Allan R. Robinson

Influence of mesoscale eddies on new production in the Sargasso Sea

It is problematic that geochemical estimates of new production — that fraction of total primary production in surface waters fuelled by externally supplied nutrients — in oligotrophic waters of the open ocean surpass that which can be sustained by the traditionally accepted mechanisms of nutrient supply., In the case of the Sargasso Sea, for example, these mechanisms account for less than half of the annual nutrient requirement indicated by new production estimates based on three independent transient-tracer techniques. Specifically, approximately one-quarter to one-third of the annual nutrient requirement can be supplied by entrainment into the mixed layer during wintertime convection, with minor contributions from mixing in the thermocline, and wind-driven transport (the potentially important role of nitrogen fixation — for which estimates vary by an order of magnitude in this region — is excluded from this budget). Here we present four lines of evidence — eddy-resolving model simulations, high-resolution observations from moored instrumentation, shipboard surveys and satellite data — which suggest that the vertical flux of nutrients induced by the dynamics of mesoscale eddies is sufficient to balance the nutrient budget in the Sargasso Sea.

General Circulation of the Eastern Mediterranean

Abstract A novel description of the phenomenology of the Eastern Mediterranean is presented based upon a comprehensive pooled hydrographic data base collected during 1985–1987 and analyzed by cooperating scientists from several institutions and nations (the POEM project). Related dynamical process and modeling studies are also overviewed. The circulation and its variabilities consist of three predominant and interacting scales: basin scale, subbasin scale, and mesoscale. Highly resolved and unbiased maps of the basin wide circulation in the thermocline layer are presented which provide a new depiction of the main thermocline general circulation, composed of subbasin scale gyres interconnected by intense jets and meandering currents. Semipermanent features exist but important subbasin scale variabilities also occur on many time scales. Mesoscale variabilities modulate the subbasin scale and small mesoscale eddies populate the open sea, especially the south-eastern Levantine basin. Clear evidence indicates Levantine Intermediate Water (LIW) to be present over most of the Levantine Basin, implying that formation of LIW is not localized but rather is ubiquitous. The Ionian and Levantine basins are confirmed to form one deep thermohaline cell with deep water of Adriatic origin and to have a turnover time of one and a quarter centuries. Prognostic, inverse, box and data assimilative modeling results are presented based on both climatological and POEM data. The subbasin scale elements of the general circulation are stable and robust to the dynamical adjustment process. These findings bear importantly on a broad range of problems in ocean science and marine technology that depend upon knowledge of the general circulation and water mass structure, including biogeochemical fluxes, regional climate, coastal interactions, pollution and environmental management. Of global ocean scientific significance are the fundamental processes of water mass formations, transformations and dispersion which occur in the basin.

