived from ocean observations. Velocity observations, however, are vector quantities that require different processing. To assimilate existing ocean velocity observations, which generally are depth limited, vertical error covariances are needed to relate the velocity errors throughout the water column. In this paper we show a velocity assimilation approach that relies on historical observations and geostrophy to relate velocity, temperature, and salinity. Geopotential calculations transform temperature and salinity covariances into corresponding geopotential covariances. This paper describes the methods and examines details of the solution process for ocean velocity data assimilation. The approach employs two separate three-dimensional variational (3DVAR) analyses combined to create one set of initial conditions during each daily forecast cycle. The first 3DVAR analysis utilizes temperature, salinity, and sea surface height anomaly data and reduces the background error variance based on these observations. The second 3DVAR analysis uses the reduced error variance and performs velocity data assimilation using surface drifter data, and these observations further reduce the error variance of temperature and salinity for the next cycle. The second 3DVAR analysis for velocity data assimilation utilizes the new full depth error covariances. We show diagnostic results from a Gulf of Mexico simulation during extensive drifter deployments in the summer of 2020. Results reveal that the two-step data assimilation cycle applies vector velocity innovations to create increments that correct the background field towards a geostrophic balance in temperature and salinity. • Surface drifter positions create Eulerian innovations for assimilation. • Empirically derived vertical covariances distribute velocity increments. • Velocity forecast error is derived from empirical vertical covariances. • Velocity increments constrain forecasted geostrophic velocities. [ABSTRACT FROM AUTHOR]

Western boundary currents (WBCs) are highly variable narrow meandering jets, making assessment of their volume transports a complex task. The required high-resolution temporal and spatial measurements are available only at a limited number of sites. In this study a method is developed for improving estimates of the East Australian Current (EAC) mean transport and its low-frequency variability, using complementary modern datasets. The present calculation is a case study that will be extended to other subtropical WBCs. The method developed in this work will reduce uncertainties in estimates of the WBC volume transport and in the interannual mass and heat budgets of the meridional overturning circulations, improving our understanding of the response of WBCs to local and remote forcing on long time scales. High-resolution expendable bathythermograph (HR-XBT) profiles collected along a transect crossing the EAC system near Brisbane, Australia, are merged with coexisting profiles and parking-depth trajectories from Argo floats, and with altimetric sea surface height data. Using HR-XBT/Argo/altimetry data combined with Argo trajectory-based velocities at 1000 m, the 2004–15 mean poleward alongshore transport of the EAC is 19.5 ± 2.0 Sv (1 Sv ≡ 106 m3 s−1) of which 2.5 ± 0.5 Sv recirculate equatorward just offshore of the EAC. These transport estimates are consistent in their mean and variability with concurrent and nearly collocated moored observations at 27°S, and with earlier moored observations along 30°S. Geostrophic transport anomalies in the EAC system, including the EAC recirculation, show a standard deviation of ±3.1 Sv at interannual time scales between 2004 and 2015. [ABSTRACT FROM AUTHOR]

Zooplankton is an important component to understand oceanographic dynamics, they are considered good indicators of environmental variability, since most species are short-lived they can be tight coupling of climate and population dynamics, and some taxa are sensitive to variables such as temperature, quality and quantity of food, oxygenation and stratification. In the Colombian Caribbean two oceanographic cruises were carried out, in order to characterize and analyze the biodiversity of the continental margin, for the periods May-June 2008 and November-December 2009. The objective of this study was to determine which oceanographic variable was the most important in the distribution of zooplankton in the oceanic waters, and how the variables structure the community. In each cruise, zooplankton samples were obtained by vertical hauls with a 1 m2 diameter conical net (200 µm mesh size). The zooplankton community structure was studied by automated image analysis (ZooImage), which combined with abundance data, allowed to calculate the slope of size spectrum, and diversity indexes of taxa and sizes. The environmental variables included temperature, salinity, oxygen and chlorophyll a of the water column, and were used to calculate the stratification with the Brunt-Vaiisala frequency; this information was complemented by satellite images of temperature, surface chlorophyll, geostrophic currents and sea level, obtained from MODIS and AQUA products. The association between environmental variables and the zooplankton community structure was determined using Spearman's correlation ranges with the Bioenv routine and a stepwise regression model, and Principal component analysis (PCA). Our results showed spatial and temporal patterns in the oceanographic conditions of the study area, such as upwelling in the Northeast region, strong stratification in the Southwest, and mesoscale activity. The abundance of zooplankton exhibited differences between the two climatic seasons and zones. Temporal differences were also evident in the size and diversity of taxa (ANOVA 2-way p <0.05). These changes were related to the moderate upwelling and the activity of mesoscale eddies. Cyclonic and anticyclonic eddies of variable size can either transport or retain zooplankton over cross-shelf and oceanic areas, thus modifying the taxonomic structure of the community. According to the Bioenv analysis, the main variables that explained the abundance and composition of zooplankton were sea level and chlorophyll a (Spearman correlation = 0.49). The stepwise regression showed that stratification, oxygen and chlorophyll a were the most important predictors of zooplankton abundance and size. [ABSTRACT FROM AUTHOR]

According to historical oceanographic data, anomalies in the dynamic topography during the winter period were calculated and two climatic stages of dynamic condition of the Arctic Ocean were defined: 1949-1993 and 2007-2013. The associativity of opposition of anomalies in the dynamic topography of the Eurasian and Amerasian basins with fluctuations in the thermal condition of the Northern Atlantic and Pacific oceans, the indexes of atmospheric circulation, river runoff, and change in the area of ice in August of the Laptev, East Siberian, and Chukchi seas is established. [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]

