Jean-Claude Gascard

The Formation of Labrador Sea Water. Part I: Large-Scale Processes

Abstract Data obtained in the western Labrador Sea during March 1976 by Hudson are analysed to show that new Labrador Sea Water was being formed at this time. On the basis of hydrographic and moored current-meter data, it is hypothesized that a 200 km scale cyclonic gyre forms in winter in the western Labrador Sea and that this gyre retains the developing deep mixed layers in this general area long enough for the transformation to Labrador Sea Water to take place. Using a model, it is demonstrated that water columns found along the western boundary of the Labrador Sea can be modified by cooling, evaporation and mixing to form deep mixed layers with the properties of Labrador Sea Water. Approximately 105 km3 of new Labrador Sea Water was formed in 1976, an estimate that is consistent with earlier estimates of mean annual production rates. This water, 2.9°C, 34.84‰, is some 0.6°C cooler and 0.06‰ fresher than that defined by Lazier (1973) from his data collected in 1966. The variation of Labrador Sea Water ...

Arctic Ocean Warming Contributes to Reduced Polar Ice Cap

Analysis of modern and historical observations demonstrates that the temperature of the intermediate-depth (150–900 m) Atlantic water (AW) of the Arctic Ocean has increased in recent decades. The AW warming has been uneven in time; a local 1°C maximum was observed in the mid-1990s, followed by an intervening minimum and an additional warming that culminated in 2007 with temperatures higher than in the 1990s by 0.24°C. Relative to climatology from all data prior to 1999, the most extreme 2007 temperature anomalies of up to 1°C and higher were observed in the Eurasian and Makarov Basins. The AW warming was associated with a substantial (up to 75–90 m) shoaling of the upper AW boundary in the central Arctic Ocean and weakening of the Eurasian Basin upper-ocean stratification. Taken together, these observations suggest that the changes in the Eurasian Basin facilitated greater upward transfer of AW heat to the ocean surface layer. Available limited observations and results from a 1D ocean column model support this surmised upward spread of AW heat through the Eurasian Basin halocline. Experiments with a 3D coupled ice–ocean model in turn suggest a loss of 28–35 cm of ice thickness after 50 yr in response to the 0.5 W m−2 increase in AW ocean heat flux suggested by the 1D model. This amount of thinning is comparable to the 29 cm of ice thickness loss due to local atmospheric thermodynamic forcing estimated from observations of fast-ice thickness decline. The implication is that AW warming helped precondition the polar ice cap for the extreme ice loss observed in recent years.

The Formation of Labrador Sea Water. Part II. Mesoscale and Smaller-Scale Processes

Abstract In a previous paper, Clarke and Gascard argued that the formation of Labrador Sea Water was taking place in a cyclonic gyre set up each winter in the western Labrador Sea. Within the gyre and at its boundaries, a number of different scales of organization are believed to be important in the formation processes. The longest of these scales is the mesoscale (50 km), which appears to be related to topographic Rossby waves generated in the Labrador Current and propagating offshore. The next smaller scale is an eddy scale (20 km) believed to arise because the mesoscale is baroclinically unstable, as shown by applying a two-layer model of Tang. This instability is believed to promote mixing by generating frontal structures and vertical motions along them, thus bringing subsurface T-S maxima nearer the surface. Then within the mesoscale and eddy-scale structures, intense vertical convective cells take place at scales which are probably of the order of 1 km in three dimensions. These events are short-liv...

Ice production and brine formation in Storfjorden, Svalbard

Brines, which appear as salty and cold bottom water layers on Arctic shelves, form by salt rejection during sea ice formation. High ice production occurs in latent heat polynyas such as the one that appears in Storfjorden under northerly winds. Using ERS-2 synthetic aperture radar imagery, we observed the ice cover in Storfjorden over the winter 1997/1998, revealing an area of over 6000 km 2 of the open water and thin ice that are characteristic of a polynya. Changes in the polynya extent correlated well with a simple wind-driven polynya size algorithm. Furthermore, using a frazil ice formation algorithm and the hypothesis of ice accumulation at the lee side of the polynya, we distinguished between open water, thin ice, and fast/pack ice. The average polynya width during the 6 winter months was about 30 km, with a maximum of 130 km in March; thus Storfjorden was composed of 5/6 fast and pack ice and 1/6 polynya. Open water occupied about half of the polynya; the other half was composed of brash ice and thin ice involved in ridging and rafting. A total ice volume of 40 km 3 was produced in Storfjorden between November 15, 1997, and May 15, 1998. About 57% of it was formed in the open water area; 9%, in the thin ice area; and 34%, in the fast/pack ice area. This ice production released more than 1 Gt (gigaton) of salt, able to increase the salinity of the Storfjorden waters by 1.4 practical salinity units (psu). Assuming that brine-enriched waters are 0.3 psu more saline than the parent waters, the above salt release is able to ventilate Storfjorden 4-5 times during the course of the productive season.