Eddies in marine science

1. Overview and Summary of Eddy Science.- 1.1 Eddy Currents in the Ocean.- 1.1.1 Observing the Eddies.- 1.1.2 Modeling the Eddies.- 1.2 Status of Eddy Science.- 1.2.1 Distribution and Generation.- 1.2.2 Physics.- 1.2.3 Role in the General Circulation.- 1.3 Influences of Eddies.- 1.3.1 Scientific Processes and Practical Consequences.- 1.3.2 Dispersion and Mixing.- 1.3.3 Description and Prediction.- 1.4 Evolution and Outlook.- 1.4.1 A Time of Transition.- 1.4.2 A Decade of Eddy Research.- 1.4.3 Eddies in Marine Science.- Regional Kinematics, Dynamics, and Statistics.- 2. Gulf Stream Rings.- 2.1 Introduction.- 2.2 History.- 2.3 Cold-Core Rings.- 2.3.1 Formation.- 2.3.2 Structure and Velocity.- 2.3.3 Distribution and number.- 2.3.4 Movement.- 2.3.5 Interaction and Coalescence with Gulf Stream.- 2.3.6 Decay.- 2.4 Warm-Core Rings.- 2.5 Gulf of Mexico Rings.- 2.6 Eastern Rings.- 2.7 Current Rings from Other Currents.- 3. Western North Atlantic Interior.- 3.1 Introduction.- 3.2 Observations.- 3.2.1 Spectrum and Time Scales.- 3.2.2 Spatial Scales.- 3.2.3 Kinetic and Potential Energy.- 3.3 Other Data.- 3.4 Some Simple Conclusions.- 4. The Western North Atlantic - A Lagrangian Viewpoint.- 4.1 Introduction.- 4.2 The Data Base.- 4.3 The Mean Field.- 4.3.1 The Lagrangian View.- 4.3.2 The Equivalent Eulerian View, and the Eddy Kinetic Energy Distribution.- 4.4 Mixing and Dispersion.- 4.5 Topographic Influences.- 4.6 The Eastward Flow in the Subtropical Gyre.- 4.7 Discrete Eddies.- 4.8 The Path of the Gulf Stream.- 4.9 Concluding Remarks.- 5. The Local Dynamics of Eddies in the Western North Atlantic.- 5.1 Introduction.- 5.2 Scientific Objectives.- 5.2.1 Dynamical Balances.- 5.2.2 Synoptic Structures.- 5.2.3 Geostrophic Fine Scales.- 5.2.4 Surface Layer Mesoscale Eddies.- 5.2.5 The General Circulation.- 5.3 The Experiment.- 5.4 The Phenomena.- 5.5 Summary.- 6. Gulf Stream Variability.- 6.1 Introduction.- 6.2 The Gulf Stream Path.- 6.3 Current Structure and Transport.- 6.4 Water Masses.- 6.5 Dynamics.- 6.6 Summary.- 7. The Northeast Atlantic Ocean.- 7.1 Introduction.- 7.2 General Oceanographic Conditions and Mean Circulation..- 7.3 Data Sources.- 7.3.1 Hydrographic Data.- 7.4 XBT Observations.- 7.5 Direct Observation of Currents.- 7.6 Satellite Observations.- 7.7 Surveys of Individual Eddies.- 7.8 Summary.- 8. Eddy Structure of the North Pacific Ocean.- 8.1 Introduction.- 8.2 Coastal Regions.- 8.3 Mid-Latitude Open Ocean.- 8.4 Tropical Region.- 8.5 Direct Current Measurements.- 9. Subpolar Gyres and the Arctic Ocean.- 9.1 Introduction.- 9.2 The Arctic Ocean.- 9.3 Labrador Sea and Northwestern Atlantic.- 9.4 The Norwegian and Greenland Seas and Their Overflows.- 9.5 The Northeast North Pacific and Bering Sea.- 9.6 Summary.- 10. Tropical Equatorial Regions.- 10.1 Latitudinal Changes of Mesoscale Processes.- 10.2 Eddy-Resolving Arrays in the Tropics.- 10.3 Mesoscale Features in Sections.- 10.4 Mesoscale Energy in the Tropics.- 11. Eddies in the Indian Ocean.- 11.1 Introduction.- 11.2 Kinetic Energy of the Seasonal Circulation.- 11.3 Arabian Sea.- 11.4 Somali Current.- 11.5 Bay of Bengal.- 11.6 Equatorial Zone.- 11.7 South Equatorial Current.- 11.8 Deep Currents in the Indian Ocean.- 11.9 Significance of Indian Ocean Eddies.- 12. The South Pacific Including the East Australian Current.- 12.1 Introduction: The Subtropical South Pacific.- 12.2 Observations: The Tasman Sea.- 12.2.1 The East Australian Current.- 12.2.2 The Tasman Front.- 12.3 Theories.- 12.3.1 A Line Vortex and a Wall.- 12.3.2 A Free Inertial Jet.- 12.3.3 Baroclinic Instability of a Western Boundary Flow..- 12.4 The Significance of Eddies.- 12.4.1 Physical Significance.- 12.4.2 Biological Significance.- 12.5 Future Research.- 12.5.1 The Mid-Ocean.- 12.5.2 The East Australian Current.- 13. Eddies in the Southern Indian Ocean and Agulhas Current.- 13.1 Introduction.- 13.2 South Eastern Indian Ocean.- 13.2.1 Vicinity of the South Equatorial Current.- 13.2.2 West of Australia.- 13.2.3 Vicinity of the Subtropical Convergence.- 13.3 South Western Indian Ocean.- 13.3.1 Eddies South of Madagascar.- 13.3.2 Larger Southwestern Indian Ocean.- 13.3.3 Adjacent to the Agulhas Current.- 13.3.4 The Agulhas Retroflection Area.- 13.3.5 The Vicinity of the Agulhas Plateau.- 13.4 Discussion.- 14. The Southern Ocean.- 14.1 Introduction.- 14.2 Observations of Eddies in the Southern Ocean.- 14.3 Eddy Generation and Decay.- 14.4 Effects Eddies and Implications for Models the Southern Ocean.- 15. Global Summaries and Intercomparisons: Flow Statistics from Long-Term Current Meter Moorings.- 15.1 Introduction.- 15.2 Global Kinetic Energy Estimates from Shift-Drift Analysis.- 15.3 The North Atlantic.- 15.3.1 Introduction.- 15.3.2 Ocean-Scale Patterns of Kinetic Energy.- 15.3.3 A Transect of the Western Basin.- 15.3.4 A Transect of the Eastern Basin.- 15.4 The North Pacific.- 15.4.1 The Kuroshio.