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High-resolution Simulations for the Bay of Bengal: Sensitivity to River Input and Wind Forcing

Jana S., A. Gangopadhyay, P. Lermusiaux, A. Chakraborty, S. Sil, and P. J. Haley, 2017. High-resolution Simulations for the Bay of Bengal: Sensitivity to River Input and Wind Forcing. Ocean Modelling, sub-judice.


We present a multiscale validation and sensitivity study of a set of high-resolution (~9 km) simulations for the Bay of Bengal (BoB) region using the Regional Ocean Modeling System (ROMS). We compare and analyze four main simulations of fifteen years duration each. They utilize different climatological monthly surface wind forcing (weaker COADS or stronger QuikSCAT) and different buoyancy inputs (river inflow with seasonally varying estuarine salinity or with zero salinity). We first complete a statistical validation to establish the model’s overall capability and specific sensitivity in reproducing basin-scale annual and seasonal variabilities. The basin-scale (large-to-mesoscale) performance is quantified in terms of biases, correlations, skills, and root-mean-square-differences (RMSD) against satellite and in situ monthly climatologies. The skill in reproducing the seasonal variability for sea surface temperature (SST), sea surface salinity (SSS), sea surface height (SSH), mixed layer depth (MLD) and depth of the 20°C isotherm (D20) is found to be heterogeneous in space, when compared to the overall annual skill. The skills for SST and SSS were high in all the simulations. The stronger winds and fresher river inflow increased the MLD skill by almost 10% each. The stronger winds however have a significant negative impact on the SSH skill while the added freshness increased the SSH skill minimally.

We then analyze the sensitivity to wind and buoyancy forcing in terms of the ability to capture a number of key processes and features: (i) surface circulation including the boundary currents and monsoonal circulation; (ii) vertical structure of temperature, salinity and stratification; (iii) freshwater plume dispersion; and (iv) coastal upwelling along the western boundary during late spring/summer. We find that the major effects of winds and river inputs are limited to the upper 50 m of the water column in a domain-average sense, with deeper and stronger influence in the northern BoB. The stronger QuikSCAT wind lowers (enhances) the upper ocean temperature (salinity), weakens the stratification, strengthens the springtime western boundary current, enhances eddy activity during summer monsoon, enhances coastal upwelling, and reduces both surface spreading and volume occupation of plume water during autumn. Increasing the coastal buoyancy (fresher river input) reduces the overall salinity at the surface by ~0.4 psu, increases the near-surface stratification in the northern BoB, and enhances the eddy activity from October through May. The lower salinity simulation prefers an eddy-dominant springtime Western Boundary Current (WBC), and enhances freshness, strength, and southward extent of the East India Coastal Current (EICC) core as well as the freshness and plume water inhibition by about 10% over the domain. The zero salinity river input better simulates the domain-wide surface salinity but significantly underestimates the SSS near the river mouths where the estuarine salinity input simulates more realistic SSS.

A Coupled-mode Shallow Water model for tidal analysis: Internal-tide reflection and refraction by the Gulf Stream

Kelly, S.M., P.F.J. Lermusiaux, T. F. Duda, and P.J. Haley Jr., 2016. A Coupled-mode Shallow Water model for tidal analysis: Internal-tide reflection and refraction by the Gulf Stream. J. Phys. Oceanogr., 46, 3661–3679, doi: 10.1175/JPO-D-16-0018.1.

