Cold Sea Upwellings on the Costa del Sol, Spain
There are many reasons why southern Spain hosts a mass exodus of holidaymakers from northern European cities during the summer months, but the seaside has always featured prominently. However, a little known, but nevertheless unpleasant, phenomenon may greet those eager tourists.
Introduction
The economy of Spain is the 5th largest in Europe and the 9th largest in the world. Tourism was worth 40 billion Euros in 2006 and, as an industry, was 2nd only in the world to France. It represented 5% of the GDP. Visitors to Andalucia, and particularly the Costa del Sol, contribute a large percentage to the wealth of the Nation, and the advertising machinery of the Spanish Tourist Board plays heavily on the sun, sand and sangria aspect of the Spanish beaches. It may then come as a great surprise to many people when they find out that upwellings of cold water occasionally reduce mid-summer sea temperatures to those normally experienced off southern Norway or around the western coasts of Britain.
Broad Scale Exchange of Water
The narrowness, and relatively shallow nature, of the Strait of Gibraltar (fig 1.), has led to the creation of the unique mass of water known to us as the Mediterranean Sea. For a while, around 6 million years ago, the access to the Atlantic Ocean was blocked and the Sea dried out considerably. It has been estimated that the water levels in the Basin could have fallen in excess of 2000 metres. The resulting climate on the edge of the well-below sea level brackish lake remnants would have been hostile to most forms of life with both air and water temperatures much above any that exist in today's world away from in areas of volcanic activity.
Figure 1. A coarse representation of currents passing through the Strait of Gibraltar
Around 5 million years ago the barrier to the Atlantic was breached, and although detailed facts on the replenishment of the Mediterranean Sea remain elusive, it appears that the opening through the Strait of Gibraltar has remained roughly in its present form ever since. Even during the last Ice Age when large amounts of ocean water were re-cycled to form enormous land-based glaciers, the sea level remained more than 100 metres above the sill separating the Mediterranean Sea from the Atlantic Ocean.
So, for several million years there has been an exchange of water between the Ocean and the Sea. Research has shown that the Mediterranean loses 1 metre in depth through evaporation each year and that takes into account rainfall and the inflow from river systems, notably the Rhone and the Ebro. As a result there is a net influx of 17,000 cubic kilometres of water from the Atlantic with more than 3 times that amount crossing the threshold between the 2 water masses due to currents (fig. 1).
The system of currents affecting the Mediterranean is complex with discoveries new to science frequently being made. In crude terms, relatively cool, low salinity water from the Atlantic flows through the Strait fairly close to the coast of Spain, but the motion eastwards has a counteracting southward push which results in a clockwise, or anticyclonic gyre (A circular, or spiral, ocean current). In fact 2 significant gyres usually exist. One with a southward extension close to Malaga, but with the main gyre occurring south of Almeria. In oceanographic terms this eastern position marks the boundary between true Mediterranean water and mixed water, but of Atlantic origin. It is known as the Almeria/Oran (Algeria) Front and is an area rich in nutrients and marine life.
Meanwhile, the current of predominantly Mediterranean water continues eastwards, eventually returning westwards along the north African coast as a cold, dense sea-bed hugging high salinity current eventually falling over the sill at the Strait of Gibraltar and then back into the Atlantic Ocean.
Mesoscale Water Mobility on the Costa del Sol
Within the general water movement through the Alboran Sea (fig.1) there are several other factors involved in determining water temperatures along the coast. The most noticeable feature (as shown in fig 2.) is the near isothermal layer from around 100-150 metres in depth and downwards. Sea temperatures show little variation through the seasons up to a depth of close to 50 metres from the surface. Above that, seasonal warming and cooling is apparent but there are many reasons why there are minor day to day changes of temperature. Precipitation, river outflow, wind-driven Atlantic water, coastal contours, both on land and at sea, and even the underwater mixing of fresh and sea water through porous limestone are major players in this complex environment.
Figure 2. A highly simplified sea temperature profile for coastal waters off Southern Spain.
Seasonal Weather Variations
The annual weather in most of the European countries bordering the Mediterranean Basin shows a similar pattern. Winter and Spring are under the influence of Atlantic frontal systems, but although some unsettled spells occur, pressure is often high and many sunny and mild days occur. The Costa del Sol, in particular, is noted for its mild and sunny winter weather. Autumn can be a time of violent thunderstorms along coastal areas of the Mediterranean as cool air destabilizes over the warm sea. Nevertheless, every Autumn has plenty of sunny and warm weather. Apart from the odd thundery outbreak, mainly in the northern part of the Mediterranean, Summer is generally dry, sunny and very warm. There are some exceptions to this bland final statement, a fact that can be attested to by residents and visitors on the Costa del Sol.
The Birth of Upwellings
For the purpose of this article, summer months can be assumed to cover any time during the period from May to October. This is because the causes of upwellings are only likely to occur during this time span. There is evidence that upwellings are more common and more dramatic to the west of the Almeria/Oran front (fig.1) due to a greater instability of the sea caused by the Atlantic water influx, variable strength gyre and a changing water density due to increasing and decreasing salinity.
