Avijit Gangopadhyay

Modeling the Gulf Stream System: How far from reality?

Analyses of a primitive equation ocean model simulation of the Atlantic Ocean circulation at 1/6° horizontal resolution are presented with a focus on the Gulf Stream region. Among many successful features of this simulation, this letter describes the Gulf Stream separation from the coast of North America near Cape Hatteras, meandering of the Gulf Stream between Cape Hatteras and the Grand Banks, and the vertical structure of temperature and velocity associated with the Gulf Stream. These results demonstrate significant improvement in modeling the Gulf Stream system using basin- to global-scale ocean general circulation models. Possible reasons responsible for the realistic Gulf Stream simulation are discussed, contrasting the major differences between the present model configuration and those of previous eddy-resolving studies.

Feature-oriented regional modeling of oceanic fronts

This paper outlines some important aspects of modeling oceanic fronts in the context of feature-oriented regional modeling for the deep sea and the Global Coastal Ocean. Previously developed forms of feature models for different types of fronts are presented in a generalized approach. The large-scale meandering frontal systems such as the Gulf Stream, Kuroshio and Brazil current can be represented by velocity-based feature models. Buoyancy forced coastal water mass fronts, such as the coastal currents, the tidal fronts, plume fronts, dense water fronts and inflow/outflow fronts can be represented by a generalized parameterized water mass feature model. The interface region of the deep ocean and the coastal region can be modeled by a melding of two water masses along and across a prescribed isobath in the form of a shelf-break front. Initialization and/or updating fields for a regional dynamical model can then be established in association with other available synoptic data sets via a feature-oriented strategic sampling approach for forecasting and dynamical balances. Example simulations from the western north Atlantic (WNA) and the strait of Sicily region are presented in support of the applicability of this approach for the Global Coastal Ocean. Simulations in the strait of Sicily region with fronts, eddies and background climatology help provide a perspective on dynamical processes in this region. Application of this methodology for rapid assessment of any regional ocean, based on limited data and resources is now possible.

Feature-oriented regional modeling and simulations in the Gulf of Maine and Georges Bank

The multiscale synoptic circulation system in the Gulf of Maine and Georges Bank (GOMGB) region is presented using a feature-oriented approach. Prevalent synoptic circulation structures, or ‘features’, are identified from previous observational studies. These features include the buoyancy-driven Maine Coastal Current, the Georges Bank anticyclonic frontal circulation system, the basin-scale cyclonic gyres (Jordan, Georges and Wilkinson), the deep inflow through the Northeast Channel (NEC), the shallow outflow via the Great South Channel (GSC), and the shelf–slope front (SSF). Their synoptic water–mass (T2S) structures are characterized and parameterized in a generalized formulation to develop temperature–salinity feature models. A synoptic initialization scheme for feature-oriented regional modeling and simulation (FORMS) of the circulation in the coastal-to-deep region of the GOMGB system is then developed. First, the temperature and salinity featuremodel profiles are placed on a regional circulation template and then objectively analyzed with appropriate background climatology in the coastal region. Furthermore, these fields are melded with adjacent deep-ocean regional circulation (Gulf Stream Meander and Ring region) along and across the SSF. These initialization fields are then used for dynamical simulations via the primitive equation model. Simulation results are analyzed to calibrate the multiparameter feature-oriented modeling system. Experimental short-term synoptic simulations are presented for multiple resolutions in different regions with and without atmospheric forcing. The presented ‘generic and portable’ methodology demonstrates the potential of applying similar FORMS in many other regions of the Global Coastal Ocean. r 2003 Elsevier Science Ltd. All rights reserved.

A parametric model for the Brazil Current meanders and eddies off southeastern Brazil

[1] The eddies off of southeastern Brazil are unique in that their water-mass composition is made up of both upwelling waters and the southward flowing Brazil Current. We present a simple asymmetric model parameterization for the temperature and salinity structure of these eddies. Previous hydrographic data and analytical expressions are used to generate the 3-D fields for the Cape Frio and Cape Sao Tome eddies. The resulting geostrophic velocity and transport estimates compare well with previous studies. An example forecast with the Princeton Ocean Model illustrates the usefulness of such parametrization in understanding the eddy-meandering activities. Citation: Calado, L., A. Gangopadhyay, and I. C. A. da Silveira (2006), A parametric model for the Brazil Current meanders and eddies off southeastern Brazil. Geoohvs. Res. Lett..