An analysis of water level time series from 20 tide gauges in Southeast Asia finds that diurnal and semidiurnal astronomical tides exhibit strong seasonal variability of both amplitude and phase, which is not caused by known modulations of the astronomical tide-generating forces. Instead, it is found that the tidal properties are coherent with the western North Pacific monsoon index (WNPMI), indicating that monsoonal mechanisms are the likely cause. The study domain includes the Malacca Strait, Gulf of Thailand, the southern SouthChina Sea, and Java Sea. The character of the geography and the tidal variability is different in each of these subregions. A new barotropic regional tide model is developed that incorporates the coupling between geostrophic currents, wind-driven (Ekman) currents, and tidal currents in the bottom boundary layer in order to examine the influence of these factors on tides. The dynamics thus preserve the frictional nonlinearities while neglecting advective nonlinearities and baroclinic tides, approximations that should be valid on the wide and shallow continental shelves in the study region. Themodel perturbation approach uses the climatological seasonal variability of wind stress and geostrophic currents, which are prescribed singly and in combination in the model, to explain the observed tidal variability. Results are most successful in the southern Gulf of Thailand and near Singapore, where it is found that the combined effect of geostrophic and Ekman currents shows increased skill in reproducing the tidal variability than individual models. Ambiguous results at other locations suggest more localized processes such as river runoff. [ABSTRACT FROM AUTHOR]

Submesoscale oceanic density fronts are structures in geostrophic and hydrostatic balance, which are prone to inertial and/or symmetric instabilities. We argue in this article that drainage of potential energy from the geostrophic flow is a significant source of their growth. We illustrate our point with two-dimensional Boussinesq numerical simulations of oceanic density fronts on the f plane. A set of two-dimensional initial conditions covers the submesoscale portion of a three-dimensional parameter space consisting of the Richardson and Rossby numbers and a measure of stratification or latitude. Because we let the lateral density gradient decay with depth, the parameter space map is nontrivial, excluding low-Rossby, low-Richardson combinations. Dissipation and the presence of boundaries select a growing mode of inertial-symmetric instability consisting of flow cells that disturb isopycnal contours. Systematically, these isopycnal displacements correspond to a drainage of potential energy from the geostrophic fronts to the ageostrophic perturbations. In the majority of our experiments, this energy drainage is at least as important as the drainage of kinetic energy from the front. Various constraints, some physical, some numerical, make the energetics in our experiments more related to inertial rather than symmetric instabilities. Our results depend very weakly on the Richardson number and more on the Rossby number and relative stratification. [ABSTRACT FROM AUTHOR]

Abstract The synoptic monthly gridded (SMG) world ocean database (WOD) were constructed with 1° × 1° horizontal resolution and 28 standard vertical levels from the surface to 3000 m deep using the optimal spectral decomposition technique. Monthly acceleration potential (AP) relative to 1000-dbar is calculated for the isopycnal surface 26.4 kg m−3 from January 1960 to December 2014 to investigate the spatial and temporal variability of the California Current. A composite analysis was conducted to obtain the total-time mean AP field and the climatological monthly mean AP variability, which is two orders of magnitude smaller than the total-time mean. Residual data were used to examine interannual variations of the AP field. An empirical orthogonal function (EOF) analysis was conducted to analyze the AP anomaly data. The EOF-1 (40% variance) represents strengthening/weakening of the North Pacific Gyre and California Current. EOF-2 (20% variance) represents onshore/offshore geostrophic flow. The first principal component was negatively correlated with the PDO; the second principal component was negatively correlated with both the PDO and ENSO, which implied that the AP anomaly contains climate variability signals. [ABSTRACT FROM AUTHOR]

Abstract: A physical‐biogeochemical model is used to produce a retrospective analysis at 3‐km resolution of alongshore phytoplankton variability in the California Current during 1988–2010. The simulation benefits from downscaling a regional circulation reanalysis, which provides improved physical ocean state estimates in the high‐resolution domain. The emerging pattern is one of local upwelling intensification in response to increased alongshore wind stress in the lee of capes, modulated by alongshore meanders in the geostrophic circulation. While stronger upwelling occurs near most major topographic features, substantial increases in phytoplankton biomass only ensue where local circulation patterns are conducive to on‐shelf retention of upwelled nutrients. Locations of peak nutrient delivery and chlorophyll accumulation also exhibit interannual variability and trends noticeably larger than the surrounding shelf regions, thereby suggesting that long‐term planktonic ecosystem response in the California Current exhibits a significant local scale (O(100 km)) alongshore component. [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]

