S. Sofianos

An Oceanic General Circulation Model (OGCM) investigation of the Red Sea circulation: 2. Three‐dimensional circulation in the Red Sea

[1] The three-dimensional circulation of the Red Sea is studied using a set of Miami Isopycnic Coordinate Ocean Model (MICOM) simulations. The model performance is tested against the few available observations in the basin and shows generally good agreement with the main observed features of the circulation. The main findings of this analysis include an intensification of the along-axis flow toward the coasts, with a transition from western intensified boundary flow in the south to eastern intensified flow in the north, and a series of strong seasonal or permanent eddy-like features. Model experiments conducted with different forcing fields (wind-stress forcing only, surface buoyancy forcing only, or both forcings combined) showed that the circulation produced by the buoyancy forcing is stronger overall and dominates the wind-driven part of the circulation. The main circulation pattern is related to the seasonal buoyancy flux (mostly due to the evaporation), which causes the density to increase northward in the basin and produces a northward surface pressure gradient associated with the downward sloping of the sea surface. The response of the eastern boundary to the associated mean crossbasin geostrophic current depends on the stratification and b-effect. In the northern part of the basin this results in an eastward intensification of the northward surface flow associated with the presence of Kelvin waves while in the south the traditional westward intensification due to Rossby waves takes place. The most prominent gyre circulation pattern occurs in the north where a permanent cyclonic gyre is present that is involved in the formation of Red Sea Outflow Water (RSOW). Beneath the surface boundary currents are similarly intensified southward undercurrents that carry the RSOW to the sill to flow out of the basin into the Indian Ocean. INDEX TERMS: 4243 Oceanography: General: Marginal and semienclosed seas; 4255 Oceanography: General: Numerical modeling; 4532 Oceanography: Physical: General circulation; KEYWORDS: Red Sea, marginal sea, Oceanic General Circulation Model, water mass formation

Heat and freshwater budgets in the Red Sea from direct observations at Bab el Mandeb

Abstract Direct oceanographic observations at the strait of Bab el Mandeb, which connects the Red Sea with the Indian Ocean, are used to estimate the heat and freshwater budgets of the Red Sea. The observations consist of 18-month records from transport resolving current-meter arrays and temperature–salinity chain moorings that allow accurate heat and salt fluxes through the strait to be determined. The new measurements account for the seasonally reversing surface-layer flow in the strait and the summertime intrusion of low-salinity thermocline water from the Gulf of Aden. The annual mean freshwater loss to the atmosphere over the Red Sea is estimated to be 2.06±0.22 m yr −1 , while the annual mean heat loss is 11±5 W m −2 . These results are compared with previous estimates of the Red Sea heat and freshwater budgets derived from both direct and indirect methods, which show large differences between each other and much larger uncertainties than the present estimates.

Wind induced sea level variability in the Red Sea

Analysis of TOPEX/Poseidon altimetry data for the Red Sea shows a large seasonal sea level signal in agreement with the few available coastal sea level stations. The results are compared with the predictions from a simple model that balances the sea surface elevation against the wind stress. The winter sea level field in the central and northern Red Sea is shown to be a response to the convergent wind pattern associated with the Indian Monsoon, consisting of NNW winds in the northern Red Sea and SSE winds in the south. There is a very good agreement between the model and observations, indicating that the basic mechanism responsible for the variability of the sea surface height inside the Red Sea on seasonal time scales is the wind pattern.

Atmospherically Forced Exchange through the Bab el Mandeb Strait

AbstractThe exchange between the Red Sea and the Indian Ocean on synoptic time scales (days to weeks) is investigated using moored current meter data collected in the strait of Bab el Mandeb from June 1995 to November 1996. Transport variations through the strait on these time scales can reach amplitudes of up to 0.6 Sv (1 Sv ≡ 106 m3 s−1), or nearly twice as large as the mean rate of exchange through the strait driven by annual evaporation over the Red Sea. The synoptic transport variability appears to be driven by two primary forcing mechanisms: 1) local wind stress variability over the strait and 2) variation in the large-scale barometric pressure over the Red Sea. Simple models of the forced response are developed and are shown to reproduce the essential features of the observations. The response to barometric pressure forcing over the Red Sea is fundamentally barotropic, whereas the response to along-strait winds is barotropic at high frequencies and tends toward a two-layer exchange at low frequenci...

Factors governing the deep ventilation of the Red Sea

A variety of data based on hydrographic measurements, satellite observations, reanalysis databases, and meteorological observations are used to explore the interannual variability and factors governing the deep water formation in the northern Red Sea. Historical and recent hydrographic data consistently indicate that the ventilation of the near-bottom layer in the Red Sea is a robust feature of the thermohaline circulation. Dense water capable to reach the bottom layers of the Red Sea can be regularly produced mostly inside the Gulfs of Aqaba and Suez. Occasionally, during colder than usual winters, deep water formation may also take place over coastal areas in the northernmost end of the open Red Sea just outside the Gulfs of Aqaba and Suez. However, the origin as well as the amount of deep waters exhibit considerable interannual variability depending not only on atmospheric forcing but also on the water circulation over the northern Red Sea. Analysis of several recent winters shows that the strength of the cyclonic gyre prevailing in the northernmost part of the basin can effectively influence the sea surface temperature (SST) and intensify or moderate the winter surface cooling. Upwelling associated with periods of persistent gyre circulation lowers the SST over the northernmost part of the Red Sea and can produce colder than normal winter SST even without extreme heat loss by the sea surface. In addition, the occasional persistence of the cyclonic gyre feeds the surface layers of the northern Red Sea with nutrients, considerably increasing the phytoplankton biomass.

