Hydrodynamic features of the South Aegean Sea as derived from Argo T/S and dissolved oxygen profiles in the area

Dimitris Kassis, Evangelia Krasakopoulou, Gerasimos Korres, George Petihakis and George S. Triantafyllou

Full paper: Kassis, D., E. Krasakopoulou, G. Korres, G. Petihakis and G. S. Triantafyllou (2016). "Hydrodynamic features of the South Aegean Sea as derived from Argo T/S and dissolved oxygen profiles in the area." Ocean Dynamics 66(11): 1449-1466.
Introduction

The South Aegean basin is a semi-enclosed basin of the Mediterranean Sea and is considered as one of the most oligotrophic, with a complex hydrology and deep sub-basins It is divided to three sub-regions namely the Myrtoan Sea at the NW part of the region, the shallow shelf of the Cyclades Plateau at NE part, and the Cretan Sea basin that occupies the southern area of the Aegean Sea and operates as a heat, salt, and dissolved oxygen reservoir. The main factors controlling its characteristics are the intensive convective mixing of the water column along with the exchanges of water and mass (diluted, suspended, or near bed) between the Aegean Sea and the adjacent Levantine and Ionian Seas.

During the last three decades, the hydrography and circulation of the South Aegean have been studied in the framework of national and international programs highlighting the significant role the basin plays in the hydrology of the Eastern Mediterranean.

For the first time, Bio-Argo floats were deployed in the area under the Greek Argo Research Infrastructure coordination. The acquired profiles cover an almost 2-year period (November 2013–July 2015) and are compared with previous Argo profiles and the reprocessed time-series data recorded from the E1-M3A POSEIDON observatory operating in the area since 2007.

Results

An important inter-annual variability has been recorded in the Cretan Sea that affects the physical and the chemical properties of the water column. This is characterized by the decrease of both T and S, together with the increase in the DO concentrations in the intermediate layers (Figure 1), which indicates an outflow from the basin of the pre-existing CIW and LIW, which are later partially replaced by TMW during the time period 2012–2013.

The Myrtoan basin is presented as a source of convection events, especially in its northern part. The increased but undersaturated DO signals that propagate deeper through mixing demonstrate the potential usage of this property as a tracer for vertical mixing (Figure 2). Low and high DO values are mostly associated with a well-stratified water column while the intermediate ones can also demonstrate deep mixing and homogenization. This is also manifested in the relation of DO field and the MLD, where the former’s distribution seems to accurately describe the latter’s variability, even in weaker mixing events. This would not be possible without the continuous profile sampling from the Argo floats since the biological processes would alternate these signals.

The interaction between the two sub-basins (Cretan and Myrtoan) is also recognized as important due to the exchanges of water masses that are reflected in the floats’ trajectories (Figure 3). Notably, in the beginning of 2015, intermediate water masses of high S and low DO are driven northwards from the Cretan to the Myrtoan basin, where DWF events dominate during the late winter–early spring period (Figure 2). The deep mixing process in the Myrtoan increases the DO concentration and the salt content of the underlying layers, resulting in densification and gradual homogenization of the water column. The southward flow that follows is the main mechanism of the northern Cretan basin’s oxygenation in the intermediate layers playing also a key role in the ecosystem functioning. Moreover, the respectively high salt content in both Myrtoan and Cretan basins demonstrates the pre-conditioned status of the wider area.

Conclusion

The Argo-DO floats, aside from the comprehensive and detailed S and T fields, provide for the first time in the South Aegean the opportunity to follow the oxygen dynamics on a large scale in both space and time and with a frequency that was formerly unavailable.