An interdisciplinary ocean prediction system: Assimilation strategies ana structured data models

Abstract An overview of ongoing research efforts for regional nowcasts, forecasts and hindcasts of physical, biogeochemical and acoustical fields is given. The estimation of oceanic fields is carried out using a modular and flexible system approach, intended to optimize data and model contact, facilitating a unified approach to interdisciplinary research. Recent developments in structured data models are presented, and generic and robust methods to combine structured data models, data streams and model fields, based upon suboptimal interpolation schemes are examined in the framework of data assimilation and dynamical interpolation.

A simple model of interannual displacements of the Gulf Stream

Interannual variations in the latitude of the north wall of the Gulf Stream 1966–1999 can be hindcast by a simple model forced solely by monthly values of the North Atlantic Oscillation (NAO) index. The model is a modified version of the one-dimensional model developed by D. Behringer et al. and utilizes the observed tendency for the centers of the Azores high and the Iceland low to move north or south as the NAO strengthens or weakens. Only one parameter, the active upper layer thickness, is changed from the original values. Although the model incorporates the thermal feedback on the wind stress used by Behringer et al., the results are almost unchanged without it. In generating the Gulf Stream position the model averages the NAO index over several months and introduces a time lag of at least a year. These delays are associated with the time taken for the lateral advection of heat. The model results are also compared with estimates of the latitude of the Gulf Stream made by A. Gangopadhyay et al. 1977–1988 and by T.M. Joyce et al. 1955–1997, and with the series of Gulf Stream transports 1954–1997 constructed by R.G. Curry and M.S. McCartney. The model reproduces the observed weakness of the seasonal variations in comparison to interannual variations. The NAO data are used to hindcast the annual latitude of the Gulf Stream for the period 1825–1999. The results show that since 1970 the Gulf Stream may have been more consistently farther south than during any period of the past 170 years.

Contrasting Response of the Eastern and Western North Atlantic Circulation to an Episodic Climate Event

AbstractRegional observational studies in the North Atlantic have noted significant hydrographical shifts in 1997–98 because of the episodic drop in the North Atlantic oscillation (NAO) during 1996. Investigation using a basin-scale model finds that, although the western North Atlantic (WNA) witnessed unusually low-salinity water by 1997, the eastern North Atlantic (ENA) simultaneously evidenced intrusions of high-salinity water at intermediate depths. This study shows that a major source of high salinity in the ENA is from the northward penetration of Mediterranean Outflow Water (MOW) that occurred concurrently with a westward shift of the subpolar front. The authors confirm that the low-salinity intrusion in the WNA is from enhanced Labrador Current flow. Results from climatological high- and low-NAO simulations suggest that the NAO-induced circulation changes that occurred in 1997–98 are a characteristic North Atlantic basin response to different forcing conditions during characteristic high- and low-N...

An Operational Circulation Modeling System for the Gulf of Maine/Georges Bank Region—Part I: Basic Elements

The basic elements of a prototype operational data assimilation modeling system that can provide near-real-time information on the ocean water property and circulation environment in the Gulf of Maine (GOM)/Georges Bank (GB) region are described in this paper. This application of the Harvard Ocean Prediction System (HOPS, Harvard University, Cambridge, MA) model includes development of protocols for the following: 1) the production of model initial fields from an objective blending of climatological and feature model (FM) hydrographic data with fishing-boat-measured bottom temperature data, 2) the ldquowarm startrdquo of the model to produce reasonably realistic initial model fields, 3) converting real-time Fleet Numerical Meteorological and Oceanographic Center (FNMOC, Monterey, CA) model nowcast and forecast winds and/or National Data Buoy Center (NDBC, Stennis Space Center, MS) operational wind measurements to model wind stress forcing fields, and 4) the assimilation of satellite-derived sea surface temperature (SST). These protocols are shown herein to evolve the initial model fields, which were dominated by climatological data, toward more dynamically balanced, realistic fields. Thus, the model nowcasts, with the assimilation of one SST field, are well positioned to produce reasonably realistic ocean fields within a few model days (MDs).