A novel hydrostatic coupled-mode shallow water model (CSW) is developed and used to simulate tides in the greater Middle Atlantic Bight region. The model incorporates realistic stratification and topography, an internal tide generating function (ITGF) that provides internal tide forcing from existing surface tide parameters, and dynamical terms that describe linearized wave- mean-flow and mean-density interactions. Several idealized and realistic simulations are used to verify the model. These verification simulations include internal-tide interactions involving topographic coupling and mean-flow coupling, and comparisons with other simpler and more complex nonlinear primitive-equation models. Then, twenty-four simulations of internal tide generation and propagation in the greater Middle Atlantic Bight region are used to identify significant internal-tide interactions with the Gulf Stream. The simulations indicate that locally generated mode-1 internal tides can refract and/or reflect at the Gulf Stream. The redirected internal tides often re-appear at the shelfbreak, where they produce onshore energy fluxes that are intermittent (i.e., noncoherent) because meanders in the Gulf Stream alter their precise location, phase, and amplitude. These results provide an explanation for the anomalous onshore energy fluxes previously observed at the New Jersey Shelfbreak and linked with the generation of nonlinear internal waves.

Internal-tide Interactions with the Gulf Stream and Middle Atlantic Bight Shelfbreak Front

Kelly, S.M. and P.F.J. Lermusiaux, 2016. Internal-tide Interactions with the Gulf Stream and Middle Atlantic Blight Shelfbreak Front. Journal of Geophysical Research - Oceans, 121, 6271–6294, doi:10.1002/2016JC011639.

Internal tides in the Middle Atlantic Bight region are noticeably influenced by the presence of the shelfbreak front and the Gulf Stream. To identify the dominant interactions of these waves with subtidal flows, vertical-mode momentum and energy partial di fferential equations are derived for small-amplitude waves in a horizontally and vertically sheared mean flow and in a horizontally and vertically variable density fi eld. First, the energy balances are examined in idealized simulations with mode-1 internal tides propagating across and along the Gulf Stream. Next, the fully-nonlinear dynamics of regional tide-mean flow interactions are simulated with a primitive equation model, which incorporates realistic summer mesoscale features and atmospheric forcing. The summer shelfbreak front, which has horizontally variable strati cation, decreases topographic internal-tide generation by about 10% and alters the wavelengths and arrival times of locally generated mode-1 internal tides on the shelf and in the abyss. The (sub)-mesoscale variability at the front and on the shelf, as well as the summer strati cation itself, also alter the internal tide propagation. The Gulf Stream produces anomalous regions of O(20 mW m2) mode-1 internal-tide energy-flux divergence, which are explained by mean-flow terms in the mode-1 energy balance. Advection explains most tide-mean flow interaction, suggesting that geometric wave theory predicts mode-1 reflection and refraction at the Gulf Stream. Geometric theory predicts that o ffshore-propagating mode-1 internal tides that strike the Gulf Stream at oblique angles (more than thirty degrees from normal) are reflected back to the coastal ocean, preventing their radiation into the central North Atlantic.

Global Analysis of Navier-Stokes and Boussinesq Stochastic Flows using Dynamical Orthogonality

Sapsis, T.P., M.P. Ueckermann and P.F.J. Lermusiaux, 2013. Global Analysis of Navier-Stokes and Boussinesq Stochastic Flows using Dynamical Orthogonality, J. Fluid Mech., 734, 83-113. doi:10.1017/jfm.2013.458

We provide a new framework for the study of fl‡uid ‡flows presenting complex uncertain behavior. Our approach is based on the stochastic reduction and analysis of the governing equations using the dynamically orthogonal field equations. By numerically solving these equations we evolve in a fully coupled way the mean fl‡ow and the statistical and spatial characteristics of the stochastic fl‡uctuations. This set of equations is formulated for the general case of stochastic boundary conditions and allows for the application of projection methods that reduce considerably the computational cost. We analyze the transformation of energy from stochastic modes to mean dynamics, and vice-versa, by deriving exact expressions that quantify the interaction among different components of the fl‡ow. The developed framework is illustrated through specifi…c fl‡ows in unstable regimes. In particular, we consider the ‡flow behind a disk and the Rayleigh–-Bénard convection, for which we construct bifurcation diagrams that describe the variation of the response as well as the energy transfers for different parameters associated with the considered ‡flows. We reveal the low-dimensionality of the underlying stochastic attractor.