- 15.4.2 Western Pacific.- 15.4.3 East-Central Pacific.- 15.4.4 Central and Eastern Tropical Pacific.- 15.5 The Equatorial Zone.- 15.6 South Atlantic.- 15.7 High Latitudes.- 15.7.1 The Antartic Circompolar Current.- 15.7.2 Weddell Sea.- 15.7.3 The West Spitsbergen Current.- 15.7.4 Arctic Ocean.- 15.8 Summary.- 15.8.1 Horizontal Distribution of Eddy Kinetic Energy at Midlatitudes.- 15.8.2 Vertical Structure of the Eddy Field at Midlatitudes.- 15.8.3 Vertical Eddy Structure in Ice-Covered Seas.- 15.8.4 Eddy Time Scales at Midlatitudes.- 15.8.5 Eddy Time Scales in the Equatorial Zone.- 15.8.6 Relationship of KE to KM Worldwide.- 16. Global Summary: Review of Eddy Phenomena as expressed in Temperature Measurements.- 16.1 Introduction.- 16.2 Standard Deviation of Temperature.- 16.3 The North and Equatorial Atlantic.- 16.4 The North and Equatorial Pacific.- 16.5 Discussion.- Models.- 17. Eddy-Resolving Numerical Models of Large-Scale Ocean Circulation.- 17.1 Introduction.- 17.2 Review of EGCM Results.- 17.2.1 What Processes Account for the Presence of Mesoscale Variability?.- 17.2.2 Do Mesoscale Phenomena Play a Fundamental Role in the Character and Dynamics of the Time-Mean Circulation?.- 17.2.3 Where the Effects of Mesoscale Circulations Are Important, Can They Be Parametrized in Terms of Mean-Field Quantities?.- 17.2.4 How Do the Answers to Questions About the Role of Mesoscale Eddies Change as the Model Physical Processes Change?.- 17.3 Comparisons with Observations.- 17.4 Unsolved Problems and Future Prospects.- 18. Periodic and Regional Models.- 18.1 Introduction.- 18.2 Two-Dimensional Turbulence.- 18.3 The Effects of ?.- 18.4 Mean Flow Generation by Localized Forcing.- 18.5 Turbulent Cascades in a Stratified Fluid.- 18.6 Scattering by Topography.- 18.7 Regional Models and Non-Local Effects.- 18.8 Comparison with Observations.- 18.9 Conclusions and Future Directions.- Effects and Applications.- 19. Eddies in Relation to Climate.- 19.1 Introduction.- 19.2 Heat Flux Carried by Eddies.- 19.3 Indirect Effects of Eddies on the Heat Balance.- 20. Eddies and Coastal Interactions.- 20.1 Introduction.- 20.2 Theoretical Considerations.- 20.2.1 Stratification, P-Effect.- 20.2.2 Abrupt Topography: Scattering and Reflection.- 20.2.3 Refractive Effects.- 20.2.4 Bottom Friction.- 20.2.5 Longshore Currents.- 20.2.6 Generating Mechanisms and Initial Value Problems.- 20.3 Observations and Analysis.- 20.3.1 The New England Continental Rise.- 20.3.2 The East Australian Shelf.- 20.3.3 The Southeastern Coast of the U.S.- 20.3.4 The West Florida Shelf.- 20.3.5 The Scotian Shelf.- 20.4 Anatomy of a Warm-Core Ring Interaction with the Continental Margin.- 20.4.1 Low-Frequency Current Data, Summer 1976.- 20.4.2 Offshore Forcing.- 20.4.3 Analysis and Interpretation.- 20.5 Conclusions.- 21. Eddy-Induced Dispersion and Mixing.- 21.1 Introduction.- 21.2 Two-Dimensional Dispersal in the Mid-Ocean.- 21.3 Streakiness.- 21.4 Particle Motions.- 21.5 Gyre-Scale Dispersal Effects.- 21.6 Tracer Evidence for Dispersal Processes.- 22. Eddies and Biological Processes.- 22.1 Introduction.- 22.2 Frontal Eddies.- 22.2.1 Formation.- 22.2.2 Air-Sea Interaction Within a Ring.- 22.2.3 Horizontal Exchanges Between the Ring and Its Surroundings.- 22.2.4 Horizontal Exchange of Plankton Between a Ring and Its Surrounding Water.- 22.3 Mid-Ocean Eddies.- 22.3.1 Biological Processes in a Single Eddy.- 22.3.2 Spatial Spectrum.- 22.4 Observational Data.- 22.4.1 Scripps Transects.- 22.4.2 Gulf Stream Ring Structures.- 22.4.3 East Australian Current Rings.- 22.4.4 Incidental Observations.- 22.5 Sampling Procedures.- 22.6 Conclusions.- 23. Eddies and Acoustics.- 23.1 Introduction.- 23.2 Dynamic Ocean.- 23.3 Sound Propagation in a Range-Dependent Environment.- 23.3.1 Geometrical Acoustics.- 23.3.2 Normal Modes.- 23.3.3 The Parabolic Approximation.- 23.4 Analytical and Numerical Studies of Sound Propagation Through Eddies.- 23.5 Acoustic Measurements in Eddies.- 23.6 Acoustic Eddy Monitoring.- 23.7 Conclusions.- 24. Instruments and Methods.- 24.1 Introduction.- 24.2 Moored Instrumentation.- 24.2.1 Moorings.- 24.2.2 Fixed Depth Moored Current Meters.- 24.3 Moored Profiling Current Meters.- 24.4 Temperature Recorders.- 24.5 Other Moored Instrumentation.- 24.6 Drifting Instruments.- 24.6.1 Surface Drifters.- 24.6.2 Deep Drifters.- 24.6.3 Problems and Interpretations.- 24.7 Profiling Instruments.- 24.7.1 Temperature and Salinity Instruments.- 24.7.2 Nansen Bottles.- 24.7.3 CTD and STD.- 24.8 Expendable Bathythermographs.- 24.9 Current Profilers.- 24.9.1 Electromagnetic Velocity Profiler.- 24.9.2 The White Horse.- 24.9.3 Schmitz-Richardson Profiler.- 24.9.4 Other Velocity Profilers.- 24.9.5 Calibration and Intercomparison.- 24.10 Remote and Inverse Techniques.- 24.10.1 Satellite and Airborne Measurements.- 24.10.2 Tomography.- 24.11 Intercomparison Between Instrument Types.- References.

