Hydrography, Circulation, and Mixing at the Calypso Deep (the Deepest Mediterranean Trough) during 2006–09 (original) (raw)
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Interbasin deep water exchange in the western Mediterranean
Journal of Geophysical Research, 1999
Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean. The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins [1988]. There, an outflow of water from the Tyrrhenian Sea into the Algero Provencal Basin was postulated in the depth range 700-1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero-Proven•al Basin. A new data[ set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero-Proven•al Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 rn to 1600-1900 rn depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv=10 era 3 8-1), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW))from the Algero-Proven•al Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero-Proven•al Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior. not considered to contribute significantly to the general intermediate and deep basin circulation [Astraldi and Gasparini, 1994]. Thus deep and intermediate water exchange (below 500 m depth) between the Tyrrhenian Sea and the Algero-Provenc]al Basin has to occur through the Sardinia Channel. The Strait of Sicily southeast of the Tyrrhenian Sea is relatively wide and shallow with a maximum depth of about 430 m, connecting the eastern Mediterranean with the western part. The deep and intermediate water exchange of the Tyrrhenian Sea has been studied by Hopkins [1988] using hydrographic data from the southern Tyrrhenian Sea and two current meters (at 347 and 1763 m depth) in the Tyrrhenian Trough, which were deployed for 70 days. Most of the time, the flow of the deep current meter was directed northeastward, into the Tyrrhenian Sea. Taking one of the two Conductivity-temperaturedepth (CTD) cross sections through the trough and adjusting the geostrophic velocity profile to the mean deep moored current meter velocity (0.9 cm s-•), a transport 23,495
Mixing over the steep side of the Cycladic Plateau in the Aegean Sea
Journal of Marine Systems, 2012
Intensive microstructure sampling over the southern slope of the Cycladic Plateau found very weak mixing in the pycnocline, centered on a thin minimum of diapycnal diffusivity with K=1.5×10 m 2 s - 1 . Below the pycnocline, K increased exponentially in the bottom 200 m, reaching 1 × 10 - 4 m 2 s - 1 a few meters above the bottom. Near-bottom mixing was most intense where the bottom slope equaled the characteristic slope of the semi-diurnal internal tide. This suggests internal wave scattering and/or generation at the bottom, a conclusion supported by near-bottom dissipation rates increasing following rising winds and with intensifying internal waves. Several pinnacles on the slope were local mixing hotspots. Signatures included a vertical line of strong mixing in a pinnacle's wake, an hydraulic jump or lee wave over a downstream side of the summit, and a 'beam' sloping upward at the near-inertial characteristic slope. Because dissipation rate averages were dominated by strong turbulence, ɛ/ νN2 > 100, the effect on K of alternate mixing efficiencies proposed for this range of turbulent intensity is explored.
Shallow rainwater lenses in deltaic areas with saline seepage
Hydrology and Earth System Sciences, 2011
In deltaic areas with saline seepage, fresh water availability is often limited to shallow rainwater lenses lying on top of saline groundwater. Here we describe the characteristics and spatial variability of such lenses in areas with saline seepage and the mechanisms that control their occurrence and size. Our findings are based on different types of field measurements and detailed numerical groundwater models applied in the south-western delta of The Netherlands. By combining the applied techniques we could extrapolate in situ measurements at point scale (groundwater sampling, TEC (temperature and electrical soil conductivity)-probe measurements, electrical cone penetration tests (ECPT)) to a field scale (continuous vertical electrical soundings (CVES), electromagnetic survey with EM31), and even to a regional scale using helicopter-borne electromagnetic measurements (HEM). The measurements show a gradual S-shaped mixing zone between infiltrating fresh rainwater and upward flowing saline groundwater. The mixing zone is best characterized by the depth of the centre of the mixing zone Dmix, where the salinity is half that of seepage water, and the bottom of the mixing zone Bmix, with a salinity equal to that of the seepage water (Cl-conc. 10 to 16 g l-1). Dmix manifests at very shallow depth in the confining top layer, on average at 1.7 m below ground level (b.g.l.), while Bmix lies about 2.5 m b.g.l. Head-driven forced convection is the main mechanism of rainwater lens formation in the saline seepage areas rather than free convection due to density differences. Our model results show that the sequence of alternating vertical flow directions in the confining layer caused by head gradients determines the position of the mixing zone (Dmix and Bmix and that these flow directions are controlled by seepage flux, recharge and drainage depth.
