Wait, What? Water can leave the Mediterranean as a dense salty overflow, plunge downslope into the Atlantic, mix violently—and then stop sinking at an intermediate depth rather than reaching the abyss.

Mediterranean Outflow Water (MOW) begins with the Mediterranean’s strong evaporation. Evaporation removes freshwater but leaves most salt behind, helping make Mediterranean water unusually saline and dense. Dense water flows westward through the Strait of Gibraltar, descends into the Gulf of Cádiz, entrains surrounding Atlantic water and then spreads into the North Atlantic at intermediate depth.

Scientific Job Claimed by This Manual

This manual owns one Ocean World process: strong Mediterranean evaporation → salty dense water → westward outflow through Gibraltar → downslope gravity current → intense entrainment of Atlantic water → modified intermediate-density Mediterranean Outflow Water → spreading into the Atlantic interior. The Estuarine Circulation Learning Manual owns generic fresh–salt exchange. The North Atlantic Deep Water Learning Manual owns northern deep-water formation. This manual owns the Gibraltar overflow, entrainment and intermediate-depth transformation.

Primary: Why Does Evaporation Make the Mediterranean Saltier?

When seawater evaporates, water molecules leave the surface but most dissolved salt remains behind. If evaporation is stronger than freshwater replacement, salinity rises.

Saltier water is generally denser, so some Mediterranean water becomes dense enough to flow outward beneath incoming Atlantic water.

Secondary: A Two-Layer Exchange at Gibraltar

The Strait of Gibraltar connects the Mediterranean Sea with the Atlantic Ocean. At broad scale, relatively fresher Atlantic water flows eastward into the Mediterranean near the surface, while denser Mediterranean water flows westward underneath.

This is a classic exchange flow, but the story does not end at the strait. Once the dense outflow reaches the Atlantic side, the seabed slopes downward into the Gulf of Cádiz.

Why the Outflow Accelerates Downslope

The Mediterranean water is denser than the Atlantic water around it. Gravity therefore drives it downslope along the seabed as a bottom-intensified current.

As the plume accelerates, strong velocity shear develops at its upper interface. That shear can generate turbulence and intense mixing.

JC: Entrainment Is the Transforming Step

NOAA AOML observations from the Gulf of Cádiz show that Mediterranean outflow transport can increase markedly downstream while its salinity decreases. The reason is entrainment: the dense plume mixes Atlantic water into itself as it descends.

This means the final MOW found in the Atlantic is not simply undiluted Mediterranean water. It is a transformed mixture created by the overflow–entrainment process.

Why It Stops Sinking

As the outflow entrains fresher and lighter Atlantic water, its density decreases. Eventually it reaches a depth where its density is similar to the surrounding Atlantic water.

At that point, the outflow stops descending strongly and begins spreading laterally at intermediate depth.

Why the Mediterranean Signature Can Be Traced Far Away

MOW carries a distinctive warm-and-salty signature compared with surrounding Atlantic intermediate waters. Oceanographers can follow that signature westward and northward using temperature–salinity diagrams and tracers.

The signal weakens with distance because mixing progressively dilutes the water mass.

Connection to Isopycnal Surfaces

After its dense downslope phase ends, Mediterranean Outflow Water spreads preferentially along density surfaces in the Atlantic interior. The Isopycnal Surfaces Learning Manual owns that along-density transport geometry.

Connection to Diapycnal Mixing

The Diapycnal Mixing Learning Manual owns irreversible exchange across density surfaces. The intense turbulent entrainment in the Gulf of Cádiz is a real example of mixing transforming a dense overflow before it settles into the Atlantic interior.

Connection to the Ocean Bottom Boundary Layer

The Ocean Bottom Boundary Layer Learning Manual owns persistent seabed friction. Mediterranean overflow is also strongly influenced by bottom stress as it descends the slope, but its defining scientific job is the dense overflow and entrainment transformation.

How Do We Know?

Scientists observe the outflow using CTD sections, current profilers, moorings, lowered instruments and tracer measurements across Gibraltar and the Gulf of Cádiz. They measure current speed, temperature, salinity and density along the plume’s path.

NOAA AOML analyses found the transport of the outflow increasing from about 0.85 Sv to about 1.9 Sv within the Gulf of Cádiz while its velocity-weighted salinity fell substantially over the first tens of kilometres—direct evidence of strong entrainment.

Observation Versus Water-Mass Reconstruction

An instrument directly measures a local current and water properties. “This is Mediterranean Outflow Water” is a water-mass interpretation that combines those properties with depth, pathway and the known source region.

The farther the water travels, the more mixing blurs its original signature, so attribution becomes a reconstruction rather than a simple label.

Can You Predict It?

• Evaporation increases while freshwater input stays similar: Mediterranean salinity and outflow density can increase.
• The dense plume entrains more Atlantic water: expect salinity and density to decrease while total transport increases.
• The surrounding Atlantic water becomes denser: the outflow may reach neutral buoyancy at a different depth.
• Bottom roughness and shear increase: turbulent mixing and entrainment can strengthen.

Transfer Test

A dense overflow begins with salinity 38 and then doubles its transport while salinity falls toward 36.7 downstream. What process most naturally explains both changes together?

Entrainment of surrounding Atlantic water. The plume gains volume while its original salty signature is diluted.

Model Boundary

Mediterranean outflow varies with tides, atmospheric forcing, Mediterranean salinity, sill hydraulics and downstream eddies. The simple pathway “dense water exits, sinks, mixes, spreads” is a robust first-order model but not a complete description of every plume filament or meddy.

Useful Misconceptions to Correct

• Mediterranean water does not leave Gibraltar unchanged.
• The outflow does not necessarily sink to the deepest Atlantic.
• Higher downstream transport can occur because ambient Atlantic water is entrained.
• Two-layer exchange at Gibraltar and intermediate-depth spreading are different stages.
• MOW is not the same water mass as NADW.

Canonical External Sources

• NOAA AOML — Overflows in the Atlantic: Mediterranean Outflow
• NOAA Atlantic Oceanographic and Meteorological Laboratory

Teaching Method

Begin with the contradiction: “How can a salty current become larger while becoming less salty?” Let students discover entrainment from conservation of volume and salinity before naming the process.

For Primary learners, use salty dense water spilling underneath fresher water. For Secondary learners, add downslope flow and mixing. For JC learners, use transport and salinity observations to infer entrainment, then require students to predict how changing density and mixing alter the final settling depth.