The eastern Mediterranean general circulation: features, structure and variability

Abstract Maps are presented for dynamic height and geostrophic flow in the upper thermocline based upon four basin-wide hydrographic surveys during 1985–1987. The data collection was coordinated, intercalibrated and pooled by the international research programme for Physical Oceanography of the Eastern Mediterranean (POEM). Objective analysis mapping was constrained to have no normal flow into the coasts. These maps reveal a new picture of the general circulation in which sub-basin-scale gyres are interconnected by jets and currents. Important variabilities occur in permanent and recurrent features but transient eddies and jets also occur. A schematic synthesis is constructed.

Data Assimilation via Error Subspace Statistical Estimation.Part I: Theory and Schemes

A rational approach is used to identify efficient schemes for data assimilation in nonlinear ocean‐atmosphere models. The conditional mean, a minimum of several cost functionals, is chosen for an optimal estimate. After stating the present goals and describing some of the existing schemes, the constraints and issues particular to ocean‐atmosphere data assimilation are emphasized. An approximation to the optimal criterion satisfying the goals and addressing the issues is obtained using heuristic characteristics of geophysical measurements and models. This leads to the notion of an evolving error subspace, of variable size, that spans and tracks the scales and processes where the dominant errors occur. The concept of error subspace statistical estimation (ESSE) is defined. In the present minimum error variance approach, the suboptimal criterion is based on a continued and energetically optimal reduction of the dimension of error covariance matrices. The evolving error subspace is characterized by error singular vectors and values, or in other words, the error principal components and coefficients. Schemes for filtering and smoothing via ESSE are derived. The data‐forecast melding minimizes variance in the error subspace. Nonlinear Monte Carlo forecasts integrate the error subspace in time. The smoothing is based on a statistical approximation approach. Comparisons with existing filtering and smoothing procedures are made. The theoretical and practical advantages of ESSE are discussed. The concepts introduced by the subspace approach are as useful as the practical benefits. The formalism forms a theoretical basis for the intercomparison of reduced dimension assimilation methods and for the validation of specific assumptions for tailored applications. The subspace approach is useful for a wide range of purposes, including nonlinear field and error forecasting, predictability and stability studies, objective analyses, data-driven simulations, model improvements, adaptive sampling, and parameter estimation.

