Cascading dense shelf-water during the extremely cold winter of 2012 in the Adriatic, Mediterranean Sea: Formation, flow, and seafloor impact Preface

Three sites of dense water formation significantly impact the deep circulation of the Mediterranean Sea. These sites are the northernmost continental shelves of the basin: the Gulf of Lion, the Northern Adriatic, and the Northern Aegean (Canals et al., 2009). While the Gulf of Lion re- sponds rapidly to the formation of dense seawater (DSW) through the flushing of its western portion and adjacent slope canyons during cyclo- genesis conditions (Canals et al., 2006; Puig et al., 2013), the Adriatic is elongated southeast-ward (800 km) and acts as a buffered system where the propagation of DSW requires at least several weeks to reach the southern slope where cascading occurs. Several hypotheses and observations have been proposed (e.g., Bignami et al., 1990a, 1990b; Vilibi? and Supi?, 2005). However, until a few years ago, the Adriatic region lacked a comprehensive, synoptic view of an event of DSW formation, its flow and modification along the shelf, and its conse- quent downslope cascading. The present special issue reports results from the impact of a major cold air outbreak on the formation of dense shelf water in the Adriatic. This occurred in February 2012, when the European region experienced a 2 weeks severe cold spell that heavily impacted the Northern Adriatic. This cold spell was accompanied by the onset of severe north-easterly Bora winds (Raicich et al., 2013), blowing al- most continuously, and being intermittently reinforced by cyclogen- esis in the western Mediterranean. A significant heat loss took place in the basin and a substantial decrease of Adriatic surface tempera- ture down to temperatures as low as 3-6 °C (Fig. 1) while the shal- low Lagoon of Venice partially froze (Fig. 2). The cold outbreak, the severe cold and dry Bora flow, accompanied by the very limited dis- charge of the Po River in the preceding autumn, caused the formation of extremely dense shelf water (Mihanovi? et al., 2013).