What is ‘risagga’? A ‘rare’ phenomenon in the Balearic Islands (and other areas) that is not so rare

Additionally it is generally known as ‘meteotsunami’, as a result of they’re harmful waves just like tsunamis, however not seismic, however meteorological in origin. The ‘rissagas’, typical of the Balearic Islands (though in addition they happen in others) are categorized as “uncommon maritime phenomena” within the AEMET Atmospheric Observations Notification System, though they’re extra frequent than one would possibly suppose. First, let’s begin with an evidence of the phenomenon itself: ‘risaggas’ (or ‘hangovers’) are extraordinary fluctuations in sea stage that may attain two meters in intervals of 10 minutes: these are gravitational waves that generate variations in atmospheric strain on the floor and trigger variations in sea stage.

At daybreak on Wednesday this phenomenon was skilled in Santa Pola (Alicante), the place the water exceeded the promenades, and in addition within the space of ​​Ciutadella (Menorca), the place they’ve been recorded ‘risaggas’ as much as 75 centimeters, as have additionally taken place in different elements of the Balearic Islands and Catalonia. The one in Santa Pola has been, as famous by the AEMET, considerably extra intense than ordinary, taking into consideration that the course of the atmospheric disturbance was from south to north, when the port of Santa Pola is open to the south, perpendicular to the course of journey of the wave.

How and why are they produced?

These marine ‘hangovers’, up to now, have been believed to be associated to astronomy, one thing just like the bizarre tide though amplified by some unknown trigger. Nonetheless, in 1979 a mathematical mannequin was developed to check this phenomenon, the novelty of which was the potential affect of gravitational waves generated in medium ranges of the troposphere on account of wind shear as a reason for atmospheric strain oscillations on the floor. . In line with this similar examine, there are three atmospheric circumstances that favor the looks of ‘rissagas’, relying on the extent of the troposphere:

  • In center and higher ranges of the troposphere, robust winds, nearly at all times from the southwest, with important velocity variations between some areas and others (shear).
  • At low ranges there should be a very heat air massDue to this fact, there’s a robust temperature inversion between this stage and the air above the ocean floor, which is colder.
  • On the floor stage there should be a weak or average stream of elements is.

Though it has not too long ago been confirmed that ‘rissagas’ might be noticed with out the latter situation occurring. What’s a ‘sine qua non’ situation is the time of yr: as Jorge González Márquez, from the Territorial Meteorological Middle of the Balearic Islands factors out, these phenomena solely happen between April and October, virtually solely in the course of the heat semester of the yr. And the way do they manifest themselves? First, it has a sudden drop within the water stage within the port, which turns into empty in a matter of minutes in shallower areas. This causes most of the boats to hit backside, many fish die of suffocation and different areas that aren’t empty pressure the boats to be stranded.

After this, the water additionally abruptly returns to the port and causes the boats to be dragged – with what this entails: they hit one another or towards the docks, sink … – and the avalanche of water causes flooding in areas close to the port, affecting parked automobiles and buildings. This phenomenon might be repeated cyclically for hours and even on successive days after intervals of calm.

How typically is there a ‘rissaga’?

It relies on the peak of the ocean stage, one can speak or not in regards to the frequency of the ‘rissagas’: if fluctuations within the stage of lower than 50 centimeters are thought-about, it’s troublesome to discriminate if the trigger is the ‘rissaga’ or every other, however the whole lot what exceeds that top is at all times thought-about to be. Within the port of Ciutadella (Menorca) they happen about 10 occasions a yr, though catastrophic swings are recorded each a number of years. In line with González Márquez, within the final three many years essentially the most critical case was one which befell within the early morning of July 20-21, 1984, which affected the city in the midst of the San Juan festivities. Though the ‘rissagas’ have been most studied within the port of Ciutadella, it isn’t unique to this space.

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