STRAIGHT OF GIBRALTAR

STRAIGHT OF GIBRALTAR
Over 100,000 ships transit through Gibraltar every year

Density of seawater in the Strait of Gibraltar One of the unique features of the Strait of Gibraltar is the inflow and outflow of the Med  which consists of layers of water with different salinity (salinity). Atlantic water is less saline and less concentrated than Mediterranean water, and flows eastward into the Mediterranean through straits as a surface layer, flowing at a speed of 2 to 3 knots at a depth of about 125 m. In contrast, heavier, cooler, and saltier water flows westward into the Atlantic Ocean. Depending on the phase of the tide, the currents flow eastward faster or slower.

Tides in the Strait of Gibraltar – 
A very distinctive feature of the Strait of Gibraltar is the continuous evaporation of water. Vertical water height in the Mediterranean Sea decreases by almost 1 meter every year. About 6.5+ million years ago, the general shape of the Mediterranean base was similar to what it is today. The movement of the plates cut off the flow of water from ocean to ocean, completely evaporating the area. But about 5+ million years ago, the plates moved again and the straits opened, allowing large amounts of water to flow out of the ocean and refill the Mediterranean basin. This evaporation continues today, and it is estimated that if the strait were to close at current high sea levels, the basin would evaporate again. Backwash and Internal Waves Adding another challenging piece to the tidal puzzle that shapes the Strait of Gibraltar near the African continent are the often narrow two-knot backwashes that interact with the Caminal Stile (the shallowest part of the strait) and cause internally generated waves. As stated by NASA; "The waves are generated when daily tidal pulses flow through the shallow Caminal Stile near Gibraltar. They flow eastward and break up the coastal topography. They can be traced up to 90 nm, and in some cases produce interference patterns due to refracted waves

Internal waves are vertical movements between two layers and can have displacements of over 100 m with wavelengths of 1-3 nm. They are so noticeable as surface wave patterns that sunlight is carefully scattered by the water surface

Alboran Gyre

As the upper-level flow pours into the Mediterranean, the Coriolis force (an effect of the earth's rotation) causes it to form a large clockwise eddy (gyre) off the North African coast called the Alboran Gyre. A smaller weak anti-clockwise eddy forms to the North. Countercurrents (westward direcction) can be seen close inshore along both shores, particularly near headlands that project into the current.

The western end of the Mediterranean, aka  Alboran Sea, is the habitat for a  largest population of dolphins in the western Mediterranean

A layer of outward-flowing dense water stays deep after exiting the Mediterranean and forms a ribbon extending along the Spanish and Portuguese coasts at about 1000m depth.

Sailing through the Strait of Gibraltar requires careful attention to weather and sea conditions due to its unique geographical and meteorological characteristics. Here are the key weather considerations:

1. Winds

• Levanter (Easterly Wind):
  • A persistent easterly wind often brings cloudy conditions and rough seas. It can funnel through the Strait and increase in intensity.
  • Best avoided unless heading westward, as it can create challenging conditions for eastbound sailors.
• Poniente (Westerly Wind):
  • A prevailing westerly wind, generally milder than the Levanter, but still capable of creating choppy seas.
  • Ideal for eastbound passages toward the Mediterranean.

2. Tidal Currents

• Eastward Currents:
  • A strong eastward current flows into the Mediterranean, influenced by the Atlantic Ocean's higher sea level.
  • This can work to your advantage when heading east but poses a challenge for westbound sailors.
• Tidal Variability:
  • Currents can reach up to 4 knots in some areas, especially during spring tides. Timing your passage with favorable tidal flows is crucial.

3. Sea State

• The Strait is narrow, creating significant wave amplification when winds oppose tidal currents.
• Crosswinds or opposing currents can result in steep, short-period waves.

4. Visibility

• Fog and mist are common, particularly in early mornings during the summer months, requiring vigilance and radar/plotter use.

5. Seasonal Variations

• Summer:
  • Higher Levanter activity. Calm mornings with wind building in the afternoon.
• Winter:
  • Westerly Poniente is more prevalent, with occasional storm systems moving through.
  • Sea conditions can be rougher, with occasional squalls.

6. Traffic

• The Strait is one of the world's busiest maritime routes.
  • Monitor AIS (Automatic Identification System) to track large vessels and maintain safe distances.
  • Cross the Traffic Separation Scheme (TSS) at right angles if crossing rather than hugging the coast.

7. Best Practices

• Check local forecasts from Spanish (AEMET) and Moroccan Meteorological Services.
• Consider timing your departure for slack tide or favorable currents.
• Have contingency plans for anchoring or sheltering in nearby ports like Tarifa, Algeciras, or Ceuta.

Modelling the Strait of Gibraltar

WINDS

CURRENTS

SWELL AND WAVES 

by oceanpossecom