Circulations and Intrusions Northeast of Taiwan – Chasing Uncertainty in the Cold Dome.

Gawarkiewicz, G., S. Jan, P.F.J. Lermusiaux, J.L. McClean, L. Centurioni, K. Taylor, B. Cornuelle, T.F. Duda, J. Wang, Y.J. Yang, T. Sanford, R.-C. Lien, C. Lee, M.-A. Lee, W. Leslie, P.J. Haley Jr., P.P. Niiler, G. Gopalakrishnan, P. Velez-Belchi, D.-K. Lee, and Y.Y. Kim. 2011. Circulation and intrusions northeast of Taiwan: Chasing and predicting uncertainty in the cold dome. Oceanography, 24(4):110-121, http://dx.doi.org/10.5670/oceanog.2011.99.

An important element of present oceanographic research is the assessment and quantification of uncertainty. These studies are challenging in the coastal ocean due to the wide variety of physical processes occurring on a broad range of spatial and temporal scales. In order to assess new methods for quantifying and predicting uncertainty, a joint Taiwan-US field program was undertaken in August/ September 2009 to compare model forecasts of uncertainties in ocean circulation and acoustic propagation, with high-resolution in situ observations. The geographical setting was the continental shelf and slope northeast of Taiwan, where a feature called the “cold dome” frequently forms. Even though it is hypothesized that Kuroshio subsurface intrusions are the water sources for the cold dome, the dome’s dynamics are highly uncertain, involving multiple scales and many interacting ocean features. During the experiment, a combination of near-surface and profiling drifters, broadscale and high-resolution hydrography, mooring arrays, remote sensing, and regional ocean model forecasts of fields and uncertainties were used to assess mean fields and uncertainties in the region. River runoff from Typhoon Morakot, which hit Taiwan August 7-8, 2009, strongly affected shelf stratification. In addition to the river runoff, a cold cyclonic eddy advected into the region north of the Kuroshio, resulting in a cold dome formation event. Uncertainty forecasts were successfully employed to guide the hydrographic sampling plans. Measurements and forecasts also shed light on the evolution of cold dome waters, including the frequency of eddy shedding to the north-northeast, and interactions with the Kuroshio and tides. For the first time in such a complex region, comparisons between uncertainty forecasts and the model skill at measurement locations validated uncertainty forecasts. To complement the real-time model simulations, historical simulations with another model show that large Kuroshio intrusions were associated with low sea surface height anomalies east of Taiwan, suggesting that there may be some degree of predictability for Kuroshio intrusions.

Special issue of Dynamics of Atmospheres and Oceans in honor of Prof. A.R. Robinson

Lermusiaux, P.F.J, A.J. Miller and N. Pinardi, 2011. Special issue of Dynamics of Atmospheres and Oceans in honor of Prof. A.R. Robinson, Editorial, Dynamics of Atmospheres and Oceans, 52, 1-3, doi:10.1016/j.dynatmoce.2011.08.001.

Professor Allan R. Robinson was one of the founding fathers of geophysical fluid dynamics. His research interests and seminal contributions have encompassed the dynamics of rotating and stratified fluids, boundary-layer flows, thermocline dynamics, the dynamics and modeling of mesoscale ocean currents, and the influence of physical processes on ocean biology. He is recognized as one of the pioneers and leading experts in modern ocean prediction, and contributed significantly to the techniques for the assimilation of data into ocean forecasting models. In the late 1950s and 1960s, Prof. Robinson’s research focused on fundamental geophysical fluid dynamics, including major contributions to thermocline theory, the wind-driven ocean circulation, coastally trapped waves, inertial currents and boundary layers. In the early 1970s, Prof. Robinson initiated investigations on realistic flow fields focusing in particular on mesoscale dynamics and forecasting, with contributions to western boundary currents, mesoscale eddies and baroclinic instabilities. He pioneered “ocean weather forecasting science” at the beginning of the 1980s, especially the development of conceptual models for the assimilation of both in situ and satellite data, specializing in the 1990s in the coupling between the deep sea and the coastal ocean. Focusing on mesoscale dynamics and coastal interactions, he also contributed to the development of new coupled physical-biological-acoustical and optical models, and he developed theories on the effects of oceanic motions on biological dynamics. Professor Robinson was also the Founding Editor of Dynamics of Atmospheres and Oceans.