We investigate the nature of the dynamo bifurcation in a configuration applicable to the Earth’s liquid outer core, i.e. in a rotating spherical shell with thermally driven motions with no-slip boundaries. Unlike in previous studies on dynamo bifurcations, the control parameters have been varied significantly in order to deduce general tendencies. Numerical studies on the stability domain of dipolar magnetic fields found a dichotomy between non-reversing dipole-dominated dynamos and the reversing non-dipole-dominated multipolar solutions. We show that, by considering weak initial fields, the above transition disappears and is replaced by a region of bistability for which dipolar and multipolar dynamos coexist. Such a result was also observed in models with free-slip boundaries in which the geostrophic zonal flow can develop and participate to the dynamo mechanism for non-dipolar fields. We show that a similar process develops in no-slip models when viscous effects are reduced sufficiently. The following three regimes are distinguished: (i) Close to the onset of convection ( Ra c ) with only the most critical convective mode (wave number) being present, dynamos set in supercritically in the Ekman number regime explored here and are dipole-dominated. Larger critical magnetic Reynolds numbers indicate that they are particularly inefficient. (ii) in the range 3 < Ra / Ra c < Ra c , the bifurcations are subcritical and only dipole-dominated dynamos exist. (iii) in the turbulent regime ( Ra / Ra c > 10 ) , the relative importance of zonal flows increases with Ra in non-magnetic models. The field topology depends on the magnitude of the initial magnetic field. The dipolar branch has a subcritical behavior whereas the multipolar branch has a supercritical behavior. By approaching more realistic parameters, the extension of this bistable regime increases. A hysteretic behavior questions the common interpretation for geomagnetic reversals. Far above the dynamo threshold (by increasing the magnetic Prandtl number), Lorentz forces contribute to the first order force balance, as predicted for planetary dynamos. When Ra is sufficiently high, dipolar fields affect significantly the flow speed, the flow structure and heat transfer which is reduced by the Lorentz force regardless of the field strength. This physical regime seems to be relevant for studying geomagnetic processes. [ABSTRACT FROM AUTHOR]

Abstract: Reliable estimates of historical and current biogeochemistry are essential for understanding past ecosystem variability and predicting future changes. Efforts to translate improved physical ocean state estimates into improved biogeochemical estimates, however, are hindered by high biogeochemical sensitivity to transient momentum imbalances that arise during physical data assimilation. Most notably, the breakdown of geostrophic constraints on data assimilation in equatorial regions can lead to spurious upwelling, resulting in excessive equatorial productivity and biogeochemical fluxes. This hampers efforts to understand and predict the biogeochemical consequences of El Niño and La Niña. We develop a strategy to robustly integrate an ocean biogeochemical model with an ensemble coupled‐climate data assimilation system used for seasonal to decadal global climate prediction. Addressing spurious vertical velocities requires two steps. First, we find that tightening constraints on atmospheric data assimilation maintains a better equatorial wind stress and pressure gradient balance. This reduces spurious vertical velocities, but those remaining still produce substantial biogeochemical biases. The remainder is addressed by imposing stricter fidelity to model dynamics over data constraints near the equator. We determine an optimal choice of model‐data weights that removed spurious biogeochemical signals while benefitting from off‐equatorial constraints that still substantially improve equatorial physical ocean simulations. Compared to the unconstrained control run, the optimally constrained model reduces equatorial biogeochemical biases and markedly improves the equatorial subsurface nitrate concentrations and hypoxic area. The pragmatic approach described herein offers a means of advancing earth system prediction in parallel with continued data assimilation advances aimed at fully considering equatorial data constraints. [ABSTRACT FROM AUTHOR]

In this paper, we consider the two-dimensional dissipative surface quasi-geostrophic equation, and establish a logarithmically improved regularity criterion. Consequently, our result extends the regularity criterion result of Dong and Pavlovié (2009). In addition, a logarithmically improved regularity criterion to the three-dimensional Navier–Stokes equations is also derived by the same arguments. Therefore, this result extends and improves many previous works. [ABSTRACT FROM AUTHOR]

The near-tropopause energy spectrum closely follows a -5/3 power law at mesoscales. Most theories addressing the mesoscale spectrum assume unbalanced dynamics but ignore the tropopause (near which the bulk of the data were collected). Conversely, it has also been proposed that the mesoscale spectrum results from tropopause-induced alterations of geostrophic turbulence. This paper seeks to reconcile these a priori mutually exclusive theories by presenting simulations that permit both unbalanced motion and tropopauseinduced effects. The model integrates the nonhydrostatic Boussinesq equations in the presence of a rapidly varying background stratification profile (an idealized tropopause). Decaying turbulence simulations were performed over a wide range of Rossby numbers. In the limit of weak flow (U ≲ 1ms-1), the essential features of the Boussinesq simulations are well captured by a quasigeostrophic version of the model: secondary roll-ups of filaments and shallow spectral slopes are observed near the tropopause but not elsewhere. However, these tropopause-induced effects rapidly disappear with increasing flow strength. For flow strengths more typical of the tropopause (U~10ms-1), the spectrum develops a shallow, near-5/3 tail associated with fast-time-scale, unbalanced motion. In contrast to weak flows, this spectral shallowing is evident at any altitude and regardless of the presence of a tropopause. Diagnostics of the fast component of motion reveal significant inertia-gravity wave activity at large horizontal scales (where the balanced flow dominates). However, no evidence points to such activity in the shallow range. That is, the mesoscale of the model is dominated by unbalanced turbulence, not waves. Implications and limitations of these findings are discussed. [ABSTRACT FROM AUTHOR]

