Hydrological Variability derived from the first Argo mission in the Cretan Sea Basin (2012)

D.Kassis*(1),  and G.Korres (1) 

 

(1) : HCMR Greece

Corresponding author: dkassis@hcmr.gr
Context

It was June 2010 when the first Greek Argo float initiated its mission in the Cretan Sea within the framework of the Greek Argo program.  The PROVOR CTS type standard CTD float (WMO 6900795) performed a complex trajectory inside the Southern Aegean basin and transmitted 102 profiles during its 20 months long operational period in which it got trapped in seasonal gyres and low bathymetry regions, got beached, recovered and redeployed. The Cretan Sea is characterized as a transitional area with complex hydrology. Water masses formed in the Levantine, Aegean and Adriatic Seas, meet and interact with water masses deriving from the Western Mediterranean Sea and entering through the Sicily straits. A subset of 75 profiles measured by the float were analyzed and combined with time series data recorded from the E1M3A multi-parametric instrumentation platform operating in the area since 2007. Data analysis shows the hydrodynamic properties of the area and reveals the dynamical behavior of the Cretan Sea upper thermocline as well as the variability of T/S characteristics in the deeper layers. Spatial analysis reveals significant variability at subsurface layers with strong, alternating signals of Modified Atlantic Water (MAW), Levantine Surface Water (LSW) and Black Sea Water (BSW).  Spatial differences at deep waters are also identified where strong temperature and salinity gradients are revealed.

 

Mission & Data

The Argo float was deployed in the centre of the basin on the 27th of June, 33nm approximately north of Heraklion port (35o.950 N, 25o.050 E) at 1500m depth. Its lifetime is separated in two operational periods since it was lost on July 2011 at the Eastern Arc Straits, recovered on Kasos Island and redeployed near its initial deployment position on November 2011 after it underwent a maintenance and calibration from the manufacturers. Its mission ended after a final transmission on March 2012 while heading towards the Western Arc Straits (Kithira & Antikithira islands) (fig.1). The float was a PROVOR CTS-3 type equipped with standard CTD sensors using Sea-Bird model SBE41/41CP41 pumped MicroCAT (http://www.seabird.com/products/ArgoCTDandFloat.htm) and its configuration were set following the recommendations of MedArgo (Poulain et al., 2007).

The float’s recorded potential temperature range was approximately 13.5 oC with minimum and maximum values of 14.001oC (800m) and 27.55oC (surface) respectively. Regarding salinity, it was ranged between 38.85 P.S.U. (700m) and 39.575 P.S.U (surface) (fig.1). The float’s orbit inside the basin revealed complex circulation features at a sub-basin scale, with a number of different in size and intensity mainly cyclonic subsystems. These gyres are varying from 5 to 8 consecutive profiles with radiuses between 10 and 30 km and estimated velocities that vary between 18 and 44 cm/s.

Figure 1. Cretan Sea basin and the 6900795 float’s trajectory (left); accumulated results from all measured profiles (right).

Results & Discussions

Regarding mesoscale circulation in the basin, the dominant eddies of the upper layers are the West Cretan anticyclone and the Cretan cyclonic gyre forming a dipole that converge in the centre of the basin (Theocharis et al., 1999). The latter is recorded from the Argo float’s orbit after its deployment during the summer of 2010 which follows a cyclonic path of an approximately 30 km radius indicating a significant westward displacement of the eastern cyclone (fig. 1). The perfect fit of the float’s path after a comparison against satellite altimetry and the derived geostrophic velocity field (fig. 2), confirms the existence of the cyclone with a signal from surface down to deeper intermediate layers where the float was drifting (350m). After a similar comparison with the Aegean Sea hydrodynamic model output, on a run that is not assimilating data from this specific float, a good representation of the eastern part of the cyclone is shown (fig.3).

