Mediterranean Sea Circulation 1

OCEAN CURRENTS/ Mediterranean Sea Circulation 1 mate dynamics involves several issues. Conceptual, Mediterranean Sea Circulation methodological, technical, and scientiRc issues include, for example, the formulation of multiscale Allan R. Robinson, Wayne G. Leslie, Division of (e.g., basin, sub-basin, mesoscale) interactive nonlin- Engineering and Applied Sciences, Department of ear dynamical models; the parameterization of Earth and Planetary Sciences, Harvard University, air}sea interactions and Suxes; the determination of 29 Oxford Street, Cambridge, MA 02138, USA speciRc regional processes of water formation and Alexander Theocharis, National Centre for Marine transformations; the representation of convection Research (NCMR), Aghios Kosmas, Hellinikon 16604, and boundary conditions in general circulation Athens, Greece models. A three-component nonlinear ocean system Alex Lascaratos, Department of Applied Physics, } Oceanography Group, University of Athens, is involved whose components are: (1) air sea inter- University Campus, Building PHYS-V, Athens 15784, actions, (2) water mass formations and transforma- Greece tions, and (3) circulation elements and structures. The focus here is on the circulation elements and ^ Copyright 2001 Academic Press their variabilities. However, in order to describe the doi:10.1006/rwos.2001.0376 circulation, water masses must be identiRed and described. Introduction 0001 The Mediterranean Sea is a mid-latitude semi-en- Multiscale Circulation and closed sea, or almost isolated oceanic system. Many Variabilities processes which are fundamental to the general circulation of the world ocean also occur within the The new picture of the general circulation in the 0004 Mediterranean, either identically or analogously. Mediterranean Sea which is emerging is complex, The Mediterranean Sea exchanges water, salt, heat, and composed of three predominant and interacting and other properties with the North Atlantic Ocean. spatial scales: basin scale (including the thermoha- The North Atlantic is known to play an important line (vertical) circulation), sub-basin scale, and role in the global thermohaline circulation, as the mesoscale. Complexity and scales arise from the major site of deep- and bottom-water formation for multiple driving forces, from strong topographic the global thermohaline cell (conveyor belt) which and coastal inSuences, and from internal dynamical encompasses the Atlantic, Southern, Indian, and Pa- processes. There exist: free and boundary currents ciRc Oceans. The salty water of Mediterranean ori- and jets which bifurcate, meander and grow and gin may affect water formation processes and shed ring vortices; permanent and recurrent sub- variabilities and even the stability of the global ther- basin scale cyclonic and anticyclonic gyres; and mohaline equilibrium state. small but energetic mesoscale eddies. As the scales 0002 The geography of the entire Mediterranean is are interacting, aspects of all are necessarily dis- shown in Figure 1A and the distribution of deep-sea cussed when discussing any individual scale. The topography and the complex arrangement of coasts path for spreading of Levantine Intermediate Water and islands in Figure 1B. The Mediterranean Sea is (LIW) from the region of formation to adjacent seas composed of two nearly equal size basins, connected together with the thermohaline circulations are by the Strait of Sicily. The Adriatic extends north- shown in Figure 2; where the entire Mediterranean ward between Italy and the Balkans, communicating is schematically shown as two connected basins with the eastern Mediterranean basin through the (western and eastern). The internal thermohaline Strait of Otranto. The Aegean lies between Greece cells existing in the western and eastern Mediterra- and Turkey, connected to the eastern basin through nean have interesting analogies and differences to the several straits of the Grecian Island arc. The each other and to the global thermohaline circula- Mediterranean circulation is forced by water ex- tion. In the western basin (Figure 3A) the basin- change through the various straits, by wind stress, scale thermohaline cell is driven by deep water and by buoyancy Sux at the surface due to fresh- formed in the Gulf of Lions and spreading from water and heat Suxes. there. Important sub-basin scale gyres in the main 0003 Research on Mediterranean Sea general circula- thermocline in the Alboran and Balearic Seas have tion and thermohaline circulations and their been identiRed. Intense mesoscale activity exists and variabilities, and the identiRcation and quantiRca- is shown by instabilities along the coastal current, tion of critical processes relevant to ocean and cli- mid-sea eddies and along the outer rim swirl Sow of RWOS 