Dense and deep water formation processes and Mediterranean overturning circulation. In: Oceanography of the Mediterranean Sea.

The opening chapters of the book set the boundaries: bottom and lateral (origin, evo- lution, shape and dimensions of the basin, Chapter 2) and surface (Climate, Chapter 3). In particular, the tectonic processes at the origin of the Mediterranean Sea are described, as well as a particular paleo-environmental event (the Messinian salinity crisis) that resulted in the almost complete desiccation of the Mediterranean Sea. The morphological characteristics of the basin floor and the tectonic and climatic processes that control sedimentation and thus the evolution of the seafloor are intro- duced to the reader. The climate is then described: the Mediterranean region exhibits a large seasonal cycle of mean temperature and total precipitation, as well as a large spatial variability. Most of the region is characterized by mild winters, though small areas with mean winter temperature below zero exist in the Alps and Caucasus, and warm or hot summers. Most areas in the central Mediterranean rely on the wet sea- son (October to March) for precipitation to provide water resources for the rest of the year, and large areas close to the north African and Middle East coasts are dry all over the year. The next chapters discuss forcings and water mass characteristics as well as gen- eral circulation, in a classical steady-state approach (Chapter 4), the air-sea interface (Chapter 5 on sea level and Chapter 6 on surface waves) and one of the most prom- inent examples of ocean-atmosphere interaction, the generation of new water masses and overturning circulation (Chapter 7). In particular, the reader is introduced to 4 CHAPTER 1 Introduction general concepts about the main forcings of the Mediterranean Sea, that are respon- sible for the peculiar characteristics of Mediterranean water masses. A general explanation on water mass analysis is given, and then the properties and vertical and horizontal distributions of the main Mediterranean water masses are described. The concept of mean sea level is introduced as well as its variability at low frequency time scales in the Mediterranean Sea. Ancillary geodetic measurements are also described, as these are relevant for coastal monitoring and sea level impacts. In addi- tion, the past evolution of Mediterranean sea level, since the Holocene up to the late 19th century, are illustrated in detail. An overview is then given of a wide range of free and forced long oceanic waves (Chapter 6) that are relevant to disentangle major ocean processes and associated coastal risk in the Mediterranean Sea: this includes tides, seiches, wind waves, storm surges, tsunamis. The physical processes, charac- teristics, and risks posed by the different phenomena are described. Chapter 7 points out the main characteristics of the dense/deep water formation processes occurring in the Mediterranean Sea and the basin-wide vertical circulation is drawn. These processes are key elements of the circulation dynamics in the Mediterranean basin. The waters are formed by open ocean convection processes and dense shelf water cascading. The meridional and zonal overturning circulation of the Mediterranean Sea are driven to a large extent by these formation and spreading processes. The Mediterranean Sea features a wide range of temporal and spatial scales (ba- sin, subbasin, and mesoscale) that interact to force a complex, highly variable gen- eral circulation. While Chapter 8 inspects mesoscale features, increasing the “spatial resolution” of the general description provided in Chapter 4, Chapter 9 discusses the recent changes and variability over time of this general picture, abandoning the steady-state view of Chapter 4. Over recent decades, many oceanographic cam- paigns have been carried out with the goal to further understand the variability at the basin scale, as well as to investigate sharp fronts and mesoscale eddies. Multi- platform experiments are being complemented with instruments able to look beyond the mesoscale, into the submesoscale. General concepts about temporal changes of water mass properties in the Mediterranean Sea are then given to the reader, focusing in particular on the second half of the 20th century and the beginning of the 21st cen- tury, and situating them in the context of the global changes occurring in the oceans. After that, Chapter 10 overviews our ever-increasing ability to observe and predict the ocean linked to the recent advancements in operational oceanography. Sustained observations and forecasting systems provide a source of invaluable new informa- tion to advance the current knowledge and understanding of the functioning of the Mediterranean Sea and its ecosystems, and to efficiently respond to maritime emer- gencies, societal needs and preservation threats. The concepts behind ocean observing and forecasting systems are introduced, the present status of the systems operating in the Mediterranean Sea is described and some recent applications are illustrated. The concluding three Chapters are devoted to the description of the major biogeochemical cycles (Chapter 11), the active geological processes in the basin (Chapter 12) and to the impact of the humans on the physical environment in the 1.2 Book structure and contents 5 ast epoch, the Anthropocene, and its most recent part, the so-called Great Acceler- ation (Chapter 13). An overview of the general biogeochemistry is given, explaining the particularities of the main biogeochemical variables and the physical, biological and geochemical processes driving their distribution (dissolved oxygen, inorganic nutrients, dissolved organic carbon, the CO 2 system). Also a brief overview on the utility of those biogeochemical variables to identify water masses is given. Future projections and threats on biogeochemistry in the Mediterranean Sea under different future climate change scenarios are discussed. Then, the geological pro- cesses that resulted in the formation of the Mediterranean Sea are described, focusing on those that control recent sedimentation at the seafloor in coastal areas and in the deep sea, on how submarine volcanoes are formed and evolve and how fluid escape at the seafloor form cold seeps and related deposits. The last Chapter follows a slightly different philosophy behind the story-telling, excluding climate change that is, by editorial choice, tackled in a distributed way throughout the book. This chapter illustrates the major anthropic impacts shaping the geological, biological and biogeochemical dynamics of the Mediterranean Sea. A synthetic description of the main modifications of deep sea floor integrity and coastal land- scapes is complemented by an in-depth picture of critical changes in the biogeo- chemical dynamics of major nutrients and pollutants (including a detailed paragraph on plastics) at Mediterranean scale.