Oceanographic and Atmospheric Conditions on the Continental Shelf North of the Monterey Bay during August 2006

Ramp, S.R., P.F.J. Lermusiaux, I. Shulman, Y. Chao, R.E. Wolf, and F.L. Bahr, 2011. Oceanographic and Atmospheric Conditions on the Continental Shelf North of the Monterey Bay during August 2006. Dynamics of Atmospheres and Oceans, 52, 192-223, doi:10.1016/j.dynatmoce.2011.04.005.

A comprehensive data set from the ocean and atmosphere was obtained just north of the Monterey Bay as part of the Monterey Bay 2006 (MB06) field experiment. The wind stress, heat fluxes, and sea surface temperature were sampled by the Naval Postgraduate School’s Twin Otter research aircraft. In situ data were collected using ships, moorings, gliders and AUVs. Four data-assimilating numerical models were additionally run, including the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) model for the atmosphere and the Harvard Ocean Prediction System (HOPS), the Regional Ocean Modeling System (ROMS), and the Navy Coastal Ocean Model (NCOM) for the ocean. The scientific focus of the Adaptive Sampling and Prediction Experiment (ASAP) was on the upwelling/relaxation cycle and the resulting three-dimensional coastal circulation near a coastal promontory, in this case Point Ano Nuevo, CA. The emphasis of this study is on the circulation over the continental shelf as estimated from the wind forcing, two ADCP moorings, and model outputs. The wind stress during August 2006 consisted of 3-10 day upwelling favorable events separated by brief 1-3 day relaxations. During the first two weeks there was some correlation between local winds and currents and the three models’ capability to reproduce the events. During the last two weeks, largely equatorward surface wind stress forced the sea surface and barotropic poleward flow occurred over the shelf, reducing model skill at predicting the circulation. The poleward flow was apparently remotely forced by mesoscale eddies and alongshore pressure gradients, which were not well simulated by the models. The small, high-resolution model domains were highly reliant on correct open boundary conditions to drive these larger-scale poleward flows. Multiply-nested models were no more effective than well-initialized local models in this respect.

The California Current System: A Multiscale Overview and the Development of a Feature-Oriented Regional Modeling System (FORMS)

Gangopadhyay, A., P.F.J. Lermusiaux, L. Rosenfeld, A.R. Robinson, L. Calado, H.S. Kim, W.G. Leslie and P.J. Haley, Jr., 2011. The California Current System: A Multiscale Overview and the Development of a Feature-Oriented Regional Modeling System (FORMS). Dynamics of Atmospheres and Oceans, 52, 131-169, doi:10.1016/j.dynatmoce.2011.04.003.

