Eastern Mediterranean water formation studied by passive tracer modelling

We have been involved in two tracer modeling studies for the Mediterranean. The first study employs an idealised tracer in order to investigate the formation and spreading of a mode water in the eastern Mediterranean. The second study examines the overflow process simulated by cartesian and isopycnic models. Model simulations of the CFC-12 distribution are included in order to compare with the observed distribution. These comparisons suggest that the isopycnic model provides a better simulation of the overflow process. The two abstracts related to these studies are shown below.


  • Tracer study of the formation, dispersal and renewal of Levantine Intermediate Water

    • K. Stratford and R. Williams
    JGR (1997), 102, 12539-12549

    The formation and dispersal of Levantine Intermediate Water (LIW) is e xamined using a series of tracer experiments using an eddy-permitting, general circulation model of the Mediterranean. LIW is formed over the Rhodes gyre and disperses through both the time-mean and time-varying circul ation. Tracer released in the Rhodes gyre is transferred westwards by the time-mean circulation, but also southwards and eastwards by the time-varying circulation. Off-line tracer experiments suggest that the time-varying circulation appears to act in an advective manner, rather than soley in a diffusive manner. The rate of invasion of tracer into the m ain thermocline suggests that the renewal timescale is 10-20 years for LIW and of order 100 years for eastern Mediterranean deep water. If advection controls this transfer of tracer, then the implied annual subduction rate is 0.6+-0.1 Sv in the Rhodes gyre, and 0.2+-0.1 Sv in the Adriatic Sea. The LIW mode is found to become cooler and fresher as a result of mixing with the Adriatic outflow, while its potential vorticity appears well conserved.

  • Comparing the overflow of dense water in isopycnic and cartesian models with tracer observations in the eastern Mediterranean

    • V. Roussenov, R. Williams and W. Roether
    Submitted to DSR (1999)

    Isopycnic and cartesian model simulations for the overflow and spreading of dense water are compared with each other and with independent transient tracer observations. This case study is performed for Adriatic dense water overflowing into the deep Eastern Mediterranean with chlorofluoromethane (CFC-12) observations used to test the model simulations. The realism of both types of model simulation depends on the representation of diapycnal mixing. In the cartesian model, convective adjustment and mixing dilute the overflow of Adriatic dense water and lead to unrealistic vertical homogenisation. Incorporating a modified convection scheme emphasising the sinking of dense fluid, rather than its mixing, leads to a more realistic penetration of the dense overflow. In the isopycnic model, there is an improved simulation of the overflow, which leads to the density contrast of the deep Mediterranean waters being maintained. However, there is too low a CFC-12 concentration at mid-depths unless explicit diapycnal mixing is incorporated. Differences in the simulated tracer fields in the two models are also partly due to differences in the barotropic circulation arising from the different interaction between the dense overflow and the topography.


    Passive tracer modelling example

    CFC-12 distribution in the Eastern Mediterranean simulated by cartesian (MOMA) and isopycnic (MICOM) models and compared with R/V "Meteor" observations in 1987 (Fig. 1a, 2a). Both models have similar resolution and sea-surface forcing.
    The source of dense waters in the Adriatic Sea is marked by high CFC-12 concentration (top-right of Fig. 1).


    Figure 1: CFC-12 distribution [pmol/kg] for a north-south section through the Ionian




    Figure 2: CFC-12 distribution [pmol/kg] for a east-west section through the Ionian



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    This page was made by V. Roussenov, University of Liverpool.
    Last changed on July 14, 2000.