Elsevier

Continental Shelf Research

Volume 44, 1 August 2012, Pages 20-29
Continental Shelf Research

On the descent of dense water on a complex canyon system in the southern Adriatic basin

https://doi.org/10.1016/j.csr.2010.11.009Get rights and content

Abstract

Using the results of a numerical model for the description of bottom-arrested currents and statistical analyses, we elucidate different characteristics of the dynamics of a southward propagating vein of North Adriatic Dense Water (NAdDW) observed to evolve within a complex canyon system of the southern Adriatic basin. The vein, monitored from March 2004 to March 2005 by three distinct mooring lines, exhibits a complex, highly time-dependent dynamics characterized by large velocity and density fluctuations. In particular, lag correlation analyses performed on the observed velocity and temperature data show that a temporal lag ranging between 7 and 10 h governs the NAdDW signal propagation along the different canyons, its magnitude inversely depending on vein downslope velocities and density anomalies. The performed model simulations reveal that, weakly depending on its initial layer thickness, exact position, and density contrast with the upper ocean, a coherent flow of dense water located upstream of the canyon system on the Italian shelf will always bifurcate at the entrance of that system; while its shallower part will disintegrate into several branches, its deeper part will continue to flow more coherently, injecting part of the bottom water downward. Regions dominated by supercritical flow regimes are simulated, which contributes to explain part of the observed flow variability. Simulated lag times between signals propagating in the canyons are consistent with observations. They are found to depend crucially on initial, upstream vein location, layer thickness, and density contrast with the upper ocean. We finally use this information, retrieved by our numerical simulations on the basis of the available observations, to infer, in a kind of inverse problem solving, possible shape, location, and density contrast possessed by the observed vein of NAdDW on the Italian continental shelf, prior to its sinking toward the Bari canyon system.

Research Highlights:

► We investigate the dynamics of a vein of dense water in the Bari canyon System. ► Upstream the canyon, the vein travels on the Italian shelf of the Adriatic basin. ► It is simulated to always bifurcate upstream the canyon system and to move downward. ► Within the canyon system, it shows a complex alongslope and downslope dynamics. ► Elements of such observed dynamics help elucidating upstream vein characteristics.

Introduction

The thermohaline circulation of the Mediterranean Sea is mainly triggered by deep penetrating convection, which is known to take place in both its western and eastern subbasins (see, e.g., Zore-Armanda, 1963, MEDOC Group, 1970, Schott and Leaman, 1991). Once convectively formed, dense water masses are often exported toward the basin abyss through bottom-arrested currents (Artegiani and Salusti, 1987, Bignami et al., 1990a, Bignami et al., 1990b, Hainbucher et al., 2006, Rubino and Hainbucher, 2007). The dynamics of these ubiquitous currents (see, e.g., Ivanov et al., 2004, Canals et al., 2006, Allen and Durrieu de Madron, 2009), induced by their excess of weight resulting from their density contrast with the overlaying waters, is deeply influenced by Earth rotation and substantially constrained by the underlying bathymetry (Smith, 1975, Jungclaus and Backhaus, 1994, Shapiro and Hill, 1997, Baines and Condie, 1998, Rubino et al., 2003). So, current penetration horizon, velocity structure, and rate of mixing derive by the relative importance of such parameters, as well as by the interaction of the bottom current with the hydrodynamics of the upper ocean.

On a regularly sloping shelf, the dynamics of bottom-arrested currents is almost geostrophic, i.e., a current deep penetration is largely impeded (Smith, 1975, Jungclaus and Backhaus, 1994). Deep ocean shelf exchange, therefore, takes place when and where ageostrophic flow dynamics occurs. This implies also that, where friction processes are large, flow nonstationarity or advection dominates (Jungclaus and Backhaus, 1994, Allen, 2004).

Over steep bathymetric constrictions, bottom-arrested currents can experience notable accelerations; they can be up to one hundred times faster than deep ocean currents normally encountered in the World Ocean and, as a result, also entrainment of ambient water in the current body will be considerably increased in such regions (Jungclaus and Backhaus, 1994, Shapiro and Hill, 1997, Ivanov et al., 2004). Connected to these flow alterations induced by irregularities in the ocean floor is also the formation of instabilities and the establishment of hydraulic supercritical regimes, which can also contribute to influence the path of the deep current, and hence the details of deep water export and its variability (see, e.g. Kämpf, 2000, Moum and Nash, 2000, Nash and Moum, 2001, Ivanov et al., 2004).

