Hurricanes, cyclones, typhoons and other tropical storms, if we look at their paths one by one, we could consider that they move randomly, or with poorly defined patterns. However as soon as we take a few the trends start to emerge. Let’s take a slightly larger sample and we can observe a curious trend, a total absence of hurricanes in a strip of thousands of kilometers along three oceans: the equator.
But what’s more, hurricanes not only seem to resist crossing the equator, but (with very rare exceptions) also don’t form in the latitude range between five degrees north and south of equatorial zero.
For a tropical storm to form, a series of circumstances related to atmospheric pressure, sea and air temperature and water depth, atmospheric humidity must occur… And also the Coriolis effectwhich dominates many of the facets of these storms.
What is the Coriolis effect? It is what is called a fictitious or apparent force, similar to centrifugal force, which we can perceive as such but cannot be considered a force in the sense in which physicists use the term. Both are caused by rotation, which implies a non-inertial frame of reference since all rotational motion implies a game of acceleration and deceleration.
The Coriolis effect is caused by the fact that the different latitudes of the Earth rotate at different speeds: if we are At the equator our speed will be almost 1,670 kilometers per hour without having to move, while if we are at one of the poles our speed will be zero. It is precisely at the poles where this apparent force takes on the greatest magnitude, while at the equator it is zero.
The implication of this for hurricanes is that this weak force at the equator is not capable of giving the wind enough rotational force to rotate it along the perimeter of a low pressure area. This is one of the basic conditions for the formation of hurricanes and it is practically impossible for it to occur near the equator.
Map with the trajectories of tropical storms observed over several decades. National Oceanic and Atmospheric Administration (NOAA).
Practically impossible is not impossible, and in this there are exceptions. In December 2001, Typhoon Vamei became the most notorious exception to this rule. Although it was not the only tropical cyclone to form in the strip near the equator, it is the closest cyclone to form, some 150 kilometers from the line, at a latitude of 1.4º N.
This exceptionality attracted the attention of meteorologists, who attributed this phenomenon to a combination of topographic and meteorological factors: an explosion of air from Asia channeled through the strait between the Malay Peninsula and the island of Borneo was the one that provided the necessary conditions to a zone of low pressures to form the hurricane.
“In the case of Vamei, everything happened in the right magnitude and place and lasted long enough. We calculate that the chances of this happening again are once every 100 to 400 years,” explained CP Chang, one of the experts who studied the phenomenon.
Neither form nor cross
That hurricanes don’t form near the equator helps a lot so they don’t cross it, but that’s not the only reason why they don’t. Although the culprit for this is, again, the Coriolis effect, or rather its variation across latitudes, also called the Beta effect.
This effect causes tropical storms to move like the prevailing winds at those latitudes: to the west and to the poles (ie northwest in the northern hemisphere and southwest on the other side of the equator).
Again here we must consider that crossing the equator is technically possible for a hurricane. It’s just so weird because of these two factors that we’ve just never observed it. It is sometimes considered that since tropical storms rotate in opposite directions in different hemispheres (due to nothing more and nothing less than the Coriolis effect), this could be behind their tendency not to cross.
What the experts consider, however, is that the “inertia” of the winds of a storm of sufficient magnitude would be enough to offset the opposite trend. “A well-developed thunderstorm has enough spin to overpower the weak Coriolis force in that environment,” explains University of Hawaii meteorologist Gary Barnes.
There are still many mysteries to solve when it comes to tropical storms and cyclones. One no less curious is that of the practice absence of this type of atmospheric phenomena in the South Atlantic and Southeast Pacific. Perhaps more important will be knowing how climate change will affect them, even more so since the variability in these phenomena makes their long-term prediction difficult.
In Xataka | Solving one of the great mysteries of meteorology: why there are more storms in the southern hemisphere
Cover image | NASA, Nilfanion
