A nested circulation model for the North Aegean Sea

International audienceA multi-nested approach has been employed for numerical simulations in the northern part of the Aegean Sea in the framework of the MFSTEP (Mediterranean Forecast System: Toward Environmental Predictions) project. The high resolution (~1.6 km) hydrodynamic model of the North Aegean Sea (NAS) has been nested within a coarser model of the Eastern Mediterranean (resolution ~3.6 km) which is also nested within a basin scale model for the Mediterranean Sea (resolution of ~7 km). The high resolution of the NAS model allows the representation of topographic details that have never been reproduced in modelling studies of the region. Such details can enhance the simulation of coastal features, but can also influence basin-scale processes, such as the pathways of waters of Black Sea origin inflowing at the Dardanelles Straits and bifurcating through island passages. We employ comparisons of the North Aegean and Eastern Mediterranean models in terms of computed flow fields and distribution of hydrodynamic properties, to evaluate the nesting procedure, the initialization requirements and the ability of a nested model to perform reliable short term simulations that employ high resolution atmospheric forcing, when initialized from a longer running coarser OGCM. We show that the topographic details of the high resolution, nested NAS model mostly affect the distribution of the Dardanelles plume, while the imposed high frequency, high resolution atmospheric forcing allows for the formation of an overall energetic flow field after a few days of spin-up period. A longer initialization procedure is suggested for the establishment of stronger currents and better developed buoyant plumes

Influence of Dardanelles outflow induced thermal fronts and winds on drifter trajectories in the Aegean Sea

The data provided by 12 drifters deployed in the Northern Aegean Sea in the vicinity of the Dardanelles Strait in August 2008 and February 2009 are used to explore the surface circulation of the basin and the connectivity to the Black Sea. The drifters were deployed within the Dardanelles outflow of waters of Black Sea origin in the Northeastern Aegean. Thanks to the particular choice of the drifter deployment positions, the data set provides a unique opportunity to observe the branching behaviour of the surface currents around Lemnos Island. Such pathways were notpossible to study with previous drifter deployments that were far from the Dardanelles Strait. In addition, the drifter tracks covered the Aegean basin quite thoroughly, mapping major circulation features and supporting the overall general circulation patterns described by previous observational and modelling studies. The collected data display cases in which drifters are driven by winds and thermal fronts. Wind products were used to estimate the influence of the atmospheric forcing on the drifter trajectories. Satellite sea surface temperature images were connected to the drifter tracks, demonstrating a high correlation between the remote and in situ observations. The waters of Black Sea origin were traced all the way to the Southern Aegean, establishing a strong connectivity link between the Aegean and Black Sea basins

Coastal Ocean Forecasting: science foundation and user benefits

The advancement of Coastal Ocean Forecasting Systems (COFS) requires the support of continuous scientific progress addressing: (a) the primary mechanisms driving coastal circulation; (b) methods to achieve fully integrated coastal systems (observations and models), that are dynamically embedded in larger scale systems; and (c) methods to adequately represent air-sea and biophysical interactions. Issues of downscaling, data assimilation, atmosphere-wave-ocean couplings and ecosystem dynamics in the coastal ocean are discussed. These science topics are fundamental for successful COFS, which are connected to evolving downstream applications, dictated by the socioeconomic needs of rapidly increasing coastal populations

Assessing the impact of observations on ocean forecasts and reanalyses: Part 2, Regional applications

The value of global (e.g., altimetry, satellite sea-surface temperature, Argo) and regional (e.g., radars, gliders, instrumented mammals, airborne profiles, biogeochemical) observation-types for monitoring the mesoscale ocean circulation and biogeochemistry is demonstrated using a suite of global and regional prediction systems and remotely-sensed data. A range of techniques is used to demonstrate the value of different observation-types to regional systems and the benefit of high- resolution and adaptive sampling for monitoring the mesoscale circulation. The techniques include Observing System Experiments, Observing System Simulation Experiments, adjoint sensitivities, representer matrix spectrum, observation footprints, information content and spectral analysis. It is shown that local errors in global and basin-scale systems can be significantly reduced when assimilating observations from regional observing systems

Cold event in the South Atlantic Bight during summer of 2003 : model simulations and implications