Figure 2. Satellite SSH (left); Sea Surface Geostrophic Velocities (right). The altimeter products were produced by Ssalto/Duacs and distributed by Aviso, with support from Cnes (http://www.aviso.altimetry.fr/duacs/)

Figure 3. Poseidon hydrodynamic model output of monthly average current field at 340m depth during July 2010. The model is based on the Princeton Ocean model (POM) and was initially developed as part of the Poseidon-I system (Korres et al., 2002).

An analysis of salinity values recorded by E1M3A mooring station at surface and subsurface layers present intense variability driven mainly from important thermohaline subsurface circulation, as the wide range of salinity field values with abrupt changes at these depths militates. High saline water is dominating at these layers measured by station with average values of 39.188, 39.114, 39.063, 39.108 and 39.163 PSU at 1, 25, 50, 75 and 100 m respectively. The low averages at 50 and 75 m reflect mainly two important incidents of a low saline water inflow during the autumn of both 2010 and 2011 with similar characteristics and slight differences. The first occurred during October of 2010 occupying the surface layers down to 50 m with a salinity minimum of 38.55 PSU while the second started earlier in 2011 (mid-September) occupying a wider layer down to 75 m with a salinity minimum of 38.22 at 50 m depth (fig.4). The origin of these water masses is supported to be northern mainly associated with (BSW) rather than of Western Mediterranean origin (MAW) due to their characteristics and the time period occurred. For the first case this is confirmed from Argo float’s data during its most North-East position where it records similar water signals while moving cyclonically south of Astypalaia Island. The low salinity signal appears at subsurface layers (40-65 m) on the 12th of September 2011, 20 days approximately before its presence at the mooring site (fig.4). The strong halocline during this set of profiles reveals the existence of LSW which dominates on the surface layers with salinities exceeding 39.5 P.S.U. The low salinity signal fades as the float exits the system and moves towards south-east (fig.4). The second salinity low incident (September 2011) coincided with the absence of the float for maintenance after its loss and recovery. Nevertheless, the analysis of satellite images shows an agreement with the hypothesis of a North-Eastern intrusion of fresher waters into the Cretan Basin (fig. 5). This signal presents seasonal characteristics since it has been also observed near the mooring site after CTD casting during the end of August of 2009.

Figure 4. Salinity distribution at surface and subsurface layers recorded at moored station between July 2010 and April 2012 (up) and Argo float during September 2010 (down).

Figure 5. Satellite SSH and Sea Surface Geostrophic Velocities for September 2010 (up) and 2011 (down).

Figure 6. West-East profile transect for Temperature, Salinity, σθ and TS diagram for the deep intermediate layers (up); float’s path between February and March 2012(down right); model representation of surface current field during the float’s 2 last profiles (down left).

During its last profiles, the float moved westward conducting a respectively high speed latitudinal transect (fig.6). These profiles reveal important spatial differences at intermediate and deep waters where the colder and fresher water layers of the centre of the basin are replaced by slightly warmer but more saline waters as the float moves towards the western Arc Straits. The strong westward flow is nicely represented from the model showing an accentuation as water masses reach the shallower Western Arc Straits (fig.6).    

 

Conclusion

The Argo network expansion into regional seas can provide very useful information regarding spatial and temporal variability of physical water properties, allowing a more detailed examination of the regional basins dynamics. Data analysis from the first Argo float’s data in South Aegean Sea revealed significant variability with signals of different origin water masses at subsurface and deep layers. Additionally, mesoscale circulation patterns together with a number of smaller mainly cyclonic subsystems are traced in the float’s trajectory. Regarding the upper layers, the westward misplacement of the Cretan cyclone together with a low salinity signal, traced all along the North-eastern entrance of the basin until the South-center, are the most important presented features. At deeper layers different water masses seem to dominate from East to West of the basin. At intermediate depths more saline and homogenous waters are recorded on the eastern part. The central part of the basin shows a stronger pycnocline with warm and cold water masses at intermediate and deep layers respectively. This picture is reversed while moving westward where a diversion of the isotherms together with more homogenous salinity fields are recorded.   

 

References
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