Exploring Arctic Transpolar Drift During Dramatic Sea Ice Retreat

The Arctic is undergoing significant environmental changes due to climate warming. The most evident signal of this warming is the shrinking and thinning of the ice cover of the Arctic Ocean. If the warming continues, as global climate models predict, the Arctic Ocean will change from a perennially ice-covered to a seasonally ice-free ocean. Estimates as to when this will occur vary from the 2030s to the end of this century. One reason for this huge uncertainty is the lack of systematic observations describing the state, variability, and changes in the Arctic Ocean.

Fate of early 2000s Arctic warm water pulse

The water mass structure of the Arctic Ocean is remarkable, for its intermediate (depth range ~150–900 m) layer is filled with warm (temperature >0°C) and salty water of Atlantic origin (usually called the Atlantic Water, AW). This water is carried into and through the Arctic Ocean by the pan-Arctic boundary current, which moves cyclonically along the basins’ margins (Fig. 1). This system provides the largest input of water, heat, and salt into the Arctic Ocean; the total quantity of heat is substantial, enough to melt the Arctic sea ice cover several times over. By utilizing an extensive archive Fate of Early 2000s Arctic Warm Water Pulse of recently collected observational data, this study provides a cohesive picture of recent large-scale changes in the AW layer of the Arctic Ocean. These recent observations show the warm pulse of AW that entered the Arctic Ocean in the early 1990s finally reached the Canada Basin during the 2000s. The second warm pulse that entered the Arctic Ocean in the mid-2000s has moved through the Eurasian Basin and is en route downstream. One of the most intriguing results of these observations is the realization of the possibility of uptake of anomalous AW heat by overlying layers, with possible implications for an already-reduced Arctic ice cover.

Observational program tracks Arctic Ocean transition to a warmer state

Over the past several decades, the Arctic Ocean has undergone substantial change. Enhanced transport of warmer air from lower latitudes has led to increased Arctic surface air temperature. Concurrent reductions in Arctic ice extent and thickness have been documented. The first evidence of warming in the intermediate Atlantic Water (AW, water depth between 150 and 900 meters) of the Arctic Ocean was found in 1990. Another anomaly, found in 2004, suggests that the Arctic Ocean is in transition toward a new, warmer state [Polyakov et al., 2005, and references therein].

Large-Scale Spreading of Deep Waters in the Western Mediterranean Sea by Submesoscale Coherent Eddies

Abstract Two large-scale free-drifting isobaric-floats experiments, “SOFARGOS”/Marine Science and Technology Programme, phase 2 (MAST2) and Mass Transfer and Ecosystem Response (MATER)/MAST3, undertaken in 1994–95 in the northwestern Mediterranean Sea and in 1997–98 in the Algerian Basin, respectively, have revealed for the first time that Western Mediterranean Deep Water, newly formed by deep convection in the Gulf of Lion (the so-called Medoc site), can be advected several hundreds of kilometers away from the formation area by anticyclonic submesoscale coherent vortices (SCVs). This behavior implies that SCVs participate actively in the large-scale thermohaline circulation and deep ventilation of the western Mediterranean Sea. These SCVs are characterized by small radius (∼5 km), very low potential vorticity, high aspect ratio (∼0.1), and extended lifetime (>0.5 yr).

The mean circulation of the southwestern Mediterranean Sea: Algerian Gyres

This is a study about the general circulation of the southwestern Mediterranean Sea based on observations of currents carried out in the southwestern Mediterranean Sea in the framework of the Mass Transfer and Ecosystem Response (MATER) program (EEC/MAST3 program). From July 1997 to August 2002, profiling floats (MEDPROF experiment), isobaric floats (LIWEX experiment), and moored current meters (ELISA experiment) give evidence of two large-scale barotropic cyclonic circulations, the here-called Western and Eastern Algerian Gyres, centered around [3730′N, 230′E] and [3830′N, 600′E], respectively. These gyres have typical horizontal scales of 100–300 km and are characterized by orbital velocities of about 5 cm/s corresponding to rotational periods of about 4 months. They are strongly related to the bottom topography of the basin and to the planetary vorticity gradient: closed f/H isocontours (f is the planetary vorticity, H the water depth) correspond to the locations of the gyres and favor such circulations as free geostrophic modes. A linear and barotropic model is used to investigate the possibility of wind driving, but the results suggest that the wind stress is not responsible for establishing such circulations. The boundary currents flowing along the continental slope of Africa, Sardinia, and the Balearic Islands are proposed to be the main drivers of these gyres.

Open Ocean Convection and Deep Water Formation Revisited in the Mediterranean, Labrador, Greenland and Weddell Seas

Abstract A number of different scales of organization are believed to be important in the deep convention and deep water formation processes. Among them we clearly identify two scales: the so called chimney and the mesoscale eddies resulting from a baroclinic instability of the chimney. Based on past and recent observations in the Mediterranean, Greenland, Labrador and Weddell Seas, we are discussing the chimney-eddy structure in detail in each of these four distinct areas. Then we analyze the dynamics of this complex structure in order to infer its importance in the context of deep convection and deep water formation in the ocean. Some elements related to bottom topography are presented dealing with the preconditioning state of the chimney-eddy structure formation.