Adriatic Deep Water (AdDW) outflowing from the South Adriatic Pit (SAP) is considered one of the main sources for the ventilation of deep Eastern Mediterranean (EM) waters (see, e.g., the excellent paper by Roether et al., 2007 for a review of the hydrologic characteristics of the EM and its variability). Most of the AdDW is formed during winter in the SAP when deep convection occurs due particularly to surface water cooling and subsequent mixing with the underlying Levantine Intermediate Water (LIW) inflowing from the Ionian Sea through the Strait of Otranto (Artegiani et al., 1997a, Artegiani et al., 1997b, Manca et al., 2002, Vilibić and Orlić, 2001, Vilibić and Orlić, 2002). However, to the formation of AdDW also contribute waters of more septentrional origin; North Adriatic Deep Water (NAdDW), a cold and fresh water with densities as high as 29.9 kg m−3 (Hendershott and Malanotte-Rizzoli, 1976, Franco et al., 1982, Manca et al., 2002, Vilibić, 2003, Vilibić et al., 2004), is in fact produced mainly by surface heat loss and evaporation driven by bora storms in the Northern Adriatic Sea (NAS) during autumn and winter. This water mass, which constitutes the densest water of the whole EM, flows mostly along the western shelf of the central Adriatic Sea as a bottom-arrested density current and arrives to the Gargano Peninsula after 2–4 months since it has been produced (Vilibić and Orlić, 2002, Vilibić, 2003, Vilibić and Supić, 2005).

As long-term oceanic and/or atmospheric variations are known to profoundly affect the production (rate and characteristics) of NAdDW, the pathways of the resulting bottom-arrested currents can vary significantly, yielding significant variability especially in regions with extremely pronounced bathymetric constrictions like, e.g., submarine channels or canyons.

Aspects of the dynamics of bottom-arrested currents made of NAdDW during their southward propagation have been observed several times (see, e.g., Zoccolotti and Salusti, 1987, Bignami et al., 1990a, Bignami et al., 1990b, Vilibić and Orlić, 2001, Manca et al., 2002, Vilibić and Supić, 2005, Sellschopp and Álvarez, 2003). Zoccolotti and Salusti (1987) found a layer of dense, cold bottom water confined to the shelf between the Gargano Peninsula and Bari at a mean depth of 100 m and a less dense water vein flowing along the Italian slope of the Strait of Otranto at depth exceeding 200 m. Bignami et al. (1990a) observed the deepening of a bottom vein of dense water off Bari in late summer 1985. In a subsequent work, Bignami et al. (1990b) found that most of the bottom flow was directed downward within an offshore-oriented canyon near Bari. The presence of this canyon caused a deepening and flattening of the original vein of dense water, downstream, the water could be observed only on the Otranto Sill, at depths of around 800 m. The subsequent flow in the Ionian Sea followed approximately the 900 m isobath in the Gulf of Taranto and further developed along the Calabrian and the eastern Sicilian coasts.

Noticeably, Manca et al. (2002), monitoring the vertical distributions of water properties at shelf and slope stations of a section between Bari and Brindisi, observed cold waters, relatively fresh and rich in oxygen, sliding diffusely along the western continental shelf and sloping down to about 800 m. Sellschopp and Álvarez (2003) shed light on the complexity inherent in the dense water outflow and mixing processes along the Italian shelf; they found that in the Gulf of Taranto, where the width of the shelf rapidly decreases, dense coastal water is released to depth and profoundly transformed by entrainment of ambient water. Very small-scale plumes are formed, which are responsible for the injection of transformed coastal water at any depth on the western slopes of the Ionian Sea.