ESTIMATION OF VERTICAL MIXING IN THE NORTH AEGEAN SEA BASED ON ARGO FLOAT DATA
The North Aegean Sea is considered as a region of deep water formation of the Mediterranean. Dense water formation events are known to take place rather infrequently and in the time intervals between such events, the bottom waters are excluded from interaction with other water masses through advection. In order to examine the evolution of deep waters during those periods at shorter-than-annual time scales, new, high-frequency data from a profiling ARGO float were analyzed. The specific MedArgo float (nr.6901884) was trapped during 2014-2015 within the deep Athos basin, and remained there for 13 months. The analyzed hydrographic profiles point out, that the Black Sea Water surface layer is an effective isolator between the deep layers of the North Aegean and the atmosphere, absorbing large amounts of heat and buoyancy and hindering dense water formation. Also, the temporal evolution of the averaged potential density anomaly possibly indicates an alternation ofreveals a short-termed dense-water formation that affected the layers shallower than 600 m, that was followed (after March 2015) and by a longer-termed stagnation period lasting till November 2015. only in the surface layer and the intermediate layer. It seems that the water masses deeper than 600m remain almost constant, without being affected by the dominant seasonal cycle. It could be possible to conclude, that the replenishment of deep basins takes place in longer-than-annual cycles, since the dominant seasonal cycle seems to affect only the upper water masses. It is noteworthy, that the vertical eddy diffusion coefficient K_S (based on the observed rate of change of salinity) indicated a positive conductivity sensor drift of the profiling float. Furthermore, the conductivity corrections verified a remarkable drift in salinity of 0,0019 to 0,0113 due to the increasing conductivity sensor drift rate of 4,6187*10-5 mmho day-1. After the calibration correcting for the sensor’s drift, the eddy diffusion coefficients K_σθ, K_S, K_T were found to range between 6-7×10-5 and 2-3×10-3 m2s-1 for the deeper than 400m waters. Considering that the deep basins of the North Aegean are practically isolated below the 400 m threshold during the stagnation period, the good agreement of the three diffusion coefficients Kc and the fact that the properties of deeper water masses slide along the θ-S curve towards lower densities suggests that the dominant process in vertical diffusion is turbulent mixing.
Distribution and mixing of intermediate water masses in the Channel of Sicily (Mediterranean Sea)
Journal of Geophysical Research, 2003
The Channel of Sicily, that represents the only connection between the western and the eastern Mediterranean sea, can be described as a three-layer system, where fresher water of Atlantic origin (MAW) flows eastward in the upper layer, Levantine Intermediate Water (LIW) leaves the eastern basin at around 250-350 m, and an outflow of Transitional Ionian Deep Water (TIDW) is identified below the LIW. SYMPLEX (SYnoptic Mesoscale PLankton EXperiment) data, collected during four surveys in the Channel of Sicily, show that LIW and TIDW are subject to major modifications when crossing the eastern sill. A significant Bernoulli aspiration is found there, and the bottom water at the sill is composed of modified deep Ionian waters coming from >800 m. An analysis of the water mass composition proves that LIW mixes essentially with TIDW, and that the TIDW flow through the Channel must be larger than the expected 0.5 Sv. The presence of a secondary circulation related to the bottom boundary layer is demonstrated to be the primary factor leading to the dilution and cooling of LIW.
Journal of Marine Systems, 2002
Hydrographic and current measurements conducted during the period from March 1997 to March 1999 in the Southern Adriatic and in the Otranto Strait provide clues to the mechanisms of dense water formation and its spreading in the Ionian Sea. The hydrographic surveys covered the successive phases of preconditioning, convection, vertical mixing and spreading. The sub-basin scale cyclonic circulation, the presence of highly saline and dense water in the intermediate layer and winter outbreaks of cold and dry continental air are verified as necessary prerequisites to open-ocean deep convection. However, in two winters, surface cooling and surface buoyancy fluxes were not of sufficient intensity, and the convective mixing reached only intermediate depth of f 400 m in 1998 and f 700 m in 1999. The ventilated convective cell in the centre of the gyre had density of f 29.16 kg m À3 , which was lower than the typical density (r h f 29.24 kg m À3) of the water that resides in the bottom layer of the southern basin (Adriatic Deep Water-ADW). In addition, dense waters from the northern shelf region contributed in filling up the Southern Adriatic deep reservoir. During more severe winter in 1999, two concomitant processes were observed: convection in the centre of the gyre and advection along the shelf of much denser waters (r h f 29.34 kg m À3), which, originating from the Northern Adriatic (NAdDW), sunk at the shelf-break and mixed with the resident bottom water in the Southern Adriatic. Finally, the fate of the ADW and its spreading into the Ionian has been investigated. Transport rates in the bottom layer across the Strait of Otranto have been estimated from long-term current-meter measurements and are related to the intensity of the ADW formation during the study period (1997-1999). Sub-inertial flow fluctuations reach as much as 1 Sv and occur during post-convection, presumably as a consequence of violent mixing processes and spreading. At longer timescales (seasonal and yearly), the low-frequency water flux is seasonally modulated and exhibits a maximum during post-convection period. It ranges from 0.1 Sv in winter 1997 to 0.4 Sv in winter 1999, showing a fairly consistent interannual variability related to the intensity of winter convection.