Gravitational attraction and loading (GAL) effects associated with ongoing long-term changes in land ice are expected to cause spatially varying trends in absolute sea level ζ, as measured by satellite altimeters. The largest spatial gradients in ζ trends, predicted from solving the sea level equation using GRACE retrievals of mass distribution over land for the period 2005-15, occur near Greenland and West Antarctica, consistent with a strong local land ice loss. Misinterpreting the estimated static GAL trends in ζ as dynamic pressure gradients can lead to substantial errors in large-scale geostrophic transports across the Southern Ocean and the subpolar North Atlantic over the analyzed decade. South of Greenland, where altimeter sea level and hydrography (Argo) data coverage is good, the residual z minus steric height trends are similar in magnitude and sign to the gravitationally based predictions. In addition, estimated GAL-related trends are as large--if not larger than--other factors, such as deep steric height, dynamic bottom pressure, and glacial isostatic rebound. Thus, accounting for static GAL effects on ζ records, which are commonly neglected in oceanographic studies, seems important for a quantitative interpretation of the observed ζ trends. [ABSTRACT FROM AUTHOR]

Previous modeling and observational studies have established that it is possible to accurately monitor the Atlantic Meridional Overturning Circulation (AMOC) at 26.5°N using a coast-to-coast array of instrumented moorings supplemented by direct transport measurements in key boundary regions (the RAPID/MOCHA/WBTS Array). The main sources of observational and structural errors have been identified in a variety of individual studies. Here a unified framework for identifying and quantifying structural errors associated with the RAPID array-based AMOC estimates is established using a high-resolution (eddy resolving at low-mid latitudes, eddy permitting elsewhere) ocean general circulation model, which simulates the ocean state between 1978 and 2010. We define a virtual RAPID array in the model in close analogy to the real RAPID array and compare the AMOC estimate from the virtual array with the true model AMOC. The model analysis suggests that the RAPID method underestimates the mean AMOC by ∼1.5 Sv (1 Sv = 10 6  m 3  s −1 ) at ∼900 m depth, however it captures the variability to high accuracy. We examine three major contributions to the streamfunction bias: (i) due to the assumption of a single fixed reference level for calculation of geostrophic transports, (ii) due to regions not sampled by the array and (iii) due to ageostrophic transport. A key element in (i) and (iii) is use of the model sea surface height to establish the true (or absolute) geostrophic transport. In the upper 2000 m, we find that the reference level bias is strongest and most variable in time, whereas the bias due to unsampled regions is largest below 3000 m. The ageostrophic transport is significant in the upper 1000 m but shows very little variability. The results establish, for the first time, the uncertainty of the AMOC estimate due to the combined structural errors in the measurement design and suggest ways in which the error could be reduced. Our work has applications to basin-wide circulation measurement arrays at other latitudes and in other basins as well as quantifying systematic errors in ocean model estimates of the AMOC at 26.5°N. [ABSTRACT FROM AUTHOR]

Three-dimensional abyssal and deep circulation of the region to the east and north of the Emperor Seamount Chain/Hawaiian Ridge is determined from a compilation of CTD and Argo float data, using a new overdetermined inverse technique for the geostrophic reference velocity and diapycnal/lateral mixing coefficients. The Northeast Pacific Basin is primarily sourced from its northern boundary, at a rate of 3.5 Sv across 47°N below 3000 m. Bottom water in the western subarctic gyre recirculates cyclonically between the Emperor Seamount Chain and 155°W. Bottom water east of 155°W takes a more direct path southward along the flank of a broad topographic slope. In the deep water, a ridge of potential vorticity lying along the Mendocino Fracture Zone separates circulation systems north and south of ∼40°N. The region has very weak diapycnal and lateral mixing, and an aspect ratio for the overturning circulation that is correspondingly flat, with bottom water parcels rising less than 1 km during their long transit from the Aleutian Trench to the latitude of Hawaii. [ABSTRACT FROM AUTHOR]

The instability of long-wave disturbances of a geostrophic current with linear velocity shear is studied with allowance for the diffusion of buoyancy. A detailed derivation of the model problem in dimensionless variables is presented, which is used for analyzing the dynamics of disturbances in a vertically bounded layer and for describing the formation of large-scale intrusions in the Arctic basin. The problem is solved numerically based on a high-precision method developed for solving fourth-order differential equations. It is established that there is an eigenvalue in the spectrum of eigenvalues that corresponds to unstable (growing with time) disturbances, which are characterized by a phase velocity exceeding the maximum velocity of the geostrophic flow. A discussion is presented to explain some features of the instability. [ABSTRACT FROM AUTHOR]

We prove the short time existence and uniqueness of smooth solutions (in $${C^{k+2,\alpha}}$$ with $${k \geqq 2}$$ ) to the 2-D semi-geostrophic system and the semi-geostrophic shallow water system with variable Coriolis parameter f and periodic boundary conditions, under the natural convexity condition on the initial data. The dual space used in analysis of the semi-geostrophic system with constant f is not available for the variable Coriolis parameter case, and we develop a time-stepping procedure in Lagrangian coordinates in the physical space to overcome this difficulty. [ABSTRACT FROM AUTHOR]

In this paper we consider a periodic 2-dimensional quasi-geostrophic equations with subcritical dissipation. We show the global existence and uniqueness of the solution [ABSTRACT FROM AUTHOR]

Underwater gliders measure high-resolution spatiotemporal oceanographic data. In April 2014, the National Institute of Ocean Technology, Chennai operated an underwater glider 'Barathi', for 127 days for observation of Bay of Bengal (BoB). In this article we present the effectiveness of the glider Barathi for high resolution temporal sampling of the surface layer in the central BoB for studying variation of temperature, salinity and density structures and acoustic characteristics on 26-27 May 2014. The results showed 'afternoon effect' on acoustic characteristics and formation of secondary sound channel. Our data set is strongly correlated (coefficient of determination r2 > 0.96) with data from a nearby Array for real-time geostrophic Oceanography (Argo) float. [ABSTRACT FROM AUTHOR]