Different investigations were performed to assess the contribution of waters originated in the northern part of the Adriatic basin to the AdDW production (see, e.g., Artegiani et al., 1997a, Artegiani et al., 1997b, Manca and Scarazzato, 2001, Vilibić and Orlić, 2002, Manca et al., 2002, Mantziafou and Lascaratos, 2004, Vilibić et al., 2004). In particular, Mantziafou and Lascaratos (2004), using numerical simulations, found the NAdDW contribution to the AdDW to amount to around 18%. However, Vilibić and Orlić (2001) assessed the NAdDW contribution as being more than 20% in some years with severe winters over the Adriatic (e.g., 1987), but only as around 2–3% during periods with scarce deep water production (e.g., 1975). Such a result was substantially confirmed by subsequent box model studies performed by Vilibić and Orlić (2002) and by realistic simulations carried out by Mantziafou and Lascaratos (2008).

So, NAdDW can contribute substantially to the production of AdDW and hence to the ventilation of the EM. The details of its southward descent toward the deep layers of the SAP and toward the Ionian Sea seems thus pivotal for a deeper understanding of the variability of the structure of the thermohaline cell of the EM, which underwent large variability during the last decades (see, e.g., Roether et al., 1996, Klein et al., 1999, Klein et al., 2000, Hainbucher et al., 2006, Roether et al., 2007, Rubino and Hainbucher, 2007). Notably, the descent of a dense vein of NAdDW in a complex canyon system located off the Italian region of Bari was recently observed by Trincardi et al. (2007) and Turchetto et al. (2007).

In this paper, for the first time, using data referring to those observations and the results of a very high resolution numerical model, we perform a sensitivity study to investigate the details of the dynamics of bottom-arrested currents of North Adriatic origin as they encounter a complex canyon system resembling that located off the Italian region of Bari. The aim is to use this information to infer, in a kind of inverse problem solving, possible shape, location, and density contrast possessed by the observed vein of NAdDW on the Italian continental shelf, prior to its sinking toward the Bari canyon system.

The paper is organized as follows: in Section 2, the observed data are presented and discussed. In Section 3, the used numerical model is introduced and idealized as well as more realistic numerical simulations are performed and discussed. Finally, in Section 4, the results of our investigation are discussed and conclusions are drawn.

Section snippets

Data

The Bari canyon system (Fig. 1, top panel), a prominent morphological structure of the southwestern Adriatic shelf (see Fig. 1, bottom panel) which breaches the outer Italian shelf with a west–east trend, is circa 10 km wide and circa 30 km long (Verdicchio and Trincardi, 2006, Trincardi et al., 2007, Turchetto et al., 2007). It is characterized by several heads at about 200 m depth and two main branches, named here B and C in accordance with previous work. The northern branch (B) is a narrow,

Numerical simulations

In order to interpret aspects of the observed bottom water dynamics and, eventually, to infer possible shape, location, and density contrast that the observed vein of NAdDW possessed on the Italian continental shelf prior to its descent toward the Bari canyon system, we carried out very high-resolution numerical simulations using a nonlinear and nonstationary hydrostatic numerical model for the description of bottom-arrested currents over realistic topography. The model solves the shallow-water

Discussion and conclusions

The measurements performed in the area of the Bari canyon system between March 2004 and 2005, together with the results of our numerical simulations, point to a large variability in the characteristics of the near-bottom circulation in the region. During spring time, events of impulsive descent of plumes of NAdDW in the canyon system are recorded, that can result in rather different velocities as well as density fields within the canyons. Nevertheless, we have demonstrated, using the

Acknowledgment

Part of this study was funded by the Euro-Mediterranean Center for Climate Change (CMCC), Venice (Italy).

References (49)

  • F. Trincardi et al.

    The impact of cascading currents on the Bari Canyon System, SW-Adriatic Margin (Central Mediterranean)

    Mar. Geol.

    (2007)
  • M. Turchetto et al.

    Particle transport in the Bari Canyon (southern Adriatic Sea)

    Mar. Geol.

    (2007)
  • G. Verdicchio et al.

    Short-distance variability in slope bed-forms along the Southwestern Adriatic Margin (Central Mediterranean)

    Mar. Geol.

    (2006)
  • I. Vilibić

    An analysis of dense water production on the North Adriatic shelf estuarine

    Coastal Shelf Sci.