Turbulent jets and eddies in the california current and inferred cross-shore transports.

The instantaneous California Current is seen to consist of intense meandering current filaments (jets) intermingled with synoptic-mesoscale eddies. These quasi-geostrophic jets entrain cold, upwelled coastal waters and rapidly advect them far offshore; this behavior accounts for the elongated, cool surface features that are seen extending across the California Current region in satellite infrared imagery. The associated advective mechanism should provide significant cross-shore transports of heat, nutrients, biota, and pollutants. The dynamics of the current system should be crucially influenced by its highly variable structure.

Analysis models for the estimation of oceanic fields

Abstract A general model for statistically optimal estimates is presented for dealing with scalar, vector and multivariale datasets. The method deals with anisotropic fields and treats space and time dependence equivalently. Problems addressed include the analysis, or the production of synoptic lime series of regularly gridded fields from irregular and gappy datasets, and the estimate of fields by compositing observations from several different instruments and sampling schemes. Technical issues are discussed, including the convergence of statistical estimates, the choice of representation of the correlations, the influential domain of an observation, and the efficiency of numerical computations.

The Atlantic Ionian Stream

Abstract This paper describes some preliminary results of the cooperative effort between SACLANT Undersea Research Centre and Harvard University in the development of a regional descriptive and predictive capability for the Strait of Sicily. The aims of the work have been to: (1) determine and describe the underlying dynamics of the region; and, (2) rapidly assess synoptic oceanographic conditions through measurements and modeling. Based on the 1994–1996 surveys, a picture of some semi-permanent features which occur in the Strait of Sicily is beginning to emerge. Dynamical circulation studies, with assimilated data from the surveys, indicate the presence of an Adventure Bank Vortex (ABV), Maltese Channel Crest (MCC), and Ionian Shelf Break Vortex (IBV). A schematic water mass model has been developed for the region. Results from the Rapid Response 96 real-time numerical modeling experiments are presented and evaluated. A newly developed data assimilation methodology, Error Subspace Statistical Estimation (ESSE) is introduced. The ideal Error Subspace spans and tracks the scales and processes where the dominant, most energetic, errors occur, making this methodology especially useful in real-time adaptive sampling.

Coupled physical and biological modeling of the spring bloom in the North Atlantic (I): model formulation and one dimensional bloom processes

This is the first of two papers that introduce a mesoscale eddy resolving coupled physical and biological model system. The physical model consists of a quasigeostrophic interior with a fully coupled surface boundary layer. The nitrogen based biological model includes nitrate, phytoplankton, heterotroph and ammonium fields. This interdisciplinary model system is used to examine aspects of the 1989 JGOFS North Atlantic Bloom Experiment data set. This paper deals mainly with one dimensional processes and a companion paper addresses three dimensional phenomena. The data set consists of two time series of observations taken from different water masses in the mesoscale environment. The general features of the two time series are well represented by a one dimensional model when the mesoscale spatial variability in the initial condition is treated explicitly within the one dimensional framework. However, a significant bias is evident in the first time series as the sampling pattern began in a warm feature and moved toward colder ones. Mistaking spatial for temporal variability in this case results in an apparent sink of heat and source of nitrate in the data. Removing this bias with the one dimensional model results in an f-ratio that is almost a factor of two higher (0.64) than computed by other authors based on nutrient inventories and primary productivity measurements (0.37). The second time series was conducted in the interior of a mesoscale feature and spatial biasing is minimal. The model forms a seasonal thermocline and nitracline that compare quite well with the data in both magnitude and vertical extent. A subsurface ammonium maximum is generated by the model from an initially homogeneous profile that also agrees well with the data. Simulated primary productivity profiles match 14C incubations except on the final day of the simulation when surface nutrients appear in to have been exhausted slightly prematurely. Computed f-ratios are consistent with independent estimates based on uptake measurements. A systematic parameter dependence and sensitivity analysis is carried out on these results. The most sensitive parameters are the phytoplankton and heterotroph maximum growth rates. Detailed analysis of the behavior of the system indicates tight coupling between phytoplankton production and heterotrophic consumption even in the early stages of the bloom.

Mediterranean Sea Circulation

Allan R. Robinson, Wayne G. Leslie, Division of Engineering and Applied Sciences, Department of Earth and Planetary Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138, USA Alexander Theocharis, National Centre for Marine Research (NCMR), Aghios Kosmas, Hellinikon 16604, Athens, Greece Alex Lascaratos, Department of Applied Physics, Oceanography Group, University of Athens, University Campus, Building PHYS-V, Athens 15784, Greece