Over the past decade, the feature-oriented regional modeling methodology has been developed and applied in several ocean domains, including the western North Atlantic and tropical North Atlantic. This methodology is model-independent and can be utilized with or without satellite and/or in situ observations. Here we develop new feature-oriented models for the eastern North Pacific from 36 to 48? – essentially, most of the regional eastern boundary current. This is the first time feature-modeling has been applied to a complex eastern boundary current system. As a prerequisite to feature modeling, prevalent features that comprise the multiscale and complex circulation in the California Current system (CCS) are first overviewed. This description is based on contemporary understanding of the features and their dominant space and time scales of variability. A synergistic configuration of circulation features interacting with one another on multiple and sometimes overlapping space and time scales as a meander-eddy-upwelling system is presented. The second step is to define the feature-oriented regional modeling system (FORMS). The major multiscale circulation features include the mean flow and southeastward meandering jet(s) of the California Current (CC), the poleward flowing California Undercurrent (CUC), and six upwelling regions along the coastline. Next, the typical synoptic width, location, vertical extent, and core characteristics of these features and their dominant scales of variability are identified from past observational, theoretical and modeling studies. The parameterized features are then melded with the climatology, in situ and remotely sensed data, as available. The methodology is exemplified here for initialization of primitiveequation models. Dynamical simulations are run as nowcasts and short-term (4-6 weeks) forecasts using these feature models (FM) as initial fields and the Princeton Ocean Model (POM) for dynamics. The set of simulations over a 40-day period illustrate the applicability of FORMS to a transient eastern boundary current region such as the CCS. Comparisons are made with simulations initialized from climatology only. The FORMS approach increases skill in several factors, including the: (i) maintenance of the low-salinity pool in the core of the CC; (ii) representation of eddy activity inshore of the coastal transition zone; (iii) realistic eddy kinetic energy evolution; (iv) subsurface (intermediate depth) mesoscale feature evolution; and (v) deep poleward flow evolution.

Multiscale Physical and Biological Dynamics in the Philippines Archipelago: Predictions and Processes

Lermusiaux, P.F.J., P.J. Haley, Jr., W.G. Leslie, A. Agarwal, O. Logutov and L.J. Burton, 2011. Multiscale Physical and Biological Dynamics in the Philippines Archipelago: Predictions and Processes. Oceanography. PhilEx Issue, 24(1), 70-89, doi:10.5670/oceanog.2011.05.

The Philippine Archipelago is remarkable because of its complex geometry, with multiple islands and passages, and its multiscale dynamics, from the large-scale open-ocean and atmospheric forcing, to the strong tides and internal waves in narrow straits and at steep shelfbreaks. We employ our multiresolution modeling system to predict and study multiscale dynamics in the region, without the use of any synoptic in situ data, so as to evaluate modeling capabilities when only sparse remotely sensed sea surface height is available for assimilation. We focus on the February to March 2009 period, compare our simulation results to ocean observations, and utilize our simulations to quantify and discover oceanic features in the region. The findings include: the physical drivers for the biogeochemical features; the diverse circulation features in each sub-sea and their variations on multiple scales; the flow fields within the major straits and their variability; the transports to and from the Sulu Sea and the corresponding balances; and finally, the multiscale mechanisms involved in the formation of the deep Sulu Sea water.

Preparing to Predict: The Second Autonomous Ocean Sampling Network (AOSN-II) Experiment in the Monterey Bay

Ramp, S.R., R. E. Davis, N. E. Leonard, I. Shulman, Y. Chao, A. R. Robinson, J. Marsden, P.F.J. Lermusiaux, D. Fratantoni, J. D. Paduan, F. Chavez, F. L. Bahr, S. Liang, W. Leslie, and Z. Li, 2009. Preparing to Predict: The Second Autonomous Ocean Sampling Network (AOSN-II) Experiment in the Monterey Bay. Special issue on AOSN-II, Deep Sea Research, Part II, 56, 68-86, doi: 10.1016/j.dsr2.2008.08.013.

The Autonomous Ocean Sampling Network Phase Two (AOSN-II) experiment was conducted in and offshore from the Monterey Bay on the central California coast during July 23-September 6, 2003. The objective of the experiment was to learn how to apply new tools, technologies, and analysis techniques to adaptively sample the coastal ocean in a manner demonstrably superior to traditional methodologies, and to use the information gathered to improve predictive skill for quantities of interest to end-users. The scientific goal was to study the upwelling/relaxation cycle near an open coastal bay in an eastern boundary current region, particularly as it developed and spread from a coastal headland. The suite of observational tools used included a low-flying aircraft, a fleet of underwater gliders, including several under adaptive autonomous control, and propeller-driven AUVs in addition to moorings, ships, and other more traditional hardware. The data were delivered in real time and assimilated into the Harvard Ocean Prediction System (HOPS), the Navy Coastal Ocean Model (NCOM), and the Jet Propulsion Laboratory implementation of the Regional Ocean Modeling System (JPL/ROMS).

