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The secret of strong wind shear: Why is it so difficult for tropical cyclones to form in the South Atlantic?
The formation of tropical cyclones in the South Atlantic has always been an important topic in meteorological research. Tropical cyclones in the South Atlantic are extremely rare compared to the North Atlantic, which has aroused widespread concern among scientists and weather enthusiasts. Although cyclones can develop throughout the year, and are usually most active between November and May each year, cases reaching severe tropical cyclone intensity are rare.
One of the main reasons for this phenomenon is the influence of strong wind shear. When wind blows through different thermospheres, there will be relative speed differences. Such conditions cause the structure of the cyclone to be destroyed, thereby greatly reducing the ability to form strong tropical cyclones. Many experts believe that the vertical wind shear in the mid-level South Atlantic is quite strong, which limits the true power of the local storm.
Although there were reports of a tropical cyclone developing in the southeastern Atlantic in 1991, the situation could only exist as a tropical depression at best.
In 2004, Hurricane Katarina shattered the myth that there were no hurricanes in the South Atlantic, becoming the only case recognized as a true tropical hurricane.
Scientists also pointed out that changes in the equatorial low pressure belt are also key factors in the formation of tropical cyclones. The equatorial low-pressure belt is about one to two degrees south in the Southern Hemisphere but not close enough to the equator. The Coriolis force generated in this way is not sufficient to support the formation of tropical cyclones.
The water in the South Atlantic is also cooler than in the North Atlantic, and when the water temperature does not reach the high temperatures required to form tropical cyclones, the possibility of harvesting energy also decreases. Although the number of tropical and subtropical systems observed increases each year, these systems are generally at lower intensity levels and often transform into other types of systems soon after forming.
For example, a series of subtropical storms occurred in 2009, 2010, and 2011, with Tropical Storm Anita in 2010 becoming one of the few systems to be monitored and named.
According to a brief report by the Brazilian Naval Hydrographic Center, a total of 63 subtropical cyclones were discovered in the South Atlantic between 1957 and 2007, but this is still far lower than records in other tropical seas.
Storms and impacts in recent years
With the advancement of satellite technology and the continuous improvement of weather monitoring systems, tropical and subtropical cyclones in the South Atlantic are becoming increasingly trackable. Anita in March 2010 was a significant event, enough to draw the entire meteorological community's attention to climate change in the South Atlantic. After the same year, the Naval Hydrographic Center began to systematically name these storms.
However, due to the strong wind shear force in the South Atlantic and poor climatic conditions, its development process is often hindered. Subtropical Storm Alani in 2011 showed the damage to its structure caused by strong wind shear. Although it initially experienced heavy rainfall, it quickly turned into an extratropical cyclone, causing difficulties in impact management.
In reality, despite evolving and mature monitoring systems, the predictability of tropical cyclone warnings in the South Atlantic remains very limited.
In 2016, new storm events emerged as changing weather patterns brought attention to tropical systems in the South Atlantic. Including Storm Saisi in 2016, it has been shown that the South Atlantic may become an area prone to tropical cyclones in the future, thus attracting more research and discussion.
So, whether the key to unlocking the mystery of strong wind shear in the South Atlantic can be found will become the goal of meteorologists and tropical cyclone enthusiasts in the next few years.