Random forcing of geostrophic motion is a common approach in idealized simulations of rotating stratified turbulence. Such forcing represents the injection of energy into large-scale balanced motion, and the resulting breakdown of quasi-geostrophic turbulence into inertia-gravity waves and stratified turbulence can shed light on the turbulent cascade processes of the atmospheric mesoscale. White noise forcing is commonly employed, which excites all frequencies equally, including frequencies much higher than the natural frequencies of large-scale vortices. In this paper, the effects of these high frequencies in the forcing are investigated. Geostrophic motion is randomly forced with red noise over a range of decorrelation time scales τ, from a few time steps to twice the large-scale vortex time scale. It is found that short τ (i.e., nearly white noise) results in about 46% more gravity wave energy than longer τ, despite the fact that waves are not directly forced. We argue that this effect is due to wave-vortex interactions, through which the high frequencies in the forcing are able to excite waves at their natural frequencies. It is concluded that white noise forcing should be avoided, even if it is only applied to the geostrophic motion, when a careful investigation of spontaneous wave generation is needed. [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]

Large scaled projects are conducted in South Yellow Sea in recent years. Topographic effect and tidal current are key issues to the coastal engineering and the ocean engineering. In this study, field surveys were conducted to investigate the tidal level, current velocity, and current direction in South Yellow Sea. A numerical model was developed to simulate the radial current field based on the field data. To investigate the mechanism of the radial current field, the actual topography and a smoothed topography were applied in the numerical model, respectively. Results show that, the current field appeares radial because of the tidal system rather than the submarine topography. Local topography centralized the radiation centre and shifted the high-velocity zones. The actual topographic effect is proposed, and results show that local topography increases the flood tide velocity and decreases the ebb tide velocity. Lagrangian residual currents are calculated to illustrate possible sediment sources and transport routes. [ABSTRACT FROM AUTHOR]

We analyse the hindcast skill for the Atlantic meridional overturning circulation (AMOC) against 10 years of RAPID/MOCHA AMOC observations, which are now long enough to remove the mean seasonal cycle prior to the hindcast skill analysis. We analyse AMOC hindcast skill in two hindcast ensembles generated with two differently initialised decadal prediction systems that are both based on the earth system model MPI-ESM. We evaluate the hindcast skill for the AMOC and its components in both prediction systems against RAPID/MOCHA observations both with and without the mean seasonal cycle removed using anomaly correlation (COR) and root-mean-square error as skill measures. We find significant hindcast skill for most lead years up to 5 for monthly-mean AMOC variations only in the newer of the two prediction systems and only using COR, but with and without the mean seasonal cycle removed. In both systems and for all analysed lead years, the two geostrophic transport components (the upper-mid-ocean transport and Florida Strait combined, that is: AMOC minus Ekman) are the main source of hindcast skill. In the present model setup and with the currently available observational time series, we cannot relate AMOC hindcast skill to the upper-mid-ocean transport alone. Yet, we can show that the seasonal variability of the upper-mid-ocean transport in the free coupled model originates from eastern boundary density variability. Overall, our results indicate modest yet robust AMOC hindcast skill above the uninitialized simulation, independent of the treatment of the seasonal cycle, although we cannot directly link this hindcast skill to the initialisation of the density field with either initialisation method. [ABSTRACT FROM AUTHOR]

Isolated anticyclones are frequently observed below the mixed layer in the Arctic Ocean. Some of these subsurface anticyclones are thought to originate at surface fronts. However, previous idealized simulations with no surface stress show that only cyclone-anticyclone dipoles can propagate away from baroclinically unstable surface fronts. Numerical simulations of fronts subject to a surface stress presented here show that a surface stress in the same direction as the geostrophic flow inhibits dipole propagation away from the front. On the other hand, a surface stress in the opposite direction to the geostrophic flow helps dipoles to propagate away from the front. Regardless of the surface stress at the point of dipole formation, these dipoles can be broken up on a time scale of days when a surface stress is applied in the right direction. The dipole breakup leads to the deeper anticyclonic component becoming an isolated subsurface eddy. The breakup of the dipole occurs because the cyclonic component of the dipole in the mixed layer is subject to an additional advection because of the Ekman flow. When the Ekman transport has a component oriented from the anticyclonic part of the dipole toward the cyclonic part then the cyclone is advected away from the anticyclone and the dipole is broken up. When the Ekman transport is in other directions relative to the dipole axis, it also leads to deviations in the trajectory of the dipole. A scaling is presented for the rate at which the surface cyclone is advected that holds across a range of mixed layer depths and surface stress magnitudes in these simulations. The results may be relevant to other regions of the ocean with similar near-surface stratification profiles. [ABSTRACT FROM AUTHOR]