Two upwelling events and one relaxation event were sampled during the experiment. The upwelling in both cases began when a pool of cold water less than 13oC appeared near Cape Ano Nuevo and subsequently spread offshore and southward across the bay as the equatorward wind stress continued. The primary difference between the events was that the first event spread offshore and southward, while the second event spread only southward and not offshore. The difference is attributed to the position and strength of meanders and eddies of the California Current System offshore, which blocked or steered the cold upwelled water. The space and time scales of the mesoscale variability were much shorter than have been previously observed in deep-water eddies offshore. Additional process studies are needed to elucidate the dynamics of the flow.

Forecasting and Reanalysis in the Monterey Bay/California Current Region for the Autonomous Ocean Sampling Network-II Experiment.

Haley, P.J. Jr., P.F.J. Lermusiaux, A.R. Robinson, W.G. Leslie, O. Logutov, G. Cossarini, X.S. Liang, P. Moreno, S.R. Ramp, J.D. Doyle, J. Bellingham, F. Chavez, S. Johnston, 2009. Forecasting and Reanalysis in the Monterey Bay/California Current Region for the Autonomous Ocean Sampling Network-II Experiment. Special issue on AOSN-II, Deep Sea Research, Part II. ISSN 0967-0645, doi: 10.1016/j.dsr2.2008.08.010.

During the August-September 2003 Autonomous Ocean Sampling Network-II experiment, the Harvard Ocean Prediction System (HOPS) and Error Subspace Statistical Estimation (ESSE) system were utilized in real-time to forecast physical fields and uncertainties, assimilate various ocean measurements (CTD, AUVs, gliders and SST data), provide suggestions for adaptive sampling, and guide dynamical investigations. The qualitative evaluations of the forecasts showed that many of the surface ocean features were predicted, but that their detailed positions and shapes were less accurate. The root-mean-square errors of the real-time forecasts showed that the forecasts had skill out to two days. Mean one-day forecast temperature RMS error was 0.26oC less than persistence RMS error. Mean two-day forecast temperature RMS error was 0.13oC less than persistence RMS error. Mean one- or two-day salinity RMS error was 0.036 PSU less than persistence RMS error. The real-time skill in the surface was found to be greater than the skill at depth. Pattern correlation coefficient comparisons showed, on average, greater skill than the RMS errors. For simulations lasting 10 or more days, uncertainties in the boundaries could lead to errors in the Monterey Bay region.

Following the real-time experiment, a reanalysis was performed in which improvements were made in the selection of model parameters and in the open-boundary conditions. The result of the reanalysis was improved long-term stability of the simulations and improved quantitative skill, especially the skill in the main thermocline (RMS simulation error 1oC less than persistence RMS error out to five days). This allowed for an improved description of the ocean features. During the experiment there were two-week to 10-day long upwelling events. Two types of upwelling events were observed: one with plumes extending westward at point Ano Nuevo (AN) and Point Sur (PS); the other with a thinner band of upwelled water parallel to the coast and across Monterey Bay. During strong upwelling events the flows in the upper 10-20 m had scales similar to atmospheric scales. During relaxation, kinetic energy becomes available and leads to the development of mesoscale features. At 100-300 m depths, broad northward flows were observed, sometimes with a coastal branch following topographic features. An anticyclone was often observed in the subsurface fields in the mouth of Monterey Bay.

A multigrid methodology for assimilation of measurements into regional tidal models

Logutov, O.G., 2008. A multigrid methodology for assimilation of measurements into regional tidal models. Ocean Dynamics, 58, 441-460, doi:10.1007/s10236-008-0163-4.