Take a typical spell of summer weather in Malaga. Days are relentlessly sunny with light sea breezes and nights are dry with very little wind. The Atlantic water still runs through the Strait of Gibraltar but evaporation increases and the surface water becomes more saline with time as it spreads eastwards. Sea temperatures rise from around 19 or 20 at the end of May to 23 or 24 Celsius towards the end of June, and then 'El Terral' appears.
El Terral or Bochorno (suffocating heat) is a northwesterly gradient wind that blows down the Guadalhorce valley towards Malaga during the summer months. It warms as it travels down from the mountains, similar to the more familiar foehn winds of central Europe. Temperatures often exceed 40 Celsius when these winds blow but fortunately they are infrequent and fairly localised, although there is reason to believe that they blow strongly at several other towns along the Spanish south coast. Apart from the very high temperatures, the winds are accompanied by extremely low relative humidity. This results in a much increased rate of evaporation. The sea is now ripe for an upwelling.
Upwelling Dynamics
The force of the offshore wind peels back layers of sea like peeling an onion. At the same time, evaporation continues apace. Eventually, a pile of dense salt water is created which becomes increasingly unstable. Finally this body of water sinks and is replaced by less dense, and very much colder water from anywhere between 25 and 100 metres in depth, depending on the intensity of the event and the coastal topography (Fig.3). This process may be enhanced if fresh water from the winter rains has permeated through the coastal limestone and diluted the saline water on the sea bed.
Figure 3. A crude representation of the forces involved during a hot and dry offshore wind event.
The overall result is a lowering of the sea temperatures on beaches by 5 to 10 degrees Celsius in just a few hours. In late September 1993, an upwelling reduced the sea temperature by about 9 degrees, and during the last week of June 2009 the sea temperature fell from 23 Celsius to between 16 and 19 Celsius along most of the Costa del Sol. After a brief recovery, 3 days of hot offshore winds produced a further upwelling in early July with sea temperatures between 16 and 19 Celsius then lasting until after mid month, perhaps helped by limestone permeation after the substantial winter rains.
Conclusion
Rapid changes in sea temperatures are very rare in the Mediterranean to the east of Almeria, although downwellings (cooling by the advection of cool water from deeper water surfaces occur due to onshore sea breezes). West of Almeria, the coast can be affected by low sea temperatures at any time between May and perhaps early October. However, events are fairly rare, maybe 2 or 3 occasions each year; and generally sea temperatures recover within a week of the cessation of the land breezes. It would be interesting to know, though, whether the Spanish Tourist Board are aware of these cold-water upwellings, or maybe the knowledge should remain part of the secret Spain.
The economy of Spain is the 5th largest in Europe and the 9th largest in the world. Tourism was worth 40 billion Euros in 2006 and, as an industry, was 2nd only in the world to France. It represented 5% of the GDP. Visitors to Andalucia, and particularly the Costa del Sol, contribute a large percentage to the wealth of the Nation, and the advertising machinery of the Spanish Tourist Board plays heavily on the sun, sand and sangria aspect of the Spanish beaches. It may then come as a great surprise to many people when they find out that upwellings of cold water occasionally reduce mid-summer sea temperatures to those normally experienced off southern Norway or around the western coasts of Britain.
Broad Scale Exchange of Water
The narrowness, and relatively shallow nature, of the Strait of Gibraltar (fig 1.), has led to the creation of the unique mass of water known to us as the Mediterranean Sea. For a while, around 6 million years ago, the access to the Atlantic Ocean was blocked and the Sea dried out considerably. It has been estimated that the water levels in the Basin could have fallen in excess of 2000 metres. The resulting climate on the edge of the well-below sea level brackish lake remnants would have been hostile to most forms of life with both air and water temperatures much above any that exist in today's world away from in areas of volcanic activity.
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Around 5 million years ago the barrier to the Atlantic was breached, and although detailed facts on the replenishment of the Mediterranean Sea remain elusive, it appears that the opening through the Strait of Gibraltar has remained roughly in its present form ever since. Even during the last Ice Age when large amounts of ocean water were re-cycled to form enormous land-based glaciers, the sea level remained more than 100 metres above the sill separating the Mediterranean Sea from the Atlantic Ocean.
So, for several million years there has been an exchange of water between the Ocean and the Sea. Research has shown that the Mediterranean loses 1 metre in depth through evaporation each year and that takes into account rainfall and the inflow from river systems, notably the Rhone and the Ebro. As a result there is a net influx of 17,000 cubic kilometres of water from the Atlantic with more than 3 times that amount crossing the threshold between the 2 water masses due to currents (fig. 1).
The system of currents affecting the Mediterranean is complex with discoveries new to science frequently being made. In crude terms, relatively cool, low salinity water from the Atlantic flows through the Strait fairly close to the coast of Spain, but the motion eastwards has a counteracting southward push which results in a clockwise, or anticyclonic gyre (A circular, or spiral, ocean current). In fact 2 significant gyres usually exist. One with a southward extension close to Malaga, but with the main gyre occurring south of Almeria. In oceanographic terms this eastern position marks the boundary between true Mediterranean water and mixed water, but of Atlantic origin. It is known as the Almeria/Oran (Algeria) Front and is an area rich in nutrients and marine life.
