Saturday, September 25

Why is the Guadalquivir valley one of the hottest inhabited places on the planet?

From Seville to Andújar, passing through Écija, Córdoba and Montoro, the irregular triangle that forms the depression of the Guadalquivir in its entire middle section, sandwiched between the mountain systems of Sierra Morena to the north, and the Subbetic Sierras to the south, is one of the most characteristic climatic regions of all continental Europe due to the torrid summers that it suffers year after year. Almost like a summer tradition, the maxims during the summer place this small corner of the world, where more than two million people coexist, under the spotlight of the warmest places on the planet, opening news programs that cross national borders.

Summer until the end: 40 degrees return to the interior of Andalusia at the beginning of September

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Historical records point to levels in the first division of planetary heat. The 46.6 ° C of Seville Airport of July 23, 1995, and the mythical 46.9 ° C of Córdoba airport and 47.4 ° C of Montoro-Vega Armijo of August 14, 2021, although far (relatively) from the maximums that usually occur in the American Death Valley, the interior of Algeria or the Arabian Peninsula, make our area one of the warmest inhabited regions on the planet that is not located in regions of extreme aridity.

It is the Guadalquivir valley, a fluvial corridor capable of sharing precipitation peaks of over 1,000 liters per year, frequent winter frosts and even occasional snowfalls, with temperature records typical of arid subtropical regions, and to which the State Meteorological Agency dedicated a study in 2018 by Nicolás Bermejo, to explain the peculiarities that make this depression the particular Spanish ‘frying pan’.

The Guadalquivir valley and its ‘privileged’ enclave

The fact that the Guadalquivir depression is just 15 kilometers from North Africa, the prelude to the hot Saharan container, makes it especially vulnerable to the warm intrusions that often occur between the months of June to September. They are very warm air inlets that tend to stabilize over the peninsula and that find in our valley an ideal orographic drawer in which to settle and reheat. An ideal starting point for the entry into play of other factors that place the entire valley as the undisputed king of daily records for maximum temperatures in the peninsula.

And what are those factors that trigger mercury more easily than in any other peninsular area? According to the study by Nicolás Bermejo, they are basically due to local orographic issues that activate breeze mechanisms facilitating surface overheating. In other words, that the valley is where it is and that it has the layout and the relief that it has, make this a natural ‘oven’ of the first order.

A game of relief and breezes

The first thing to understand why the summer in the valley resides in a barometric phenomenon completely imperceptible to its inhabitants, the summer thermal low. A meteorological phenomenon that covers the entire middle course, and part of the lower, of the Guadalquivir River. It consists of the formation of a center of low surface pressures in the early hours of the day. Thanks to the morning insolation, the surface air expands and the pressure decreases, forming first on the vertical of Andújar and Montoro, and extending its domain down the valley until it almost reaches the mouth with the passing of the hours. A kind of invisible bubble that envelops the lower layers of the valley from Andújar to Seville, and that serves as a stabilizer of the wind on the surface to facilitate heating in the first meters above the ground.

It is a perfect area for air boxing, which facilitates reheating on the surface during the central hours of the day.

This drop follows almost perfectly the orographic evolution, downstream, of the valley relief. A perfect area for air boxing, which facilitates surface reheating during the central hours of the day. Thus, it is usual that, during hot episodes, it is precisely the medium and flat areas of medium elevations that are the first to heat up. Something that happens from noon in the bottoms of the valleys, when the boxing acts as a “natural oven” for a stable and stagnant air mass.

And it is precisely here where local peculiarities make points such as Montoro, especially sensitive locations to achieve record records. That the maximum temperature recorded in a Spanish automatic station is where Sierra Morena and the subbetic systems approach is no coincidence. Even though this is a valley area, still at a very low altitude, the proximity of the two mountain slopes means that the volume of surface air to be heated is less than, for example, that of Córdoba airport. The lower the volume, the higher the warming, which largely explains the almost constant daily differences between the records from one season to another.

Nicolás Bermejo in his study also refers to the game of valley and mountain breezes that takes place during days of atmospheric stability in the Guadalquivir depression, and that play a fundamental role in triggering or containing temperatures throughout the region. . These games are perfectly appreciated in the wind direction records of any meteorological station in the middle course. East wind that increases the overheating on the surface, and that after noon rolls rapidly to “south”, “south-east” and “west” to contain the rise in temperature, either due to the contribution of humidity from the mouth to the border of the course medium, either because of the increase in turbulence in the lower layers of the atmosphere. Thus, on days and points where the wind does not alter the stability conditions, surface overheating acquires greater importance.

Peninsular hot spots and the myth of 50 degrees

That the orographic and latitude peculiarities make the Guadalquivir valley the particular ‘pan’ of Spain, especially due to its regularity and constancy, does not mean that there are no other points of interest in hot peninsular episodes. Until now, the maximum temperature record in the Iberian Peninsula was located in Amareleja, in the Guadiana valley in Portuguese territory. On August 1, 2003, the Portuguese town also reached 47.4 ° C, a record shared with the Cordoba town of Montoro since last August.

And it is that almost all the valleys and hydrographic depressions of the southern half of the peninsula meet the conditions to reach extreme heat records in favorable episodes. The Portuguese Alentejo, with the depression formed by the Tagus at its Portuguese mouth, and the aforementioned Guadiana valley, both in Spanish and Portuguese territory, have shown the warm potential of these basins in the recent past. The 46.6 ° C that was recorded at the El Granado meteorological station, in the province of Huelva, or the 46 ° C in Badajoz reached in August 2018, give a good example of the ‘baking’ characteristics of the Guadiana.

Recently, during the heat wave from August 13 to 16, the Genil valley left a historic record of 46 degrees in the Granada capital, at an altitude of 700 meters. In Murcia, thanks to the local winds that affect the Vega del Segura, a new historical maximum of 46.1 ° C was set on August 15. Just nine kilometers away, at the meteorological observatory at the Alcantarilla base area, 46.8 ° C was reached for the first time for a historical series of 80 years.

Extreme records that give an idea of ​​the potential of the Iberian Peninsula to reach values ​​that touch or exceed the border of 48 ° C. A priori ‘affordable’ barrier in warm advections such as July 2017 or August 2021, where the particularities of the local breezes between Córdoba and Montoro prevented even higher records. A barrier that if overcome would still be far from the myth of the peninsular 50 degrees, and of which there is only written evidence in records of doubtful reliability.

A ceiling, that of 50 degrees, typical of arid desert and sub-desert regions, difficult to reach even in extremely warm countries. This is the case of Riyadh, the capital of Saudi Arabia, in the heart of the Arabian peninsula. The 48.2 ° C of August 7, 1998 constitutes its absolute maximum, only 9 tenths above the Spanish historical maximum. A good reference to know where we are, and where we could be.

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