A process-oriented numerical modeling study was conducted to investigate the formation and underlying forcing of an anticyclonic eddy train observed in the northern South China Sea. Observations showed that long-lived anticyclonic eddies formed an eddy train along an eastward separated jet across the northern South China Sea in summer. The eddy train plays a critical role in regulating ocean circulation in the region. Forced by the southwesterly monsoon and prevailing dipole wind stress curl in the summer, the northward coastal jet separates from the west boundary of the South China Sea basin and overshoots northeastward into the basin. The anticyclonic recirculation of the separated jet forms the first anticyclonic eddy in the eddy train. The jet meanders downstream with a strong negative shear vorticity that forms a second and a third anticyclonic eddy along the jet's path. These three eddies form the eddy train. These eddies weaken gradually with depth from surface, but they can extend to approximately 500 m deep. The inherent stratification in the region regulates the three-dimensional scale of the anticyclonic eddies and constrains their intensity vertical extension by weakening the geostrophic balance within these eddies. Analyses of the vorticity balance indicate that the eddy train's negative vorticity originates from the beta effect of northward western boundary current and from the subsequent downstream vorticity advection in the jet. The jet separation is a necessary condition for the formation of the eddy train, and the enhanced stratification, increased summer wind stress, and associated negative wind stress curl are favorable conditions for the formation of the anticyclonic eddies. [ABSTRACT FROM AUTHOR]

We numerically investigate single-layer shallow-water beta-plane quasi-geostrophic turbulence in a doubly periodic domain with emphasis onwavenumber-frequency spectra.We conduct a broad parameter sweep, varying the deformation radius (Ld ), the narrow-band forcing wavenumber (kf), and the meridional gradient of the Coriolis parameter (β). Out of 54 simulations we present ten in detail spanning slowly propagating vortices to strong jets.We define a nondimensional parameter γβ in terms of β, Ld, and the energy injection rate. The moderately low γβ case is characterized by westward propagating coherent vortices and zonal wavenumber-frequency spectra dominated by a nondispersive line (NDL) corresponding to uniform propagation at or near the long-wave Rossby speed. The moderately high γβ case is characterized by jets, and the NDL persists even when there are no coherent vortices. The jets have large meridional excursions (meanders) that propagate westward nearly uniformly at a speed slower than the long-wave Rossby speed. Also at moderately high γβ, a second dispersion relation appears, roughly corresponding to linear waves on a zonal potential vorticity (PV) staircase. At very high γβ, during the slow evolution to a PV staircase, the structure of the linear waves is altered by the small perturbations to a constant potential vorticity gradient. A simple model treating the small perturbation as a sinusoid accurately predicts the meridional wavenumber-frequency spectra in the very high γβ simulations. [ABSTRACT FROM AUTHOR]

The Lofoten Vortex (LV), a quasi-permanent anticyclonic eddy in the Lofoten Basin of the Norwegian Sea, is investigated with an eddy-permitting primitive equation model nested into the ECCO2 ocean state estimate. The LV, as simulated by the model, extends from the sea surface to the ocean bottom at about 3000 m and has the subsurface core between 50 m and 1100 m depths. Above and below the vortex core the relative vorticity signal decreases in amplitude while the radius increases by as much as 25–30% relative to the values in the core. Analyzing the model run, we show that the vertical structure of the LV can be casted into four standard configurations, each of which forms a distinct cluster in the parameter space of potential vorticity anomalies in and above the LV core. The stability of the LV for each of the configurations is then studied with three-layer and a two-layer (in winter) quasi-geostrophic (QG) models over a flat bottom as well as over a realistic topography. The QG results show a number of common features with those of the primitive equation model. Thus, among the azimuthal modes dominating the LV instability, both the QG model and the primitive equation model show a major role the 2nd and 3rd modes. In the QG model simulations the LV is the subject of a rather strong dynamic instability, penetrating deep into the core. The results predict 50–95% volume loss from the vortex within 4–5 months. Such a drastic effect is not observed in the primitive equation model, where, for the same intensity of perturbations, only 10–30% volume loss during the same period is detected. Taking into account the gently sloping topography of the central part of the Lofoten basin and the mean flow in the QG model, brings the rate of development of instability close to that in the primitive equation model. Some remaining differences in the two models are discussed. Overall, the LV decay rate obtained in the models is slow enough for eddy mergers and convection to restore the thermodynamic properties of the LV, primarily re-building its potential energy anomaly. This justifies the quasi-permanent presence of the LV in the Lofoten Basin. [ABSTRACT FROM AUTHOR]

Argo floats are used to investigate Labrador Sea overturning and its variability on seasonal time scales. This is the first application of Argo floats to estimate overturning in a deep-water formation region in the North Atlantic. Unlike hydrographic measurements, which are typically confined to the summer season, floats offer the advantage of collecting data in all seasons. Seasonal composite potential density and absolute geostrophic velocity sections across the mouth of the Labrador Sea assembled from float profiles and trajectories at 1000 m are used to calculate the horizontal and overturning circulations. The overturning exhibits a pronounced seasonal cycle; in depth space the overturning doubles throughout the course of the year, and in density space it triples. The largest overturning [1.2 Sv (1 Sv ≡ 106 m3 s−1) in depth space and 3.9 Sv in density space] occurs in spring and corresponds to the outflow of recently formed Labrador Sea Water. The overturning decreases through summer and reaches a minimum in winter (0.6 Sv in depth space and 1.2 Sv in density space). The robustness of the Argo seasonal overturning is supported by a comparison to an overturning estimate based on hydrographic data from the AR7W line. [ABSTRACT FROM AUTHOR]

