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R. Allyn Clarke
and
Jean-Claude Gascard

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 and its rate of production over the last 50 years is discussed.

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Jean-Claude Gascard
and
R. Allyn Clarke

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-lived and occur in response to particularly intense air-sea exchanges.

Most of these processes have already been recognized in the Mediterranean Sea (MEDOC): that is, baroclinic instability of mesoscale features generating mixing at an eddy scale which is quite small because the scale is related to the internal radius of deformation (5–10 km). What is new is the link between the unstable mesoscale structures and the large-scale general circulation through the generation of topographic Rossby waves.

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Fabienne Gaillard
,
Jean-Claude Gascard
, and
Patrice Klein

Abstract

In order to access the statistical properties of the mesoscale dynamics in the Western Mediterranean, and its associated transport and heat fluxes during the postconvection period, the authors have applied data combination methods for analyzing a wide range of in situ measurements collected during the Thetis 1 and Convhiv experiments. CTD and XBT profiles were merged with times series at a fixed or moving point and also with integral time series obtained from acoustic tomography data. Estimates of temperature and currents within a box of approximately one degree square, over a time period of 35 days during the postconvection period, were produced. During this winter, convection has been only partial, rarely penetrating deeper than 1200 m. The analysis concentrates on the upper 1000 m, where most changes occur. Geostrophy is used as a dynamical constraint relating the parameters. The time evolution is controlled by a Kalman filter using simple persistence.

The contribution of the different datasets to the estimation indicates their complementarity in the time and space dimensions. Hydrography and Eulerian measurements provide a major contribution to the estimation of the baroclinic modes. Tomography data complement the estimate at all horizontal scales. Float data bring some information on the barotropic mode but the major contribution on this mode comes from the reciprocal tomography data, particularly at the largest scales. The period analyzed mostly covers the postconvection. Estimation of the kinetic energy indicates that the barotropic contribution represents 85% of the total energy. Horizontal advection transfers heat toward the central area at a mean rate of 50 W m−2 compensating for the heat losses through the surface. The mesoscale flow field observed is characterized by strongly barotropic coherent vortices with a size O(30–40 km). These barotropic eddies are present during all phases of convection.

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Gilles Reverdin
,
Jean-Claude Gascard
,
Bernard Le Cann
,
Louis Prieur
,
Michel Assenbaum
, and
Pascale Lherminier

Abstract

An anticyclonic mode water vortex and its environment were investigated from November 2000 to September 2001 in the northeast Atlantic (near 43.5°N, 15°–19°W) with neutrally buoyant drifting floats, moored current meters, satellite altimetric sea surface height, and several hydrological surveys and sections. These observations reveal a coherent inner core (∼30 km in diameter) made of very oxygenated northeast Atlantic central waters (11°–12.7°C and 35.5–35.7 on the 1978 practical salinity scale) from 150 m down to about 750-m depth. The core presents high relative vorticity (up to approximately −0.5 times the Coriolis frequency f ) within at least 10 km of its center, near 400–700 m. Peak velocity along the core rim is located deeper than 600 m bordering the deepest and densest (σθ = 27.175 kg m−3) northeast Atlantic mode water found during the Programme Océan Multidisciplinaire Méso Echelle (POMME) project. This water likely originates north of 47°N, where it could have been in contact with the sea surface in early 1999. Below the core, large near-inertial internal waves are found. At least during spring and summer 2001, the core was embedded in a much larger anticyclonic eddy that extends to 100 km from its center, with azimuthal velocity decreasing from the sea surface to 1500 m. This eddy does not trap floats for a long time and is associated with a sea level anomaly on the order of 10 cm. From January through August 2001, both the core and the larger eddy moved anticyclonically around a shallow part of the Azores–Biscay ridge. The core trajectory also exhibits smaller anticyclonic loops on shorter time scales, suggesting that at least at times it is not located at the center of the larger eddy.

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Igor V. Polyakov
,
Leonid A. Timokhov
,
Vladimir A. Alexeev
,
Sheldon Bacon
,
Igor A. Dmitrenko
,
Louis Fortier
,
Ivan E. Frolov
,
Jean-Claude Gascard
,
Edmond Hansen
,
Vladimir V. Ivanov
,
Seymour Laxon
,
Cecilie Mauritzen
,
Don Perovich
,
Koji Shimada
,
Harper L. Simmons
,
Vladimir T. Sokolov
,
Michael Steele
, and
John Toole

Abstract

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.

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Igor V. Polyakov
,
Vladimir A. Alexeev
,
Igor M. Ashik
,
Sheldon Bacon
,
Agnieszka Beszczynska-Möller
,
Eddy C. Carmack
,
Igor A. Dmitrenko
,
Louis Fortier
,
Jean-Claude Gascard
,
Edmond Hansen
,
Jens Hölemann
,
Vladimir V. Ivanov
,
Takashi Kikuchi
,
Sergey Kirillov
,
Yueng-Djern Lenn
,
Fiona A. McLaughlin
,
Jan Piechura
,
Irina Repina
,
Leonid A. Timokhov
,
Waldemar Walczowski
, and
Rebecca Woodgate

No Abstract available.

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