The Physical Characteristics and Dynamics of the Aegean Water Masses Dimitris Velaoras, Vassilis Zervakis, and Alexander Theocharis Contents 1 Introduction 2 Hydrology and Circulation 2.1 Surface and Intermediate Water Masses and Circulation 2.2 Dense Water Formation Processes and Deep Water Masses 3 The Role of the Aegean Sea in the Eastern Mediterranean Deep Thermohaline Functioning 3.1 The Eastern Mediterranean Transient 3.2 The Aegean Sea as Driver of Eastern Mediterranean Quasi-Decadal Thermohaline Oscillations 4 Recent Aegean Sea Status; Post-EMT to Date 4.1 The North Aegean Sea 4.2 The South Aegean Sea 4.3 Cretan Straits 5 Unresolved Issues and Future Research References Abstract The Aegean Sea’s hydrological and circulation characteristics are as complex as its geomorphological setting. All major Eastern Mediterranean (EMed) water masses can be found inside the basin, while the Aegean Sea is a producer of dense intermediate and in some cases deep water masses that are exported to the EMed. The role of the Aegean Sea as a dense/deep water source for the EMed has been revised following the Eastern Mediterranean Transient event of the 1990s decade. The local dense water production has been recognized as having a strong D. Velaoras (*) and A. Theocharis Hellenic Centre for Marine Research, Institute of Oceanography, Anavyssos, Greece e-mail: dvelaoras@hcmr.gr V. Zervakis Department of Marine Sciences, School of the Environment, University of the Aegean, Mytilene, Greece e-mail: zervakis@marine.aegean.gr Christos L. Anagnostou, Andrey G. Kostianoy, Ilias D. Mariolakos, Panayotis Panayotidis, Marina Soilemezidou, and Grigoris Tsaltas (eds.), The Aegean Sea Environment: The Natural System, Hdb Env Chem, DOI 10.1007/698_2020_730, © Springer Nature Switzerland AG 2021