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): C05022, doi:10.1029/2006JC003903.A set of model simulations are used to determine the principal forcing mechanisms that resulted in anomalously cold water in the South Atlantic Bight (SAB) in the summer of 2003. Updated mass field and elevation boundary conditions from basin-scale Hybrid Coordinate Ocean Model (HYCOM) simulations are compared to climatological forcing to provide offshore and upstream influences in a one-way nesting sense. Model skill is evaluated by comparing model results with observations of velocity, water level, and surface and bottom temperature. Inclusion of realistic atmospheric forcing, river discharge, and improved model dynamics produced good skill on the inner shelf and midshelf. The intrusion of cold water onto the shelf occurred predominantly along the shelf-break associated with onshore flow in the southern part of the domain north of Cape Canaveral (29° to 31.5°). The atmospheric forcing (anomalously strong and persistent upwelling-favorable winds) was the principal mechanism driving the cold event. Elevated river discharge increased the level of stratification across the inner shelf and midshelf and contributed to additional input of cold water into the shelf. The resulting pool of anomalously cold water constituted more than 50% of the water on the shelf in late July and early August. The excess nutrient flux onto the shelf associated with the upwelling was approximated using published nitrate-temperature proxies, suggesting increased primary production during the summer over most of the SAB shelf.The preparation of this paper was primarily supported by the Southeast Atlantic Coastal Ocean Observing System (SEACOOS) and the South Atlantic Bight Limited Area Model (SABLAM). SEACOOS is a collaborative, regional program sponsored by the Office of Naval Research under award N00014-02-1-0972 and managed by the University of North Carolina-General Administration. SABLAM was sponsored by the National Ocean Partnership Program (award NAG 13-00041). Data from ship surveys were collected and processed with the support from NSF grant OCE-0099167 (J. R. Nelson), NSF grant OCE-9982133 (J. O. Blanton, SkIO), NASA grant NAG-10557 (J. R. Nelson), and SEACOOS. NOAA NDBC buoy data and NOS coastal water level records were obtained through NOAA-supported data archives and web portals. Moored instrument data from the Carolina Coastal Ocean Observation and Prediction System (Caro-COOPS) were acquired from the system’s website (http://www.carocoops.org). Caro-COOPS is sponsored by NOAA grant NA16RP2543

Storm-driven sediment transport in Massachusetts Bay

This paper is not subject to U.S. copyright. The definitive version was published in Continental Shelf Research 28 (2008): 257-282, doi:10.1016/j.csr.2007.08.008.Massachusetts Bay is a semi-enclosed embayment in the western Gulf of Maine about 50 km wide and 100 km long. Bottom sediment resuspension is controlled predominately by storm-induced surface waves and transport by the tidal- and wind-driven circulation. Because the Bay is open to the northeast, winds from the northeast (‘Northeasters’) generate the largest surface waves and are thus the most effective in resuspending sediments. The three-dimensional oceanographic circulation model Regional Ocean Modeling System (ROMS) is used to explore the resuspension, transport, and deposition of sediment caused by Northeasters. The model transports multiple sediment classes and tracks the evolution of a multilevel sediment bed. The surficial sediment characteristics of the bed are coupled to one of several bottom-boundary layer modules that calculate enhanced bottom roughness due to wave–current interaction. The wave field is calculated from the model Simulating WAves Nearshore (SWAN). Two idealized simulations were carried out to explore the effects of Northeasters on the transport and fate of sediments. In one simulation, an initially spatially uniform bed of mixed sediments exposed to a series of Northeasters evolved to a pattern similar to the existing surficial sediment distribution. A second set of simulations explored sediment-transport pathways caused by storms with winds from the northeast quadrant by simulating release of sediment at selected locations. Storms with winds from the north cause transport southward along the western shore of Massachusetts Bay, while storms with winds from the east and southeast drive northerly nearshore flow. The simulations show that Northeasters can effectively transport sediments from Boston Harbor and the area offshore of the harbor to the southeast into Cape Cod Bay and offshore into Stellwagen Basin. This transport pattern is consistent with Boston Harbor as the source of silver found in the surficial sediments of Cape Cod Bay and Stellwagen Basin.We gratefully acknowledge support from the USGS Mendenhall Post-Doctoral Research Program for John C. Warner

Mediterranean Sea response to climate change in an ensemble of twenty first century scenarios

The Mediterranean climate is expected to become warmer and drier during the twenty-first century. Mediterranean Sea response to climate change could be modulated by the choice of the socio-economic scenario as well as the choice of the boundary conditions mainly the Atlantic hydrography, the river runoff and the atmospheric fluxes. To assess and quantify the sensitivity of the Mediterranean Sea to the twenty-first century climate change, a set of numerical experiments was carried out with the regional ocean model NEMOMED8 set up for the Mediterranean Sea. The model is forced by air–sea fluxes derived from the regional climate model ARPEGE-Climate at a 50-km horizontal resolution. Historical simulations representing the climate of the period 1961–2000 were run to obtain a reference state. From this baseline, various sensitivity experiments were performed for the period 2001–2099, following different socio-economic scenarios based on the Special Report on Emissions Scenarios. For the A2 scenario, the main three boundary forcings (river runoff, near-Atlantic water hydrography and air–sea fluxes) were changed one by one to better identify the role of each forcing in the way the ocean responds to climate change. In two additional simulations (A1B, B1), the scenario is changed, allowing to quantify the socio-economic uncertainty. Our 6-member scenario simulations display a warming and saltening of the Mediterranean. For the 2070–2099 period compared to 1961–1990, the sea surface temperature anomalies range from +1.73 to +2.97 °C and the SSS anomalies spread from +0.48 to +0.89. In most of the cases, we found that the future Mediterranean thermohaline circulation (MTHC) tends to reach a situation similar to the eastern Mediterranean Transient. However, this response is varying depending on the chosen boundary conditions and socio-economic scenarios. Our numerical experiments suggest that the choice of the near-Atlantic surface water evolution, which is very uncertain in General Circulation Models, has the largest impact on the evolution of the Mediterranean water masses, followed by the choice of the socio-economic scenario. The choice of river runoff and atmospheric forcing both have a smaller impact. The state of the MTHC during the historical period is found to have a large influence on the transfer of surface anomalies toward depth. Besides, subsurface currents are substantially modified in the Ionian Sea and the Balearic region. Finally, the response of thermosteric sea level ranges from +34 to +49 cm (2070–2099 vs. 1961–1990), mainly depending on the Atlantic forcing

