Fast deep sinking in Mediterranean eddies

Abstract

[1] Large (up to 0.03 m s À1 ) downward vertical water velocities (w) are observed using yearlong moored ADCP at $2400 m in the deep Mediterranean Sea's Algerian basin. Once every 2 -3 months, jwj rapidly increases to O(0.01 ± 0.002 m s À1 ) before slowly decreasing during a few weeks, in association with the passage of mesoscale eddies formed nearby. These amplitudes of negative w are O(100) times larger than those commonly linked to mesoscale eddies near the surface (subduction), and O(10) times larger than settling velocities for marine snow. Our observations suggest that mesoscale eddies, which are important for biological productivity near the surface, can also convect nearly fresh material down to the bottom ($3000 m there) within a few days. Citation: van Haren, H., C. Millot, and I. Taupier-Letage (2006), Fast deep sinking in Mediterranean eddies, Geophys. Res. Lett., 33, L04606,

Deposition processes from echo-character mapping along the western Algerian margin (Oran–Tenes), Western Mediterranean

“…This superficial current is 200-400 m thick. In addition, some mesoscale cyclonic and/or anticyclonic eddies (100-200 km diameter) sometimes form along the coast and cause westward flows that create upwelling or downwelling, which can transport sediment particles down to the bottom with a velocity of 0.05 m/s (Van Haren et al, 2006). Because we observe migrating sediment waves on the Chirp lines, we need to understand the role of these currents, if any, in the deep-basin sedimentation at depths of 2000-3000 m. According to Millot et al (1997), at 2000 m depth, the main currents are eastward and up to 4 cm/s.…”

Mesoscale eddies: hotspots of prokaryotic activity and differential community structure in the ocean

“…Mesoscale eddies are ubiquitous features in the ocean (Cheney and Richardson, 1976;Arístegui et al, 1997;van Haren et al, 2006), with strong implications on regional biogeochemistry and productivity. Anticyclonic eddies (AEs) have been seen to accumulate organic matter within their cores (for example Arístegui et al, 2003;Mathis et al, 2007) and to exhibit elevated microbial respiration (Arís-tegui and Montero, 2005;Mouriñ o-Carballido and McGillicuddy, 2006) and heterotrophic production (Baltar et al, 2007;Ewart et al, 2008).…”

Acoustic and optical variations during rapid downward motion episodes in the deep north-western Mediterranean Sea

“…Such w-value has been observed using upward looking 75 kHz ADCPs (Schott et al, 1991;van Haren et al, 2006). Part of this w can be attributed to sinking materials, although their typical downward speeds are commonly -1×10 -3 m s -1 or less (Passow, 1991;Lampitt et al, 1993).…”

“…We speculate that one can relate sea surface satellite images with deep-sea ADCP observations, as seems justifiable for quasi-barotropic Gulf Stream and Algerian mesoscale eddies, with >2000 m vertical extent (Richardson, 1983;Millot et al, 1997;van Haren et al, 2006), and also for sub-mesoscale eddies with >1000 m vertical uniformity (Testor and Gascard, 2006). For the presumed more baroclinic NC, the present observations compare to some extent with models on dipoles, which have near-zero phase speed due to their interaction with the sheared current deep below (Griffiths and Pearce, 1985;Crépon et al, 1989).…”

“…ii) Advection due to horizontal density gradient, frontal zones, such as confirmed in the upper 1000 m off Nice (Stemmann et al, 2008), and mesoscale eddies, such as occur in the Algerian subbasin (van Haren et al, 2006 North-South (v), vertical (w), focussing on episodic relatively large downward w. The echo intensity data are interpreted qualitatively as suspended material, mainly zooplankton. These data are compared with certain observations from PMTs, which are related to bioluminescence.…”