0376 Editor: Indira Operator: Indira NP Scan: Yoga 2 OCEAN CURRENTS / Mediterranean Sea Circulation a sub-basin scale gyre. The basin scale thermohaline sion, and mixing of these water masses. These in- cell of the eastern basin is depicted generically in clude: sources of forced and internal variabilities; Figure 3B and discussed in more detail in the next the spectrum and relative amounts of water types section. The basin scale general circulation of the formed, recirculating within the Mediterranean main thermocline is composed of dominantly ener- basins, and Suxing through the straits, and the ac- getic sub-basin scale gyres linked by sub-basin scale tual locations of upwelling. jets. The active mesoscale is shown by a Reld of A basin-wide qualitative description of the ther- 0006 internal eddies, meanders along the border swirl mohaline circulation in the western basin of the Sow of a sub-basin scale gyre, and as meandering jet Mediterranean Sea has recently been provided by segments. The Atlantic Water jet with its instabili- Millot (see Further Reading). Results based on ties, bifurcations, and multiple pathways, which cruises in December 1988 and August 1989 in- travels from Gibraltar to the Levantine is a basin dicated that the deep layer in the western Mediterra- scale feature not depicted in Figure 3 this also per- nean was 0.123C warmer and about 0.33 PSU more tains to the intermediate water return Sow. saline than in 1959. Analysis of these data together with those from earlier cruises has shown a trend Large-scale Circulation of continuously increasing temperatures in recent decades. Based on the consideration of the heat and 0005 Processes of global relevance for ocean climate dy- water budget in the Mediterranean, the deep-water namics include thermohaline circulation, water mass temperature trend was originally speculated to be formation and transformation, dispersion, and the result of greenhouse gas-included local warming. mixing. These processes are schematically shown in A more recent argument considers the anthropo- Figure 4A and B for the western and the eastern morphic reduction of river water Sux into the east- basins. The Mediterranean basins are evaporation ern basin to be the main cause of this warming basins (lagoons), with freshwater Sux from the At- trend. lantic through the Gibraltar Straits and into the Since the beginning of the twentieth century, 0007 eastern Mediterranean through the Sicily Straits. when the Rrst investigations in the Mediterranean Relatively fresh waters of Atlantic origin circulating Sea took place (1908), up to the mid-1980s, both in the Mediterranean increase in density because the intermediate and deep conveyor belts of the evaporation (E) exceeds precipitation (advective sa- eastern basin presented rather constant character- linity preconditioning), and then form new water istics. The Adriatic has been historically considered masses via convection events driven by intense local as the main contributor to the deep and bottom cooling (Q) from winter storms. Bottom water is waters of the Ionian and Levantine basins, thus produced: for the western basin (WMDW) in the indicating an almost perfectly repeating cycle in Gulf of Lions (Figure 4A) and for the eastern basin both water mass characteristics and formation rates (Figure 4B) in the southern Adriatic (EMDW, which during this long period. Roether and Schlitzer found plunges down through the Otranto Straits). Recent in 1991 that the thermohaline circulation in the observations also indicate deep water (LDW) forma- eastern basin consists of a single coherent convective tion in the north-eastern Levantine basin during cell which connects the Levantine and Ionian basins exceptionally cold winters, where intermediate and has a turnover time of 125 years below 1200 m. water (LIW) is regularly formed seasonally. Evid- Their results indicated that the water formed in the ence now shows that LIW formation occurs over Adriatic is a mixture of surface water (AW) and much of the Levantine basin, but preferentially in intermediate Levantine water (LIW) from the Medi- the north, probably due to meteorological factors. terranean. The Aegean has also been reported as The LIW is an important water mass which circu- a possible secondary source, providing dense waters lates through both the eastern and western basins to the lower intermediate and/or deep layers, name- and contributes predominantly to the efSux from ly Cretan Intermediate Water (CIW), that affected Gibraltar to the Atlantic, mixed with some EMDW mainly the adjacent to the Cretan Arc region of the and together with WMDW. Additionally, intermedi- eastern Mediterranean. Since 1946 increased densit- ate and deep (but not bottom) waters formed in the

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