A three-dimensional coupled model of the western Black Sea plankton dynamics: Seasonal variability and comparison to SeaWiFS data

The main physical and biological processes that control the seasonal cycle of the plankton dynamics over the Western Black Sea were explored by means of a three‐dimensional, 7‐compartment, on‐line coupled biophysical model that was developed for this study. Adopting high frequency forcing in terms of air‐sea interaction and Danube river inputs, we performed a simulation of the coupled model during the 2002–2003 period. A series of 8‐day Chl‐a SeaWiFS images provided a validation tool that guided us, along with available in situ measurements, to the improvement of model parameterizations and the calibration of the biological parameters. The simulation of the seasonal phytoplankton variability over the entire Western Black Sea, extending from the highly eutrophic river influenced area to the open sea area, was a major challenge that made necessary the representation of both the spatial and time variability of several processes. Despite the model simplicity, the simulated Chl‐a patterns presented a good agreement as compared to the SeaWiFS and in situ data. During winter, phytoplankton in coastal areas was shown to be limited by light availability, primarily due to the increased particulate matter concentrations, as a result of resuspension from the sediment and the increased river loads. During summer, the primary production was mostly sustained by riverine nutrients and regeneration processes and thus was strongly linked to the evolution of the Danube plume. The limiting nutrients showed deviations from the observed concentrations, indicating the necessity for a more realistic phytoplankton growth model

Mississippi River and Sea Surface Height Effects on Oil Slick Migration

Millions of barrels of oil escaped into the Gulf of Mexico (GoM) after the 20 April, 2010 explosion of Deepwater Horizon (DH). Ocean circulation models were used to forecast oil slick migration in the GoM, however such models do not explicitly treat the effects of secondary eddy-slopes or Mississippi River (MR) hydrodynamics. Here we report oil front migration that appears to be driven by sea surface level (SSL) slopes, and identify a previously unreported effect of the MR plume: under conditions of relatively high river discharge and weak winds, a freshwater mound can form around the MR Delta. We performed temporal oil slick position and altimeter analysis, employing both interpolated altimetry data and along-track measurements for coastal applications. The observed freshwater mound appears to have pushed the DH oil slick seaward from the Delta coastline. We provide a physical mechanism for this novel effect of the MR, using a two-layer pressure-driven flow model. Results show how SSL variations can drive a cross-slope migration of surface oil slicks that may reach velocities of order km/day, and confirm a lag time of order 5–10 days between mound formation and slick migration, as observed form the satellite analysis. Incorporating these effects into more complex ocean models will improve forecasts of slick migration for future spills. More generally, large SSL variations at the MR mouth may also affect the dispersal of freshwater, nutrients and sediment associated with the MR plume

Sediment dispersal in the northwestern Adriatic Sea

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): C11S03, doi:10.1029/2006JC003868.Sediment dispersal in the Adriatic Sea was evaluated using coupled three-dimensional circulation and sediment transport models, representing conditions from autumn 2002 through spring 2003. The calculations accounted for fluvial sources, resuspension by waves and currents, and suspended transport. Sediment fluxes peaked during southwestward Bora wind conditions that produced energetic waves and strengthened the Western Adriatic Coastal Current. Transport along the western Adriatic continental shelf was nearly always to the south, except during brief periods when northward Sirocco winds reduced the coastal current. Much of the modeled fluvial sediment deposition was near river mouths, such as the Po subaqueous delta. Nearly all Po sediment remained in the northern Adriatic. Material from rivers that drain the Apennine Mountains traveled farther before deposition than Po sediment, because it was modeled with a lower settling velocity. Fluvial sediment delivered to areas with high average bed shear stress was more highly dispersed than material delivered to more quiescent areas. Modeled depositional patterns were similar to observed patterns that have developed over longer timescales. Specifically, modeled Po sediment accumulation was thickest near the river mouth with a very thin deposit extending to the northeast, consistent with patterns of modern sediment texture in the northern Adriatic. Sediment resuspended from the bed and delivered by Apennine Rivers was preferentially deposited on the northern side of the Gargano Peninsula, in the location of thick Holocene accumulation. Deposition here was highest during Bora winds when convergences in current velocities and off-shelf flux enhanced delivery of material to the midshelf.The authors are grateful for funding and support from the Office of Naval Research’s Coastal Geosciences and Marine Modeling programs, the U.S. Geological Survey, and NATO’s SACLANT-CEN