An Operational Circulation Modeling System for the Gulf of Maine/Georges Bank Region—Part II: Applications

This paper describes the implementation of a prototype operational system for providing near-real-time information on the ocean water property and circulation environment in the Gulf of Mexico (GOM)/Georges Bank (GB) region. This application of the Harvard Ocean Prediction System (HOPS) model to the Advanced Fisheries Management Information System (AFMIS) was developed and tested during a 52-week sequence of weekly nowcast/forecasts. The initial assimilation format for a single sea surface temperature (SST) image was expanded to accommodate the assimilation of a trio of SST images, which produced a more realistic thermocline. The model system was applied to the winter-like conditions of March 2002 and the summer-like conditions of August 2002, when the model temperature and velocity fields could be compared with moored time-series measurements at several locations. This data assimilation model system produced qualitatively correct ocean temperature and flow patterns. During late winter, the wind dominated the variability of the model ocean surface Ekman transport, GB north flank jet, and Maine Coastal Current. During late summer, the assimilation of very warm SST dominated the variability of the model coastal upwelling, a stronger and more stable GB north flank jet, and Maine Coastal Current. The model did reveal a persistent anticyclone in the western central GOM that has not been well documented. Quantitatively, the surface temperatures from the model converged with those measured by the Gulf of Maine Ocean Observing System (GoMOOS) within 4-6 d from the beginning of the model run. Further, while the variability of the model and observed temperatures generally tracked each other, there were cases in which differences between contemporary and climatological temperatures led to systematic offsets. The model/observation velocity comparisons, while in general not as good as those for temperature, did improve after 6-8 d and the assimilation of two SST images. Thus, future work is needed to better understand the quantitative model/observation differences revealed by intercomparisons and to improve the initialization and assimilation protocols.

A Feature Oriented Regional Modeling System for the North Brazil Current Rings Migration after Retroflection

Southeast of the Trinidad-Venezuela region, the North Brazil Current (NBC) retroflects and forms about 5-8 rings annually. The ensemble of trajectories of rings extends offshore of the 500 m isobath, with a mean translation speed of approximately 14 km/day and a mean length scale of about 100 km (Goni and Johns, 2001, 2003). At least two distinct ring types exist: surface-intensified and thermocline-intensified, with differences evident in both azimuthal velocity and water masses. This paper presents a recent implementation of an operational modeling system for this region. The key to this modeling effort is to implement the feature oriented regional modeling methodology for the NBC rings with an advanced initialization scheme to incorporate the varying ring structure, shape, and associated currents made possible by regular surveillance and deployment of instruments into the ring. Multiple observations provide input and guidance to the ring initial conditions for the feature model system. Based on previous studies and data, a water-mass based feature model system for two distinct NBC rings is developed. The parametric models for temperature and salinity are built to capture the main features observed in vertical structure of those rings. For example, in the case of the thermocline-intensified ring, different empirical-analytical functions with tunable parameters are used to represent (i) the thermocline shoaling up in the intermediate depth of the ring, (ii) the dipping down in the inshore and offshore edges, and (iii) the presence of the maximum salinity water at 50-100 m. These feature models are first calibrated with available sea surface temperature (SST) data and then melded with background climatology in a featureoriented multiscale objective analysis to develop a three-dimensional description of the regional ocean. The feature oriented scheme is used to initialize an operational forecasting system using the Harvard Ocean Prediction System (HOPS) framework. Implementation, calibration, and validation of this system were carried out for multiple case studies during 2006 and 2007 when a number of drifter data sets were available. A hindcast study for 27 January 2010 is used for verifying the forecast system in a semi-operational mode. A fully operational system was launched in July 2010. Introduction The NBC is an intense western boundary current and a dominant circulation feature in the western tropical Atlantic. The NBC separates from the South American coastline near 6° – 8° N and retroflects to feed the eastward North Equatorial Counter Current (NECC). The tip of the retroflection grows and stalls typically in the region around 4-5° N, 50-52° W and pinches off to form a closed circulating eddy every 6-8 weeks between July and March. Between late spring (April) and midsummer (June), the retroflection weakens and the northwestward flow may extend to about 7° N and beyond. The resulting eddies, called NBC rings, migrate northwestward parallel to the shelf/slope contours and eventually reach the Lesser Antilles and impact active drilling sites in the region. Previous studies indicate that ring trajectories extend offshore of the 500 m isobath, with a mean translation speed of approximately 14 km/day and a mean length scale of about 100 km and azimuthal velocities from 70–200 cm/s (Goni and Johns, 2001, 2003; Wilson et al., 2002). Fratantoni and Richardson (2006) observed at least two distinct ring-to-ring differences in the three kinds of observed ring structures; one is in the azimuthal velocity structure and the other is in their water mass composition. In other words, two distinct ring types exist: surfaceintensified and thermocline-intensified.