This paper presents a rigorous, yet practical, method of multigrid data assimilation into regional structured-grid tidal models. The new inverse tidal nesting scheme, with nesting across multiple grids, is designed to provide a fit of the tidal dynamics to data in areas with highly complex bathymetry and coastline geometry. In these areas, computational constraints make it impractical to fully resolve local topographic and coastal features around all of the observation sites in a stand-alone computation. The proposed strategy consists of increasing the model resolution in multiple limited area domains around the observation locations where a representativeness error is detected in order to improve the representation of the measurements with respect to the dynamics. Multiple high-resolution nested domains are set up and data assimilation is carried out using these embedded nested computations. Every nested domain is coupled to the outer domain through the open boundary conditions (OBCs). Data inversion is carried out in a control space of the outer domain model. A level of generality is retained throughout the presentation with respect to the choice of the control space; however, a specific example of using the outer domain OBCs as the control space is provided, with other sensible choices discussed. In the forward scheme, the computations in the nested domains do not affect the solution in the outer domain. The subsequent inverse computations utilize the observation-minus-model residuals of the forward computations across these multiple nested domains in order to obtain the optimal values of parameters in the control space of the outer domain model. The inversion is carried out by propagating the uncertainty from the control space to model tidal fields at observation locations in the outer and in the nested domains using efficient low-rank error covariance representations. Subsequently, an analysis increment in the control space of the outer domain model is computed and the multigrid system is steered optimally towards observations while preserving a perfect dynamical balance. The method is illustrated using a real-world application in the context of the Philippines Strait Dynamics experiment.

Data driven simulations of synoptic circulation and transports in the Tunisia-Sardinia-Sicily region

Onken, R., A.R. Robinson, P.F.J. Lermusiaux, P.J. Haley Jr. and L.A. Anderson, 2003. Data driven simulations of synoptic circulation and transports in the Tunisia-Sardinia-Sicily region. Journal of Geophysical Research, 108, (C9), 8123-8136.

Data from a hydrographic survey of the Tunisia-Sardinia-Sicily region are assimilated into a primitive equations ocean model. The model simulation is then averaged in time over the short duration of the data survey. The corresponding results, consistent with data and dynamics, are providing new insight into the circulation of Modified Atlantic Water (MAW) and Levantine Intermediate Water (LIW) in this region of the western Mediterranean. For MAW these insights include a southward jet off the east coast of Sardinia, anticyclonic recirculation cells on the Algerian and Tunisian shelves, and a secondary flow splitting in the Strait of Sicily. For the LIW regime a detailed view of the circulation in the Strait of Sicily is given, indicating that LIW proceeds from the strait to the Tyrrhenian Sea. No evidence is found for a direct current path to the Sardinia Channel. Complex circulation patterns are validated by two-way nesting of critical regions. Volume transports are computed for the Strait of Sicily, the Sardinia Channel, and the passage between Sardinia and Sicily.

Features of dominant mesoscale variability, circulation patterns and dynamics in the Strait of Sicily

Lermusiaux, P.F.J. and A.R. Robinson, 2001. Features of dominant mesoscale variability, circulation patterns and dynamics in the Strait of Sicily. Deep Sea Research. 48, (9), 1953-1997.

