Eastern Mediterranean Transient
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| Eastern Mediterranean Transient | |
|---|---|
| Name | Eastern Mediterranean Transient |
| Location | Eastern Mediterranean Sea |
| First | 1990s |
| Outcome | Altered deep-water formation and circulation |
Eastern Mediterranean Transient The Eastern Mediterranean Transient describes a major, rapid reorganization of deep-water formation and thermohaline circulation in the Mediterranean Sea, notably affecting the Aegean Sea, Ionian Sea, Levantine Basin, and adjoining basins. Observed from the late 1980s through the 1990s, the event altered exchanges with the Atlantic Ocean through the Strait of Gibraltar, influenced the Black Sea outflow, and had implications for regional interactions with the North Atlantic Oscillation and Madden–Julian Oscillation. It has been studied by oceanographers associated with institutions such as the National Oceanic and Atmospheric Administration, Institut Français de Recherche pour l'Exploitation de la Mer, and Scripps Institution of Oceanography.
Overview
The phenomenon involved a shift in the principal site of deep-water formation from the Gulf of Lion and Adriatic Sea toward the Aegean Sea and eastern basins, producing a cascade of physical changes that propagated westward and influenced the Western Mediterranean Sea. Key observational programs included cruises by the Royal Netherlands Institute for Sea Research, experiments run by Mediterranean Forecasting System groups, and monitoring by the Global Ocean Observing System. The Transient modified properties tracked by instruments developed at Woods Hole Oceanographic Institution, Lamont–Doherty Earth Observatory, and CSIC laboratories.
Causes and Mechanisms
Drivers implicated in the reorganization include anomalous atmospheric forcing from the North Atlantic Oscillation, episodic cold-air outbreaks linked to the Arctic Oscillation, and moisture anomalies connected to the Mediterranean Storms and Saharan Air Layer. Enhanced evaporation in the Levantine Basin and cooling in the Aegean Sea increased surface density, promoting convection akin to processes observed in the Gulf Stream and Eastern North Atlantic Current. Freshwater flux variations from rivers such as the Nile River and pulses related to the Bosporus and Dardanelles straits, modulated by interactions with the Black Sea, also influenced stratification. Wind-driven mesoscale variability associated with features like the Myrtoan Sea jets, Etesian winds, and exchanges across the Cretan Arc contributed to vertical mixing and lateral advection.
Observations and Evidence
Evidence derives from repeated hydrographic sections, moored time series, and tracer studies using properties recorded by instruments from Argo floats, CTD profiles aboard research vessels like RV Pelagia, and current meters deployed by programs including Euro-Argo and Mediterranean Science Commission. Chemical tracers such as chlorofluorocarbons measured in campaigns by teams from University of Barcelona, National and Kapodistrian University of Athens, and University of Athens provided age constraints comparable to studies in the Southern Ocean and North Pacific. Satellite altimetry from missions like TOPEX/Poseidon and Jason-1, together with sea surface temperature data from NOAA-AVHRR and ERS satellites, documented surface signatures that matched in situ changes. Paleoceanographic proxies studied by laboratories at University of Cambridge and ETH Zurich offered longer-term context.
Oceanographic and Climatic Impacts
The Transient altered deep-water mass characteristics including temperature, salinity, and oxygen content, with consequences for benthic habitats in regions studied by scientists from Hellenic Centre for Marine Research and National Institute of Oceanography and Applied Geophysics (OGS). Changes in thermohaline circulation affected nutrient distributions impacting plankton communities monitored by Plymouth Marine Laboratory and fisheries dynamics relevant to stakeholders such as the Food and Agriculture Organization offices for the Mediterranean. The event modulated sea-level signals measured by tide gauges in Venice, influenced water mass exchanges linked to the Gibraltar Sill, and interacted with climate modes including the Atlantic Multidecadal Oscillation and El Niño–Southern Oscillation.
Timeline and Spatial Extent
The onset occurred in the late 1980s with crescendo in the early 1990s, recorded in multidisciplinary programs coordinated by centers including IFREMER and CNR with subsequent evolution through the 2000s. Spatially, the perturbation originated in the eastern basins—Levantine Basin, Cretan Sea, Aegean Sea—propagating westward across the Ionian Sea into the Adriatic Sea and influencing the Alboran Sea on decadal time scales, analogous to propagation pathways observed in circulation studies of the Gulf of Mexico outflow and the Red Sea exchange processes.
Modeling and Predictive Studies
High-resolution numerical simulations by groups at Princeton University, ETH Zurich, Plymouth Marine Laboratory, and Barcelona Supercomputing Center used versions of models such as MITgcm, ROMS, and NEMO to reproduce the Transient when forced with atmospheric fields from reanalyses like ECMWF ERA-Interim and NCEP/NCAR. Sensitivity experiments examined the roles of surface fluxes, lateral boundary conditions at the Strait of Gibraltar, and river inputs from the Nile River and Rhône. Ensemble forecasting frameworks developed with support from European Space Agency and Horizon 2020 projects explored predictability and teleconnections to phenomena studied by IPCC assessments and climate centers including Met Office and Météo-France.
Significance and Research Directions
The Eastern Mediterranean Transient serves as a natural experiment informing understanding of abrupt oceanographic shifts relevant to studies by IPCC Working Group I and World Meteorological Organization initiatives. Ongoing priorities include integrating biogeochemical cycles investigated by Max Planck Institute for Meteorology, improving coupled ocean–atmosphere models at NOAA Geophysical Fluid Dynamics Laboratory, and enhancing observational networks via EU Copernicus and Global Ocean Observing System collaborations. Future work aims to refine projections of similar reorganizations under climate change scenarios examined in reports by Intergovernmental Panel on Climate Change and to assess ecological consequences for jurisdictions including Greece, Turkey, Egypt, and Italy.