Please select your home edition
North Sails 2019 - NSVictoryList - Leaderboard

Navigating the Strait of Gibraltar tides

by Tidetech 4 Sep 2020 23:34 PDT
Navigating the Strait of Gibraltar tides © Tidetech

Suppose you're familiar with oceanography and some of the more treacherous shipping routes of the world. In that case, you'll know about the challenges for the shipping industry that the Strait of Gibraltar poses. This narrow passage that connects the Atlantic Ocean to the Mediterranean Sea has many elements that contribute to the complexity of tides and currents in this area, and this article explores how it all works.

From the rapid evaporation of water that exceeds the influx of water from rivers and rain to the layers of seawater, each of a different density and salinity, there are several ever-changing elements to be aware of when transiting the Strait of Gibraltar.

Here's an outline of just how and why these infamous straits are so complex to navigate.

The density of seawater in the Gibraltar Strait

One of the singular features of the Strait of Gibraltar is the Mediterranean inflow and outflow, which consists of layers of water that have different salinity (levels of salt).

The water in the Atlantic is less salty and less dense than water in the Mediterranean, and it flows eastwards into the Mediterranean through the Straits as a surface layer, about 125m deep with a speed of two to three knots.

In contrast, there is a westward outflow of heavier, colder, and more salty water, which works its way out into the Atlantic. The tidal flow will either speed up or slow down the eastern flowing current, depending on the phase of the tide.

An ever-evaporating sea

A very distinctive feature of the Strait of Gibraltar is it's continually evaporating body of water. The Mediterranean Sea loses nearly a meter of vertical water height every year.

Around seven million years ago, the general shape of the Mediterranean basis was nearly the same as today. Due to the movement of tectonic plates, the inflow of water from the ocean to the sea was cut off, leaving the area to evaporate completely. However, around 5.3 million years ago, the tectonic plates moved again, opening the Strait and allowing an immense amount of water to flow from the ocean, filling the Mediterranean basin once again.

This evaporation is still occurring today, and it's estimated that were the Strait closed today at our current higher sea level, the basin would evaporate once again.

Counter currents and internal waves

To add another challenging piece to the tidal puzzle that makes up the Strait of Gibraltar, close to the African continent, there is often a narrow counter current of two knots that interacts with the Camarinal Still (the shallowest part of the Strait) and causes internal waves. As noted by NASA;

"The waves are generated as a diurnal tidal pulse flows over the shallow Camarinal Sill at Gibraltar. The waves flow eastward, refract around coastal features; can be traced for as much as 150 km, and sometimes create interference patterns with refracted waves." - Earth Observatory, NASA

These internal waves are a vertical motion between the two layers and can have a displacement sometimes exceeding 100m with a wavelength of two to four km. They're so distinct as a surface wave pattern that sunlight is deferentially scattered off the water surface and the waves can be observed by astronauts in space!

You can see exactly how this looks in the video below.

The 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 flows) can be seen close inshore along both shores, particularly near headlands that project into the current.

The western end of the Mediterranean, the Alboran Sea, is the habitat for the largest population of bottlenose dolphins in the western Mediterranean, is home to the last population of harbour porpoises in the Mediterranean, and is the most important feeding ground for loggerhead sea turtles in Europe.

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. It splits into two at Cape St Vincent, with one branch going west and the other branch going North. Evidence of this flow is seen as far north as the Greenland-Scotland sill.

Amazingly, the westward flowing Mediterranean water reaches the American continent and travels south to Antarctica and the Weddell Sea.

Modelling the Strait of Gibraltar

Over 100,000 ships transit through Gibraltar every year for commercial purposes, so modelling of the complexity of the Strait is key for safe passage.

You can browse ocean data modelling covering Biscay, the Iberian Peninsula and the Gibraltar Strait here.

Related Articles

Youth Foiling Gold Cup day 4
RHKYC Team Agiplast 2nd after qualifying series The light, shifty wind brought the flying Persico 69Fs back down to earth, but the unstable conditions allowed for plenty of opportunities for wily opportunists to move up through the fleet, or for others to slip down. Posted today at 1:40 am
America's Cup: 'Look at the bloody clouds'
Profile of the weather guru who's helped Team NZ skippers call the shots for more than two decades The weather guru who's helped Team NZ skippers call the shots for more than two decades hopes his final forecasts will be winners. Posted today at 1:07 am
2021 JJ Giltinan 18ft Skiff Championship day 2
An epic, race long battle for tech2 and Yandoo Winning Group tech2's team overcame an epic, race long battle with her arch rival Yandoo Winning Group to score a sensational, and possibly controversial, win in Race 2 of the JJ Giltinan 18ft Skiff Championship on Sydney Harbour today. Posted on 7 Mar
2021 Islands Race at San Diego Yacht Club
'io takes the win A full year ago in Spring 2020, Newport Harbor Yacht Club and San Diego Yacht Club hosted the Islands Race with a just a hint of the Covid-19 influence over our lives, one of the last "normal" regattas before the pandemic took over. Posted on 7 Mar
Bahamas Optimist National Championship
The Championship fleet battled it out until the final seconds of the very last race The Bahamas Optimist National Championship 2020 was sailed February 27-28 in George Town, Exuma, The Bahamas, hosted and organized by the Exuma Sailing Club on the waters of beautiful Elizabeth Harbor. Posted on 6 Mar
Mainsail hoisting system by Colligo Marine
This system allows you to decouple the top car from your mainsail easily Tired of climbing up on your boom to release the top car on your mainsail so you can cover it completely? Posted on 6 Mar
2021 JJ Giltinan 18ft Skiff Championship day 1
An impressive start for the tech2 team Australian 18ft Skiff champion tech2 team of Jack Macartney, Charlie Wyatt and Lewis Brake took the first step towards adding the JJ Giltinan Championship to an already impressive record this season when they came from behind to take out Race 1. Posted on 6 Mar
Ready to go at Bacardi Cup Invitational Regatta
The 2021 Bacardi Cup Invitational Regatta is all set to welcome over seventy-five teams Every year the Bacardi Cup and Bacardi Invitational Regatta attract a dazzling mix of professional rock star racers and super-talented Corinthian teams from around the globe. Posted on 6 Mar
Youth Foiling Gold Cup day 3
No wind, no fly No foiling today in the Youth Foiling Gold Cup. "We tried our best, we went out looking for the wind, but it was hopeless, so we cancelled for the day.", said Dede De Luca, Chief Sailing Officer. Posted on 5 Mar
San Diego to start Helly Hansen NOOD Regatta 2021
Competitive sailors in the region can expect top-quality races on San Diego Bay For nearly three decades, the Helly Hansen NOOD Regatta national series kicked off in St. Petersburg, Fla., in February, but a shift in the 2021 calendar will put Southern California sailing front and center with the Helly Hansen NOOD Regatta San Diego. Posted on 5 Mar