    (2003)
  • I. Vilibić et al.

    Least-squares tracer analysis of water masses in the south Adriatic (1967–1990)

    Deep Sea Res. I

    (2001)
  • I. Vilibić et al.

    Adriatic water masses, their rates of formation and transport through the Otranto Strait

    Deep-Sea Res. I

    (2002)
  • L. Zoccolotti et al.

    Observations of a vein of very dense marine water in the southern Adriatic Sea

    Cont.. Shelf Res.

    (1987)
  • A. Artegiani et al.

    Field observation of the flow of dense water on the bottom of the Adriatic Sea during the winter of 1981

    Oceanol. Acta

    (1987)
  • S.E. Allen

    Restrictions on deep flow across the shelf-break

    Surv. Geophys.

    (2004)
  • S. Allen et al.

    A review of the role of submarine canyons in deep-ocean exchange with the shelf

    Ocean Sci.

    (2009)
  • A. Artegiani et al.

    The Adriatic Sea general circulation. Part I: Air–sea interactions and water mass structure

    J. Phys. Oceanogr.

    (1997)
  • A. Artegiani et al.

    The Adriatic Sea general circulation. part II: Baroclinic circulation structure

    J. Phys. Oceanogr.

    (1997)
  • P.G. Baines et al.

    Observations and modelling of Antarctic downslope flows: A review in ocean, ice and atmosphere: interactions at the Antarctic Continental Margin

    Antarct. Res. Ser.

    (1998)
  • F. Bignami et al.

    Observations on a bottom vein of dense water in the southern Adriatic and Ionian Seas

    J. Geophys. Res.

    (1990)
  • Cited by (40)

    • Intruding gravity currents and their recirculation in a rotating frame: Numerical results

      2022, Ocean Modelling
      Citation Excerpt :

      The thickening of an accelerating boundary layer in a rotating frame was analytically shown by Huthnance (2009) for an idealised case. For a discussion of cascading in the ocean from observations and its representation in realistic numerical simulations, I refer to Luneva et al. (2020) for the Arctic and to Rubino et al. (2012) for the Mediterranean. Both works emphasise the importance of the topographic structure on the cascading, a subject that is not considered here.

    • Megafauna distribution along active submarine canyons of the central Mediterranean: Relationships with environmental variables

      2019, Progress in Oceanography
      Citation Excerpt :

      Most of the canyons explored are located in the western Mediterranean, in relation both to their geological and sedimentary processes (e.g. Berné and Loubrieu, 1999; Canals et al., 2004, 2009; Puig et al., 2008) as well as to their biodiversity and fishery resources (e.g. Cartes et al., 1994; Stefanescu et al., 1994; Gili et al., 2000; Palanques et al., 2005; Sabatini et al., 2007; Sardà et al., 2009; Ramirez-Llodra et al., 2010b; Farrugio, 2012; Gori et al., 2013; Fabri et al., 2014). Differently, in the central Mediterranean only the Bari Canyon in the southern Adriatic Sea (e.g. Ridente et al., 2007; Trincardi et al., 2007; Canals et al. 2009; Rubino et al., 2010; Carniel et al., 2012; Bo et al. 2012; Angeletti et al. 2014; D’Onghia et al., 2015a, 2015b, 2016) and the Gioia Canyon in the southern Tyrrhenian sea (Gamberi and Marani, 2007, 2008; Pierdomenico et al., 2016, 2018; Casalbore et al., 2014, 2018) have been studied in relation to their morpho-sedimentary and ecological characterization. Moreover, it is necessary to consider that shelf-indenting canyons can also represent a main pathway for the transport of anthropogenic debris from coastal to deeper areas, thus becoming hot-spots of marine litter (Galgani et al., 1996; Ramirez-Llodra et al., 2013; Pham et al., 2014; Tubau et al., 2015).

    View all citing articles on Scopus
    1

    Permanent address: P.P. Shirshov Institute of Oceanology, Pervaya Liniya 30, St. Petersburg 199053, Russia.

    2

    Present address: Max Planck Institute for Meteorology, Ocean in the Earth System Department, Bundesstraße 53, 20146 Hamburg, Germany.

    View full text