Wayne G. Leslie

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.

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.

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.

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

Mesoscale and upper ocean variabilities during the 1989 JGOFS bloom study

Abstract Altimetric data from Geosat and some critical hydrographic measurements were used to estimate in real time the mesoscale physical oceanographic environment surrounding the Joint Global Ocean Flux Study (JGOFS) 1989 North Atlantic Bloom Experiment. Three cyclonic eddies, including an exceptionally large one, evolved and interacted over the 10 weeks of observations. Subsequent analysis of all available hydrographic data confirmed the real time estimates and provided further quantitative information concerning the mesoscale and submesoscale structure of the upper ocean. Remotely sensed indicators of near-surface chlorophyll content reveal significant biological variability on these wavelengths. The altimetric and hydrographic data have been assimilated into a dynamical model to produce optimal estimates of physical fields of interest as they evolve in time for use in physical and biological process studies.

Estimation and Prediction of Oceanic Eddy Fields

Abstract A system for the optimal estimation and forecasting synoptic/mesoscale ocean currents is presented consisting of an observational system, a statistical model and a dynamical model. The methodology is developed for the case when the dynamical model is a baroclinic, quasigeostrophic open-ocean model. Issues involved include the synoptic and regular gridding of asynoptic non-uniform data, the construction of composite fields from several data types, and the initialization and verification of the dynamical model. Models for the error fields required for weighted combinations of independent field estimates are obtained from the analysis of an observational model; a barotropic example is presented. The statistical model is applied to the POLYMODE Synoptic-Dynamic Experiment (SDE) XBT and current meter data which is successfully assimilated in the dynamical model. Dynamical forecasting experiments are carried out and the difference fields between analysis fields and forecast fields are studied and attributed to: error sources in the analysis, in initial and boundary data, and to the dynamical adjustment of those fields by the model. Quantitative results indicate that useful accuracies can be efficiently obtained.

Data assimilation and dynamical interpolation in GULFCAST experiments

Abstract GULFCAST is a forecast system for the Gulf Stream meander and ring region consisting of a dynamical open-ocean model and an observational network comprised of remotely sensed sea-surface temperatures (and recently, sea-surface height) and critically located air-dropped expendable bathythermographs (AXBTs). We present here the case study of a real-time forecast system out for 2 weeks in the Spring of 1986 during the development of GULFCAST methodology. The AXBT data from successive flights were assimilated and a frontal location was ‘nowcast’ and forecast within the error bounds of navigation, AXBT sampling and model resolution during a multiple ring-stream interaction event.

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.

Advanced interdisciplinary data assimilation: filtering and smoothing via error subspace statistical estimation

The efficient interdisciplinary 4D data assimilation with nonlinear models via error subspace statistical estimation (ESSE) is reviewed and exemplified. ESSE is based on evolving an error subspace, of variable size, that spans and tracks the scales and processes where the dominant error occurs. A specific focus here is the use of ESSE in interdisciplinary smoothing which allows the correction of past estimates based on future data, dynamics and model errors. ESSE is useful for a wide range of purposes which are illustrated by three investigations: (i) smoothing estimation of physical ocean fields in the Eastern Mediterranean, (ii) coupled physical-acoustical data assimilation in the Middle Atlantic Bight shelfbreak, and (iii) coupled physical-biological smoothing and dynamics in Massachusetts Bay.

Experiment in eastern Mediterranean probes origin of deep water masses

During the last decade the oceanography community has focused much attention on the Mediterranean Sea. One reason for the growing interest is that the Mediterraneans impact on the Northern Atlantic Ocean is more significant than previously realized. The warm, salty Mediterranean water tongue exits the Gibraltar Straits and spreads throughout the North Atlantic at all depths between 1000 and 2500 m. The second reason for the surge in interest is the well-recognized role of the Mediterranean Sea as a laboratory for studying ocean processes that are important in global climate dynamics [Malanotte-Rizzoli and Robinson, 1991; Malanotte-Rizzoli and Robinson, 1994].