Meanwhile, the current of predominantly Mediterranean water continues eastwards, eventually returning westwards along the north African coast as a cold, dense sea-bed hugging high salinity current eventually falling over the sill at the Strait of Gibraltar and then back into the Atlantic Ocean.
Mesoscale Water Mobility on the Costa del Sol
Within the general water movement through the Alboran Sea (fig.1) there are several other factors involved in determining water temperatures along the coast. The most noticeable feature (as shown in fig 2.) is the near isothermal layer from around 100-150 metres in depth and downwards. Sea temperatures show little variation through the seasons up to a depth of close to 50 metres from the surface. Above that, seasonal warming and cooling is apparent but there are many reasons why there are minor day to day changes of temperature. Precipitation, river outflow, wind-driven Atlantic water, coastal contours, both on land and at sea, and even the underwater mixing of fresh and sea water through porous limestone are major players in this complex environment.
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Figure 2. A highly simplified sea temperature profile for coastal waters off Southern Spain.
Seasonal Weather Variations
The annual weather in most of the European countries bordering the Mediterranean Basin shows a similar pattern. Winter and Spring are under the influence of Atlantic frontal systems, but although some unsettled spells occur, pressure is often high and many sunny and mild days occur. The Costa del Sol, in particular, is noted for its mild and sunny winter weather. Autumn can be a time of violent thunderstorms along coastal areas of the Mediterranean as cool air destabilizes over the warm sea. Nevertheless, every Autumn has plenty of sunny and warm weather. Apart from the odd thundery outbreak, mainly in the northern part of the Mediterranean, Summer is generally dry, sunny and very warm. There are some exceptions to this bland final statement, a fact that can be attested to by residents and visitors on the Costa del Sol.
The Birth of Upwellings
For the purpose of this article, summer months can be assumed to cover any time during the period from May to October. This is because the causes of upwellings are only likely to occur during this time span. There is evidence that upwellings are more common and more dramatic to the west of the Almeria/Oran front (fig.1) due to a greater instability of the sea caused by the Atlantic water influx, variable strength gyre and a changing water density due to increasing and decreasing salinity.
Take a typical spell of summer weather in Malaga. Days are relentlessly sunny with light sea breezes and nights are dry with very little wind. The Atlantic water still runs through the Strait of Gibraltar but evaporation increases and the surface water becomes more saline with time as it spreads eastwards. Sea temperatures rise from around 19 or 20 at the end of May to 23 or 24 Celsius towards the end of June, and then 'El Terral' appears.
El Terral or Bochorno (suffocating heat) is a northwesterly gradient wind that blows down the Guadalhorce valley towards Malaga during the summer months. It warms as it travels down from the mountains, similar to the more familiar foehn winds of central Europe. Temperatures often exceed 40 Celsius when these winds blow but fortunately they are infrequent and fairly localised, although there is reason to believe that they blow strongly at several other towns along the Spanish south coast. Apart from the very high temperatures, the winds are accompanied by extremely low relative humidity. This results in a much increased rate of evaporation. The sea is now ripe for an upwelling.
Upwelling Dynamics
The force of the offshore wind peels back layers of sea like peeling an onion. At the same time, evaporation continues apace. Eventually, a pile of dense salt water is created which becomes increasingly unstable. Finally this body of water sinks and is replaced by less dense, and very much colder water from anywhere between 25 and 100 metres in depth, depending on the intensity of the event and the coastal topography (Fig.3). This process may be enhanced if fresh water from the winter rains has permeated through the coastal limestone and diluted the saline water on the sea bed.
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Figure 3. A crude representation of the forces involved during a hot and dry offshore wind event.
The overall result is a lowering of the sea temperatures on beaches by 5 to 10 degrees Celsius in just a few hours. In late September 1993, an upwelling reduced the sea temperature by about 9 degrees, and during the last week of June 2009 the sea temperature fell from 23 Celsius to between 16 and 19 Celsius along most of the Costa del Sol. After a brief recovery, 3 days of hot offshore winds produced a further upwelling in early July with sea temperatures between 16 and 19 Celsius then lasting until after mid month, perhaps helped by limestone permeation after the substantial winter rains.
Conclusion
Rapid changes in sea temperatures are very rare in the Mediterranean to the east of Almeria, although downwellings (cooling by the advection of cool water from deeper water surfaces occur due to onshore sea breezes). West of Almeria, the coast can be affected by low sea temperatures at any time between May and perhaps early October. However, events are fairly rare, maybe 2 or 3 occasions each year; and generally sea temperatures recover within a week of the cessation of the land breezes. It would be interesting to know, though, whether the Spanish Tourist Board are aware of these cold-water upwellings, or maybe the knowledge should remain part of the secret Spain.