Combining an intensive hydrographic data survey with a numerical primitive equation model by data assimilation, the main features of dominant mesoscale to subbasin-scale variability in the Strait of Sicily (Mediterranean Sea) during the summer of 1996 are estimated, revealed and described, and several hydrographic and dynamical properties of the #ow and variabilities discussed. The feature identi”cation is based on two independent real-time analyses of the variability. One analysis `subjectivelya evaluates and studies physical “eld forecasts and their variations. The other more `objectivelya estimates and forecasts the principal components of the variability. The two independent analyses are found to be in agreement and complementary. The dominant dynamical variations are revealed to be associated with “ve features: the Adventure Bank Vortex, Maltese Channel Crest, Ionian Shelfbreak Vortex, Messina Rise Vortex, and temperature and salinity fronts of the Ionian slope. These features and their variations are found to have links with the meanders of the Atlantic Ionian Stream. For each feature, the characteristic physical scales, and their deviations, are quanti”ed. The predominant circulation patterns, pathways and transformations of the modi”ed Atlantic water, Ionian water and modi”ed Levantine intermediate water, are then identi”ed and discussed. For each of these water masses, the ranges of temperature, salinity, depth, velocity and residence times, and the regional variations of these ranges, are computed. Based on the estimated “elds and variability principal components, several properties of the dynamics in the Strait are discussed. These include: general characteristics of the mesoscale anomalies; bifurcations of the Atlantic Ionian Stream; respective roles of topography, atmospheric forcings and internal dynamics; factors controlling (strengthening or weakening) the vortices identi”ed; interactions of the Messina Rise and Ionian Shelfbreak vortices; and, mesoscale dynamics and relatively complex features along the Ionian slope. For evaluation and validation of the results obtained, in situ data, satellite sea surface temperature images and trajectories of surface drifters are employed, as well as comparisons with previous studies.

Estimation and study of mesoscale variability in the Strait of Sicily

Lermusiaux, P.F.J., 1999b. Estimation and study of mesoscale variability in the Strait of Sicily. Dynamics of Atmospheres and Oceans, 29, 255-303.

Considering mesoscale variability in the Strait of Sicily during September 1996, the four-dimensional physical fields and their dominant variability and error covariances are estimated and studied. The methodology applied in real-time combines an intensive data survey and primitive equation dynamics based on the error subspace statistical estimation approach. A sequence of filtering and prediction problems are solved for a period of 10 days, with adaptive learning of the dominant errors. Intercomparisons with optimal interpolation fields, clear sea surface temperature images and available in situ data are utilized for qualitative and quantitative evaluations. The present estimation system is shown to be a comprehensive nonlinear and adaptive assimilation scheme, capable of providing real-time forecasts of ocean fields and associated dominant variability and error covariances. The initialization and evolution of the error subspace is explained. The dominant error eigenvectors, variance and covariance fields are illustrated and their multivariate, multiscale properties described. Five coupled features associated with the dominant variability in the Strait during August-September 1996 emerge from the dominant decomposition of the initial PE variability covariance matrix: the Adventure Bank Vortex, Maltese Channel Crest, Ionian Shelf Break Vortex, Strait of Messina Vortex, and subbasin-scale temperature and salinity fronts of the Ionian slope. From the evolution of the estimated fields and dominant predictability error covariance decompositions, several of the primitive equation processes associated with the variations of these features are revealed, decomposed and studied. In general, the estimation of the evolving dominant decompositions of the multivariate predictability error and variability covariances appears promising for ocean sciences and technology. The practical feedbacks of the present approach which include the determination of data optimals and the refinements of dynamical and measurement models are considered.

The Atlantic Ionian Stream

Robinson, A.R., J. Sellschopp, A. Warn-Varnas, W.G. Leslie, C.J. Lozano, P.J. Haley Jr., L.A. Anderson and P.F.J. Lermusiaux, 1999. The Atlantic Ionian Stream. Journal of Marine Systems, 20, 129-156.

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. q1999 Elsevier Science B.V. All rights reserved.

A Topographic-Rossby mode resonance over the Iceland-Faeroe Ridge.

Miller, A.J., P.F.J. Lermusiaux and P.-M. Poulain, 1996. A Topographic-Rossby mode resonance over the Iceland-Faeroe Ridge. Journal of Physical Oceanography, 26 (12), 2735-2747. doi: http://dx.doi.org/10.1175/1520-0485(1996)026<2735:ATMROT>2.0.CO;2.