Interannual Variability of Deep-Water Formation in the Northwestern Mediterranean

Abstract

In the Gulf of Lions, observations of deep convection have been sporadically carried out over the past three decades, showing significant interannual variability of convection activity. As long time series of meteorological observations of the region are available from coastal stations, heat flux time series for the Gulf of Lions for the individual winters from 1969 to 1994 are derived by calibrating these observations against direct measurements obtained over the convection site. These heat fluxes are also compared against heat fluxes obtained by the French PERIDOT weather model for the winter of 1991/92. A Kraus-Turner one-dimensional mixed layer model is initialized by climatological mean temperature and salinity profiles and then driven by the heat flux time series of the individual years. Resulting convection depths are in satisfactory agreement with existing observational evidence, showing the dominance of interannual variability of local forcing on convection variability. The interannual variability of convection depth causes interannual variations in deep-water properties, and these are also compared with the hydrographic database.

Factors controlling interannual variability of vertical organic matter export and phytoplankton bloom dynamics – a numerical case-study for the NW Mediterranean Sea

“…In particular, the absolute magnitude of the marked interannual variations predicted by our model in both MLD and HF (Table 1) agree with those reconstructed by observations and numerical modelling by Mertens and Schott (1998) Gulf of Lions that roughly coincides with an area of well documented deep-water formation, often referred to as the MEDOC area (MEDOC, 1970). In this and other successive studies (Gascard, 1978;Marshall and Schott, 1999) the authors observed deep convection and a complete homogenization of the water column down to the bottom (>2000 m) over an area of 50-100 km in diameter.…”

“…The cyclonic circulation leads to an uplift of the isopycnals forming a dome centred on the south of the Gulf of Lions at 42 • N 5 • E. During winter the Mistral from the Rhone Valley and the Tramontana from the north side of the Pyrenees blow over the area. These winds are cold and dry and typically occur in strong bursts, lasting for a few days, that are able to erode the near-surface stratification and expose the weakly stratified waters underneath (Leaman and Schott, 1991;Mertens and Schott, 1998). Deep convection can then occur with a consequent nutrient enrichment of the upper layers that will fuel the next spring bloom.…”

“…Deep convection can then occur with a consequent nutrient enrichment of the upper layers that will fuel the next spring bloom. However, the deep-water formation process is very irregular and can be completely absent during some years (Mertens and Schott, 1998). This variability is likely to be reflected also in bloom dynamics.…”

Recent observations of seasonal variability of the Mediterranean outflow in the Strait of Gibraltar

“…As far as the observed seasonal signal has its origin in the formation of deep water in the Gulf of Lions, a process with clear interannual variability [Mertens and Schott, 1998], this variability must be also important in the outflow through the Strait.…”

Coupled atmosphere ocean climate model simulations in the Mediterranean region: effect of a high-resolution marine model on cyclones and precipitation

“…Its peculiar latitudinal location, in a transitional area between tropical and mid-latitude variability and its complex geometry are important drivers for local phenomena characterizing its climate, such as lee cyclogenesis (Trigo et al, 2002), local winds of sustained speed such as Bora, Mistral, Etesian winds (Zecchetto and De Biasio, 2007), and deep-water formation (Mertens and Schott, 1998;MEDOC Group, 1970).…”