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 Medย 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 โย
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, akaย Alboran Sea, is the habitat for aย 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 WAVESย
Entering and Exiting Atolls in the South Pacific
Entering and Exiting Atolls in the South Pacific
1) enter during daylight hours
2) go in a slack tide or near slack tide – use interpolation to calculate slack tide – interploation and other tools are NOT perfect – be prepared to enter with ripples coming through
3) many times the atoll will still “empty out like a teacup” even though you are near slack tide as trade winds continuously fill the atoll due to elevated trade winds / big swell braking over the S / SE portion of the atolls
4) the most dangerous part are standing waves if you see those do NOT enter and wait
5) when entering with and out-flowing atoll your steering is enhanced as there is more water flowing over your rudders so in many case going in is easier than going out.
6) use satellite charts in addition to sonar charts to validate where shoaling and coral heads are as you enter the atoll.
7) if you can avoid entering during a squall, during low visibility and do no enter at night
8) The bigger the Atoll and the fewer the openings ( outflows ) the more ripping the current will be and the later the entry will be from slack tide
THE LONG OF IT
Where tide is of interest is around the atolls, explicitly when you are attempting to enter or leave one. Each pass is a flowing gateway.ย . The thing that matters is the passes here are undeniably more impacted by wind and wave impacts.
In the wake of visiting our third atoll and paying attention to the day to day dramatizations of othersย , we areย examinging out when that subtle leeway will happen includes as much art as seamanship.
A few different rules for individual passes, again corresponding to a decent time from an essential port. Different tidal forecast distributions have a similar issue. They need to give you direction yet are restricted by the way that any direction they really do give is probably going to be erroneous.
There are not many essential ports here and the distance between where you are attempting to sort out your tides and where the essential is can rush to many miles. The two nearest to usย Rangiroa (160miles away) and
Tahanea (around 100 miles), neither precisely close. Not an issue, I hear you say.
The enormous issue with the Tuamotos is that there are extra factors that can essentially modify the hour of slack waterย Despite the fact that tides are little, seldom north of one meter even at springs, the base outward momentum speed at passes in times of clear, actually weather conditions runs somewhere in the range of 4 and 6kts, subject to the atoll. With gigantic measures of water hustling all through the frequently restricted passes with reef tight either side, races, bubbles, standing waves and overfalls are normal external the brief time of slack, importance travel during slack remaining parts ideal. Nonetheless, the key proclamation that you should comprehend is that the active momentum speed is affected by climate and can, anytime of the tide, be essentially higher a speed than the approaching flowing stream.
Wind over tide is a significant thought ย In the Tuamotus, Atolls for the most part run SE to NW in locations. Most atolls are comprised of motus (the more modest islands making up the atoll’s edge) on the E side, where millennia of the ordinary tradesย ย have kept sand and ruin within the reef to shape them. The safeguarded W side is undeniably more infertile and most frequently comprises basically of reef with a couple of little segregated motus as it were. The S and N finishes of atolls will regularly have a motus because of the wave wash over impact of the ocean.
A few atolls (Makemo for example) have not many extremely lengthy motus on its E side which gives brilliant security to the tidal pond. Be that as it may, the more prominent the quantity of motus making up the E side of an atoll (Raroia has parts),
When the water is in the tidal pond, it necessities to get away and most atolls have yet a couple of fundamental passes where the water can escape from. Restricted measures of water will stream back out through the holes between the motus or back over the reef.
The course a pass faces is an element. One that faces E, into the Exchanges, will constantly have a breeze over tide impact during an active stream, keeping down and dialing the outward stream back
A decent wide, profound channel permits more water to drop of the tidal pond.
Also he size of the atoll thus the size of the catch region for water streaming in to the tidal pond. The most grounded of flowing races is at Hao, an exceptionally huge atoll. More than 30 miles in length it has a solitary pass confronting N. With such a lot of water coming into the tidal pond tossed over the reef, the race can work out positively past 10kts on an outward stream.
Thus, how much water in the tidal pond attempting to escape through the pass anytime of tide is the amount of what the weather conditions has unloaded across the reef and the typical tide. The more water there is in the tidal pond, the more prominent the momentum strength and speed of the active stream. This then, at that point, impacts a contrast among determined and genuine pants times. Furthermore, assuming that the tidal pond is exceptionally high after a time of terrible climate, you might find that the outward stream refutes the approaching tide, meaning no leeway period by any stretch of the imagination.
The Well known Guestimator gives the accompanying augmentations to current outward stream speeds:
1. Add 1 kt for each day the breeze has been blowing north of 20 kts from a S or W part
2. Add 0.5 kt for each day the breeze has been blowing more than 15 kts from a S or W part
3. Add 0.5 kt for each 1/2 meter augmentation of southerly swell over 1.5 meters (ie 3 meter grow = +1.5 kt)
4. Cap the Breeze Wave factor at 1.5 times the Typical Max Current
5. Take away 0.5 kt for wide/profound passes and for every additional pass that an atoll has