2023-07-22 20:30:00
The month of July 2023 presented a rare weather scenario, with two occurrences of continental extratropical cyclones in the Southern Region of Brazil. The maturation condition of a cyclone of this type over the continent by itself is not so rare at this time of the winter season, but two consecutive occurrences, almost analogous, are worth noting.
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It is noteworthy that only one of them, the occurrence verified around July 12, was totally independent. The occurrence centered on the 8th of the month was dependent, that is, it arose from another giant extratropical oceanic cyclone that began to appear four days earlier over the extreme south of Argentina.
This month’s second extratropical cyclone was so large and intense that it plagued the South Atlantic area with other dependent extratropical cyclones of all sizes and stages of development. Typical situation around Antarctica over the last 20 years.
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It is important to emphasize that the geographic settings also need to be remembered, since the maturation of continental extratropical cyclones in the northern hemisphere is more common, especially over Canada and Russia, precisely because of the large continental extension at high latitudes. If, on the one hand, the Northern Hemisphere is much more continental, in the Southern Hemisphere, we have the opposite, especially South America, which presents a different profile, where the emerged lands taper as we advance in latitudes, towards the south.
Cyclones are meteorological phenomena described as large centers of low atmospheric pressure on the surface that fit the synoptic scale (something that has a size between 300 and approximately 2000 km in length). Some can be framed in the meso-scale, reaching slightly smaller proportions. They are then classified in order to be divided into two large groups, where the type of process for their formation becomes the main factor in this division, according to the classical schools. We still have a new category that would make room for sub-tropicals. It is still in the evaluation stage, as it needs a more elaborate theoretical framework, which did not prevent it from being put into practice.
The classic division of cyclones
Photo: Reproduction/NOA/NASA
Following the classic division, we have the tropical cyclones, whose formation in essence occurs by the convective process originating from gigantic clouds of the type cumulonimbus (Cb) that act in cohesive groups, forming the great center of low pressure on the surface of the cyclone, distributed around it in the form of multiple spiral nebulous arms, very similar to a galaxy. In general, they have varying sizes between 300 and 500 km in diameter, with some peripheral disturbances that can still extend for more than 100 km, especially in their front sector, in the direction of their displacement.
The second group deals with extratropical cyclones. Its formation is essentially described by two basic schools of meteorology and geographic climatology: the Norwegian and the French, respectively. Although they have some points in common, they are quite different in terms of the maturation process of an extratropical cyclone. It will not fit here to have a wide discussion, so let’s stick with the point that interests us, which is the formation of the center of low atmospheric pressure of the extratropical cyclone on the surface, since this, in addition to being much larger than the tropics, arises from the difference in the spatial distribution of air temperature by the medium latitudes (from 30 to 60º N or S).
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This difference becomes marked by the advance of a cold air mass from regions of high latitudes, such as Antarctica. Due to any disturbance due to the high potential generated between the temperature values, especially if it is on the hottest continent, as was one of the recent cases, then there is the beginning of the process that will alter the atmospheric pressure fields.
Because it is stable, cold air tends to stay on the surface as it moves to lower latitudes as a huge lens. In this way, it removes the warmer air that was in front of it, forcing it to rise. If there is a lot of moisture in this forcibly raised air, it will tend to form clouds, including cumulonimbusdistributed in a broad line called cold front (Norwegian school) or depression channel (French school).
The advance of the cold air mass over the warm one has great extension. Thus, the inertial factors of the planet are applied due to its rotation. In the Southern Hemisphere, this causes a clockwise circulation between the cold air and the hot air, generating a low pressure center that will be intensified, in particular, because the hot air is removed very quickly from the surface, the closer it is to the original disturbance that caused the cyclone generation.
More about extratropical cyclone
Photo: Playback/Twitter
In general, in our quadrant involving South America, the South Atlantic and the Antarctic peninsula region, the maturation of extratropical cyclones at lower latitudes occurs between 100 and 300 km away from the east coast of South America, if not much further. However, when the initial disturbance of the extratropical cyclone occurs over the continent, it may mature well at the coastal continental interface, precisely where we can have more abrupt variations between air temperatures over the continent and the ocean.
In recent cases, we can still add an aggravating factor to the presence of the extension of the South Atlantic anticyclone, which, during these winter months, occupies Brazilian geographic areas, taking over a large part of the Southeast and Midwest states. Its presence intensifies the amplitude of continental temperatures, enhancing the process.