Recent progress in performance evaluations and near real-time assessment of operational ocean products

Operational ocean forecast systems provide routine marine products to an ever-widening community of users and stakeholders. The majority of users need information about the quality and reliability of the products to exploit them fully. Hence, forecast centres have been developing improved methods for evaluating and communicating the quality of their products. Global Ocean Data Assimilation Experiment (GODAE) OceanView, along with the Copernicus European Marine Core Service and other national and international programmes, has facilitated the development of coordinated validation activities among these centres. New metrics, assessing a wider range of ocean parameters, have been defined and implemented in real-time. An overview of recent progress and emerging international standards is presented here.

Water Mass Formation, Overturning Circulation, and the Exchange of the Red Sea with the Adjacent Basins

The Red Sea experiences strong atmospheric forcing through both wind stress and air–sea buoyancy fluxes. Direct observations and modeling experiment show a robust response that consists of a strong and complicated three-dimensional circulation pattern with intense seasonal variability, involving water masses that are locally formed in the Red Sea or enter it from adjacent basins. Two thermohaline cells are identified related to intermediate (Red Sea Outflow Water—RSOW) and deepwater (Red Sea Deep Water—RSDW) formation processes. Results from numerical simulations indicate that the permanent cyclonic gyre in the northern end of the basin is the most probable location for the RSOW formation to take place, but further investigation with observations and numerical modeling techniques is needed to better understand the processes involved as well as the role of the Gulfs of Suez and Aqaba in the regional thermohaline circulation. The Red Sea circulation is closely linked to the flow pattern in the Strait of Bab-al-Mandab where the exchange of the Red Sea with the Indian Ocean takes place. The exchange is of an inverse estuarine type, which compensates for the strong heat and freshwater fluxes in the basin, but with very strong seasonal and synoptic variability related to remote and local forcing. Although important progress has been achieved during the last two decades in describing and understanding basic processes of the Red Sea dynamics, several features are not yet understood and explained. Further observational and modeling activities are required to improve our understanding of these processes and should be combined in an interdisciplinary approach to improve our monitoring and forecasting capabilities for environmental management and protection.

Long Term Variability of Sea Surface Temperature in Mediterranean Sea

The long term variability of the sea surface temperature (SST) of the Mediterranean basin and its sub-basins for the period 1869-2006 (138 years) is investigated using the International Comprehensive Ocean-Atmosphere Data Set (I-COADS). Analysis of the SST time-series revealed a positive trend in both basin and sub-basin scale. During the last century, the highest positive SST trend is found in the Adriatic Sea (0.0141 o C/y) and the lowest one in the Aegean sea (0.0011 o C/y). This difference in the SST evolution in the two sub-basins can be related to the shift of the Eastern Mediterranean deep water formation site during the 90s, known as Eastern Mediterranean Transient (EMT). The SST variations of the Eastern Mediterranean sub-basins (Adriatic Sea, Ionian Sea, Aegean Sea, Levantine Sea) are highly correlated to each other, in contrast to the poor correlation of the SST variations between the Eastern and Western Mediterranean Sea. Harmonic analysis has shown that a dominant period of the Mediterranean variability is similar to the deep water turnover time of the basin. Comparison with climatic indices points out a high correlation of the Western Mediterranean and Adriatic Sea SST with the NAO index, while the Eastern Mediterranean SST variations are highly correlated to the Indian Summer Monsoon Index.

The Summer Circulation in the Gulf of Suez and Its Influence in the Red Sea Thermohaline Circulation

AbstractThe Gulf of Suez is accepted as an important location for Red Sea Deep Water formation, but the circulation and exchange with the Red Sea around the year remains elusive. A summer cruise in the area gives the opportunity to investigate features of the summertime hydrological structure and exchange with the Red Sea. An inverse estuarine circulation and exchange with the Red Sea is evident. The topographic patterns of the gulf play an important role in the circulation. Two sills, one in midbasin and a second at the mouth of the gulf, inhibit the bottom flow, topographically trapping waters that were formed in the cold season. Although the water mass characteristics of the outflowing waters during the other seasons are not directly related to the deep waters, they can influence the water column structure of the northern Red Sea. A simple box model shows that their contribution can have a significant influence in the formation of the intermediate layer. A hypersaline (40.6 psu) but relatively warm (23...

Physical and biological characteristics of the winter‐summer transition in the Central Red Sea

The Central Red Sea (CRS) lies between two distinct hydrographic and atmospheric regimes. In the southern Red Sea, seasonal monsoon reversal regulates the exchange of water between the Red Sea and the Indian Ocean. In the northern Red Sea intermediate and occasionally deep water are formed during winter to sustain the basins overturning circulation. Highly variable mesoscale eddies and the northward flowing eastern boundary current (EBC) determine the physical and biogeochemical characteristics of the CRS. Ship-based and glider observations in the CRS between March and June 2013 capture key features of the transition from winter to summer and depict the impact of the eddy activity on the EBC flow. Less saline and relatively warmer water of Indian Ocean origin reaches the CRS via the EBC. Initially, an anticyclonic eddy with diameter of 140km penetrating to 150m depth with maximum velocities up to 30-35 cm s−1 prevails in the CRS. This anticyclonic eddy appears to block or at least redirect the northward flow of the EBC. Dissipation of the eddy permits the near-coastal, northward flow of the EBC and gives place to a smaller cyclonic eddy with a diameter of about 50km penetrating to 200m depth. By the end of May, as the northerly winds become stronger and persistent throughout the basin, characteristic of the summer southwest monsoon wind regime, the EBC and its associated lower salinity water became less evident, replaced by the saltier surface water that characterizes the onset of the summer stratification in the CRS.