The Kuroshio south of Japan shows bimodal path fluctuations between the large meander (LM) path and the nonlarge meander (NLM) path. The transition from the NLM path to the LM path is triggered by a small meander generated off southwestern Japan. The small meander first propagates eastward (downstream) along the Kuroshio and then rapidly amplifies over Koshu Seamount, located about 200 km south of Japan, leading to the formation of the LM path of the Kuroshio. Although Koshu Seamount is essential for the rapid amplification of the small meander, the underlying physical mechanism is not fully understood. In this study, the role of Koshu Seamount is revisited using a two-layer quasi-geostrophic model that takes into account the effects of bottom topography. Numerical experiments show that the transition from the NLM path to the LM path can be successfully reproduced only when bottom topography mimicking Koshu Seamount is incorporated. In this case, the upper-layer meander trough is rapidly amplified together with a lower-layer anticyclone by baroclinic instability during their passage over the northern slope of Koshu Seamount. A linear stability analysis shows that baroclinic instability over a seamount is caused by resonant coupling between the upper-layer Rossby wave in the eastward background flow and the lower-layer seamount-trapped wave during their eastward propagation over the northern slope of the seamount. The spatial scale and structure of this baroclinically unstable mode are close to those of the numerically reproduced small meander in its early amplification stage over the seamount. [ABSTRACT FROM AUTHOR]

Hydrographic measurements together with Ship mounted Acoustic Doppler Current Profilers and Lowered Acoustic Doppler Current Profilers (LADCP) obtained in October 2014 are used to describe water masses, geostrophic circulation and mass transport of the Canary Current System, as the Eastern Boundary of the North Atlantic Subtropical Gyre. Geostrophic velocities are adjusted to velocities from LADCP data to estimate an initial velocity at the reference layer. The adjustment results in a northward circulation at the thermocline layers over the African slope from an initial convergent flow. Final reference velocities and consequently absolute circulation are estimated from an inverse box model applied to an ocean divided into 13 neutral density layers. This allows us to evaluate mass fluxes consistent with the thermal wind equation and mass conservation. Ekman transport is estimated from the wind data derived from the Weather Research and Forecasting model. Ekman transport is added to the first layer and adjusted with the inverse model. The Canary Current located west of Lanzarote Island transports to the south a mass of − 1.5 ± 0.7 Sv (1 Sv = 10 6 m 3 s − 1 ≈ 10 9 kg s − 1 ) of North Atlantic Central Water at the surface and thermocline layers (~ 0–700 m). In fall 2014, hydrographic data shows that the Canary Current in the thermocline (below at about 80 m depth to ~ 700 m) recirculates to the north over the African slope and flows through the Lanzarote Passage. At intermediate layers (~ 700–1400 m), the Intermediate Poleward Undercurrent transports northward a relatively fresh Antarctic Intermediate Water in the range of 0.8 ± 0.4 Sv through the Lanzarote Passage and west of Lanzarote Island beneath the recirculation of the Canary Current. [ABSTRACT FROM AUTHOR]

The problem of localized dense water formation over a sloping bottom is considered for the general case in which the topography forms a closed contour. This class of problems is motivated by topography around islands or shallow shoals in which convection resulting from brine rejection or surface heat loss reaches the bottom. The focus of this study is on the large-scale circulation that is forced far from the region of surface forcing. The authors find that a cyclonic current is generated around the topography, in the opposite sense to the propagation of the dense water plume. In physical terms, this current results from the propagation of low sea surface height from the region of dense water formation anticyclonically along the topographic contours back to the formation region. This pressure gradient is then balanced by a cyclonic geostrophic flow. This basic structure is well predicted by a linear quasigeostrophic theory, a primitive equation model, and in rotating tank experiments. For sufficiently strong forcing, the anticyclonic circulation of the dense plume meets this cyclonic circulation to produce a sharp front and offshore advection of dense water at the bottom and buoyant water at the surface. This nonlinear limit is demonstrated in both the primitive equation model and in the tank experiments. [ABSTRACT FROM AUTHOR]

The mean subthermocline and intermediate zonal circulation in the tropical Pacific is investigated using a compilation of shipboard ADCP measurements and absolute geostrophic velocities constructed from a high-resolution 0-2000-m Argo climatology referenced to a 1000-m velocity field derived from Argo float drifts. This reference field is dominated by basinwide alternating zonal jets with a meridional wavelength of about 3°. In regions where the sampling of SADCP data is sufficient, the consistency between the two independent datasets is striking; using the Argo drift reference is crucial to capture the current structures. Two apparently distinct systems of alternating westward and eastward zonal jets are seen in both datasets equatorward of 10°: a series of low-latitude subthermocline currents (LLSCs) below the thermocline, extending from about 200 to 800 m, including the eastward Tsuchiya jets; and a series of low-latitude intermediate currents (LLICs), extending from about 700 to at least 2000 m. These systems seem to merge poleward of 10°. Both series shoal to lighter densities eastward. The subthermocline currents and their associated potential vorticity structures undergo a major shift near 155°W, suggesting some difference in the dynamic regime between the regions west and east of this longitude. Differing behaviors (the LLSCs tend to angle poleward to the east, whereas the LLICs angle slightly equatorward) suggest that these jets may be dynamically distinct, with different forcing mechanisms. [ABSTRACT FROM AUTHOR]