What is an anticyclone
Photo: Playback/Canva
The anticyclone is the reverse of the cyclone. It has high atmospheric pressure on the surface. As it inhibits the formation of clouds, it allows the surface where it is positioned to receive a high load of solar radiation, even in winter, warming the atmospheric air in the troposphere (the first layer of the atmosphere from bottom to top). Its circulation in the southern hemisphere is counterclockwise. It is precisely for this reason that winter rains now occur in several states in the Northeast region, as a good part of its center is located in the Southeast region, the periphery of the anticyclone forces air circulation to come from the Atlantic over the continent. The air well laden with moisture will favor the occurrence of heavy rains mainly in Bahia, Sergipe and Alagoas.
The severity of the extratropical cyclone system is linked to the greater temperature difference
The duration of the anticyclone, its intensity (or resistance) and the temperature of the tropical South Atlantic waters will define both the intensity of the rains, as well as its persistence in the Northeastern states under its influence. These are the factors that control this recurring weather pattern every year and not “climate change” or “global warming”. For this reason, each year we see these conditions being different, within the classic climatic variability and not alarmism.
Diametrically opposite, on the extreme south side of this anticyclone, whose circulation encourages rainfall in the states of the Northeast, we will have the clash with the cold air that has been advancing, as described above. It will be precisely here that we should focus, since the severity of the extratropical cyclone system is linked to the greater temperature difference, in its initial stage, causing a greater difference in atmospheric pressure at the surface. If this pressure difference is large, the winds blowing towards the low surface pressure will be stronger, trying to compensate for this imbalance.
For spatial reasons, the center of the low, although large, within the synoptic scale, is still smaller when compared to the system as a whole, when we observe the extensive cloud arm of the associated cold front. Thus, the winds will be very intense near the center of the cyclone and tend to decrease in speed as we move away from this center.
In the cases that occurred in July 2023, the two systems matured in this center still within the continent or at the coastal limit, bringing the unpleasantness of strong winds to the populated areas of the Southern States, in particular Rio Grande do Sul and Santa Catarina, with slightly smaller performances on Paraná and even São Paulo, especially in coastal regions (the ferry that makes the Guarujá–Bertioga connection had to be out of operation for many hours due to the large waves generated by the winds over the sea).
The “Big S”
In a didactic way, this very recurrent combination during the winter is described as “The Big S”, where the circulation of winds on the surface contours the anticyclone that is very pronounced over a good part of Brazil (centered approximately in the Southeast Region). This anticyclonic system would be the upper part of the “S”, while the extratropical cyclone (continental or maritime, close to the Brazilian coast) would be the lower part of the “S”, as this letter would represent the circulation of winds that comes from the combined anticyclone in the interior of Brazil, passing from Rondônia, Mato Grosso and Mato Grosso do Sul, following through the Southern States and ending up inside the low pressure center of the extratropical cyclone.
Pictorial scheme of the anticyclonic circulation (blue H – high pressure) and the connection with an extratropical cyclone over the Brazil area (red L – low pressure center); the figure of the ‘Big S’ (semi-transparent blue line) indicates the peripheral circulation that connects the two systems; once inside the cyclone, the air tends to rise, causing occlusion (central spiral) where the hot air that was removed from the surface reaches higher levels to participate in other flow processes in the atmosphere | Photo: Playback/Inmet
There are many geographical combinations of the position of the “S” that clearly vary according to the years. As this circulation is loaded with moisture since it starts in the ocean, it will also present varying intensities annually, bearing in mind that it can be influenced by issues of The boy or La Niña, hot or cold Atlantic, air temperature differences over the continent, etc. However, what does not change are two things: our low resilience to the occurrences of this phenomenon when the cyclone matures very close to the continent and the nonsense trumpeted by the brown media that this is an effect of “climate changes”.
Be sure that during the winter in the Southern Hemisphere, the “Trail of the Depressions”, as I called it in my master’s thesis, can extend to lower latitudes. Thus, with the record values of low temperatures verified in recent years in Antarctica, as observed in 2021, we may have several cold polar masses, generating the necessary potential for the maturation of intense extratropical cyclones. Luckily for us, the rule is that the maturation of these cyclones occurs much more often over the oceans.
Also read: “The year is not over in 2023”
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