Mediterranean Intermediate Circulation (2010)

M.Menna(1)* and P.-M.Poulain(1)

  

(1) : OGS Italy

* Corresponding author : M.Menna
Full paper: M. Menna and P. M. Poulain, 2010: Mediterranean intermediate circulation estimated from Argo data in 2003–2010, ocean Science 6: 331-343, doi:10.5194/os-6-331-2010, 2010.

 

The trajectories of 38 Argo floats in the Mediterranean Sea are used to determine the float surface and sub-surface displacements, and create a dataset of velocities at 350m in order to study the intermediate circulation in regions of good data coverage. The float subsurface velocities and the pseudo-Eulerian statistics computed from them show typical circulation pathways, which can be related to the motion of the Levantine Intermediate Water (LIW).

Data & Method

Data from 38 Argo floats (14 Apex and 24 Provor), representing 3431 cycles from October 2003 to January 2010, are selected to estimate the Mediterranean subsurface circulation near 350 m corresponding to their parking depth. Each float descents from the surface to 350m, where it remains about 4 days before reaching the profile depth (700m, extended to 2000 m every ten cycles). At the end of each cycle, the float remains about 6 hours at the sea surface, where it transmits its geographical position and data to the Argos satellite system (Poulain et al., 2007).

Figure 1. Schematic diagram of depth versus time of a float cycle and two subsequent float transmissions.

Those Argos positions are used to determine the subsurface currents through three steps (Menna and Poulain, 2010):

  1. In first approximation, the intermediate velocities (Vold) are estimated using the last position of cycle (n-1) and the first position of cycle n (grey stars in figure 1, Lebedev et al. 2007). But this method doesn’t take into account the unknown drift (while floats are at the surface) which may induce errors in sub-surface circulation estimation, mainly considering that currents are generally much stronger at and close to the surface than at parking depth.
  2. The second approximation involves the computation of diving (DS) and surfacing (AE) times and positions to obtain more accurate estimation of the float surface displacements (Menna and Poulain, 2009). Provor floats record AE and DS times, but Apex floats do not, so that we estimate it from the Argos message information and TS times. Then, AE and DS positions are extrapolated from those AE and DS times, and from the Argos satellite fixes combined in a simple model based on linear displacement and inertial motion (Park et al.,2005). The diving and surfacing times and positions are used to evaluate the intermaediate currents (V350).
  3. The third step provides the comparison between rough (first approximation - Vold) and fine (second approximation - V350) datasets of intermediate currents in order to select only the best estimates of velocities (variance explained/total variance > 0.8) and to produce the final dataset V350end.

From the results of those three steps, the Mediterranean sub-surface circulation is estimated and pseudo-Eulerian statistics are computed to emphasize the main typical pathways of the basin.

Results

The better sub-surface displacement velocity data-set (V350) obtained in second approximation, is compared with the first approximation displacement data-set (Vold), showing a negligible mean difference, a standard deviation less than 5.3 cm/s and a high correlation coefficient (about 0.93). From this comparison, the best estimates of V350 data-set is used to produce the final dataset V350end and investigate the sub-surface Mediterranean circulation.

Figure 2.Mean circulation field at 350 m, derived from 1o×1o binned floats velocities data (V350end).

The amplitude of the V350end velocities reach the maximum values of about 33 cm/s northeast of the Balearic Islands (western basin) and in the Ierapetra eddy (eastern basin). High speeds (larger than 20 cm/s) are also found in the north and central regions of the Liguro-Provençal-Catalan basin and in some eddies of the eastern basin, whereas the Liguro-Provençal, Algerian and Libyan-Egyptian Currents are characterised by maximum speeds of 15 cm/s.

In the central and western part of Mediterranean basin, the pseudo-Eulerian statistics computed with V350end show typical intermediate circulation pathways related to the motion of the Levantine Intermediate Water (LIW). The mean circulation mapped by floats is almost everywhere in agreement with the description of intermediate circulation provided in the framework of various international projects and by several authors in the past. There is an important exception in the southern Ionian, along the Libyan coast, where floats follow a north westward current, which is in opposite direction compared to the circulation patterns described by Millot and Taupier-Letage (2005) and depicted in Figure 3.

Figure 3. Circulation of the Levantine Intermediate Water; Mediterranean Sea geography and nomenclature of the major sub-basins and straits (adapted from Millot and Taupier-Letage, 2005).

 

Details in the best sampled regions of the pseudo-Eulerian statistics (figure 4) confirm typical circulation pathways related to the LIW :

  • in the Western region, the velocity field follows the Liguro-Provençal-Catalan and the Algerian currents; this area is characterised by high values of EKE and by principal axes of variance oriented in the direction of mean currents
  • in the Eastern region the mesoscale and sub-basin scale circulation is dominated by eddies (diameters of 100-150 km); the maximum value of EKE and variance is located south of Crete (Ierapetra eddy).

 

Figure 4 : (a) - Detail of the Western Mediterranean basin (bins of 0.5o×0.5o) and location of the Algerian Gyres (AG1, AG2). (b) - Detail of the Eastern Mediterranean basin (bins of 0.5o×0.5o) and location of the main sub-basin eddies and gyres: Cretan Gyre (CG), Ierapetra Eddy (IE), Egyptian Eddy (EE), Libyan Eddies (LE), Herodotus Trough Eddy (HTE), Rhodes Gyre (RG), Eratosthenes Seamount Eddy (ESE). 

 

Conclusion

The method described provides a novel quantitative description of the intermediate Mediterranean currents, based on Argo float displacements at 350 m depth, partially related to the pathways of the LIW. This description confirms, contradicts (in the southern Ionian) and expands the results of previous qualitative studies, mostly based on hydrographic, satellite and mooring data.

It is hoped that the continuation of the Argo program with the standard parameters (cycles of 5 days, parking depth near 350 m) will provide more abundant data in the whole Mediterranean Sea in order to improve the results, and eventually monitor long-term changes in the mean intermediate circulation related to climate change.

 

References
  • Lebedev, K. V., Yoshinari, H., Maximenko, N. A., and Hacker, P. W.: Velocity data assessed from trajectories of Argo floats at parking level and at the sea surface, IPRC Technical Note, 4(2), 1–16, 2007
  • Menna, M. and P. M. Poulain : Mediterranean intermediate circulation estimated from Argo data in 2003-2010. Ocean Science, 6, 331-343, 2010
  • Menna M., and P.-M. Poulain : Valutazione delle correnti subsuperficiali nel Mediterraneo da float Argo, Technical Report 2009/95-OGA-15-SIRE, pp. 1-63, Trieste, 2009
  • Millot, C. and Taupier-Letage, I.: Circulation in the Mediterranean Sea, Handb. of Environ. Chem., 5, 29–66, 2005
  • Park, J.J., K. Kim, B.A. King, and S.C. Riser, An Advanced Method to Estimate Deep Currents from Profiling Floats, J. Atmos. Ocean. Tech., 22 (8), 1294-1304, 2005
  •  Poulain P.-M., Barbanti R., Font J., Cruzado A., Millot C., Gertman I., Griffa A., Molcard A., Rupolo V., Le Bras S., and Petit de la Villeon L. : MedArgo: a drifting profiler program in the Mediterranean Sea, Ocean Sci., 3: 1901-1943. 2007