Nitrate interleaving structures were observed below the mixed layer during a cruise to the Kuroshio Extension in October 2009. In this paper, we investigate the formation mechanisms for these vertical nitrate anomalies, which may be an important source of nitrate to the oligotrphoc surface waters south of the Kuroshio Extension Front. We found that nitrate concentrations below the main stream of the Kuroshio Extension were elevated compared to the ambient water of the same density ( σ = 23.5-25). This appears to be analogous to the 'nutrient stream' below the mixed layer, associated with the Gulf Stream. Strong turbulence was observed above the vertical nitrate anomaly, and we found that this can drive a large vertical turbulent nitrate flux $>\mathcal {O}$ (1 mmol N m day). A realistic, high-resolution (2 km) numerical simulation reproduces the observed Kuroshio nutrient stream and nitrate interleaving structures, with similar lateral and vertical scales. The model results suggest that the nitrate interleaving structures are first generated at the western side of the meander crest on the south side of the Kuroshio Extension, where the southern tip of the mixed layer front is under frontogenesis. Lagrangian analyses reveal that the vertical shear of geostrophic and subinertial ageostrophic flow below the mixed layer tilts the existing along-isopycnal nitrate gradient of the Kuroshio nutrient stream to form nitrate interleaving structures. This study suggests that the multi-scale combination of (i) the lateral stirring of the Kuroshio nutrient stream by developed mixed layer fronts during fall to winter, (ii) the associated tilting of along-isopycnal nitrate gradient of the nutrient stream by subinertial shear, which forms vertical interleaving structures, and (iii) the strong turbulent diffusion above them, may provide a route to supply nutrients to oligotrophic surface waters on the south side of the Kuroshio Extension. [ABSTRACT FROM AUTHOR]

The southern annular mode (SAM) in the atmosphere and the Antarctic circumpolar current (ACC) in the ocean play decisive roles in the climatic system of the mid- to high-latitude southern hemisphere. Using the time-variable gravity data from the GRACE satellite mission, we find the link between the space-time variabilities of the ACC and the SAM. We calculate the empirical orthogonal functions (EOF) of the non-seasonal ocean bottom pressure (OBP) field in the circum-Antarctic seas from the GRACE data for the period from 2003 to 2015. We find that the leading EOF mode of the non-seasonal OBP represents a unison OBP oscillation around Antarctica with time history closely in pace with that of the SAM Index with a high correlation of 0.77. This OBP variation gives rise to a variation in the geostrophic flow field; the result for the same EOF mode shows heightened variations in the zonal velocity that resides primarily in the eastern hemispheric portion of the ACC and coincided geographically with the southernmost boundary of the ACC's main stream. Confirming previous oceanographic studies, these geodetic satellite results provide independent information toward better understanding of the ACC-SAM process. [ABSTRACT FROM AUTHOR]

Parker's two-dimensional (2D) dynamo model with an algebraic form of nonlinearity for the α-effect is considered. The model uses geostrophic distributions for the α-effect and differential rotation, which are derived from the three-dimensional (3D) convection models. The resulting configurations of the magnetic field in the liquid core are close to the solutions in Braginsky's Z-model. The implications of the degree of geostrophy observed in the 3D dynamo models for the behavior of the mean magnetic field are explored. It is shown that the reduction in geostrophy leads to magnetic field reversals accompanied by the relative growth of the nondipole component of the field on the surface of the liquid core. The simulations with a random α-effect which causes turbulent pulsations are carried out. The approach is capable of producing realistic sequences of magnetic reversals. [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]

The circulation structure in the western tropical Indian Ocean (WTIO) was investigated based on surface drifters and Simple Ocean Data Assimilation (SODA), with particular attention to the characteristics of the circulation structure associated with seasonal monsoon transitions. Surface drifter trajectories manifest three distinct seasonally dependent patterns of western boundary currents in the WTIO. The northward Somali Current (SC) flows across the equator to ∼10 °N in the boreal summer (June to August; JJA). The northward SC turns offshore near the equator during monsoon transitions. The southward SC feeds the South Equatorial Countercurrent together with the northward East African Coast Current in the boreal winter (December to February; DJF). While during the monsoon transition (MAM and SON), drifters prefer to turn eastward and feed the strong equatorial currents. Based on satellite observations, the results of a balanced diagnostic model demonstrate that the geostrophic component U ζ and the wind-driven component U τ predominate the mean state and the seasonal variations of circulation in the WTIO. The vertical motion shows that upwelling primarily occurs along the coasts of Somali and Oman and is induced by offshore Ekman transport. Ekman transport and associated vertical motions constitute meridional overturning circulation. The SODA results confirm the existence of an equatorial roll in the boreal summer and winter. The results elucidate the dynamic relationships between 3-D circulation and monsoon transitions in the WTIO. • Drifter trajectories describe three seasonally dependent circulation patterns in the western tropical Indian Ocean. • The geostrophic component U ζ and wind-driven component U τ predominate the mean state and the soasonal variability of circulation. • The 3-D structures, show seasonal variability associated with monsoon transitions. [ABSTRACT FROM AUTHOR]

The article reports that Wind is the main driver of general ocean circulation, using satellite data and a multiscale process analysis method, Shikhar Rai and colleagues at the University of Rochester that wind deposits kinetic energy in geostrophic ocean flow with wind extracts energy.

A quasi-geostrophic three-level T63 model of the wintertime atmospheric circulation of the Northern Hemisphere has been applied to investigate the impact of Arctic amplification (increase in surface air temperatures and loss of Arctic sea ice during the last 15 years) on the mid-latitude large-scale atmospheric circulation. The model demonstrates a mid-latitude response to an Arctic diabatic heating anomaly. A clear shift towards a negative phase of the Arctic Oscillation (AO−) during low sea-ice-cover conditions occurs, connected with weakening of mid-latitude westerlies over the Atlantic and colder winters over Northern Eurasia. Compared to reanalysis data, there is no clear model response with respect to the Pacific Ocean and North America. [ABSTRACT FROM AUTHOR]