Oscillation ENSO is a cyclical climatic event which has 3 phases: El Nino, La Nina and a neutral phase. Despite a regional location in the Tropical Pacific of its determinants, the oscillation ENSO has an impact on a good part of the planet and can profoundly modify the cycles of drought and rainfalls in equatorial Africa, in America or in Asia. And this is only one consequence among many of its effects.
We will discuss in this post more specifically the impacts of the oscillation ENSO on the hurricane season in the Atlantic basin.
Cyclonic genesis
To develop and strengthen, a cyclone needs different favorable parameters (meteorological, oceanic and physical):
- Hot water on the surface (at least 27 °)
- Atmospheric instability (stormy tropical wave)
- Humid surrounding air (and not charged with dust)
- Un shear weak wind
- The Coriolis effect
Other parameters can have influence but are not specifically necessary for the genesis of a cyclonic system. Among the important parameters, we will avoid the Coriolis effect which is physical and due to the rotation of the earth, therefore permanent and not impacted by any regional or global climatic situation. We therefore have 4 parameters related to the regional climatic situation.
The effect of ENSO oscillation on the parameters inducing cyclones
Depending on the phase, the oscillation ENSO will have marked effects on 2 to 3 of these parameters:
- In cycle El Nino, the temperature of the tropical Atlantic will be a little lower and the shear more marked wind with an altitude wind from the westerly sector prevailing during summer and autumn, as for the rest of the year. The monsoon activity over tropical West Africa will also be a little weaker, which induces a less unstable atmosphere in the major development zone (MDR) in the tropical Atlantic. We can clearly see in the statistical study a fairly clear correlation between the episodes El Nino and the cyclonic seasons in the Atlantic not very active.
- In cycle La Nina, it is exactly the opposite. The ocean temperature is above the climatological averages, the shear the wind is weaker with a very weak or zero western sector altitude current and the African monsoon is more marked, which implies more tropical waves passing from the African continent to the Atlantic Ocean. Here too, we can detect a clear correlation between the episodes. La Nina and cyclonic seasons more active than average in the Atlantic.
- In neutral cycle, the conditions are less marked and can be quite variable depending on the situation of other global or regional oscillations (which also impact the effects of the phases El Nino et La Nina one way or the other but without inverting them) such as the North Atlantic Oscillation (NAO) or the Multidecadal Atlantic Oscillation (AMO) or the Madden-Julian Oscillation (MJO).
Beyond these direct effects, the phases ENSO also interact with other atmospheric or oceanic elements such as for example the equatorial Kelvin waves which move around the globe from west to east in the northern hemisphere and which also impact the temperature of the water. And there are also periods of overlap between 2 cycles ENSO which mean that we have not yet officially passed from one to the other but where certain effects are already being felt in the conditions of development of cyclones, which can make understanding not always easy.
The ENSO oscillation is still very mysterious
Even today, climate science has difficulty in pinpointing all the ins and outs of this oscillation which impacts the weather over a large part of the globe (and therefore largely cyclones in the Atlantic) over periods of several months. Our analytical and forecasting capacity is essentially empirical and therefore linked to the observation of oceanic and atmospheric conditions in the tropical Pacific. Forecasting models find it difficult to anticipate developments beyond a few months. We are still today unable to understand in depth why this or that other cycle is underway and why it lasts longer or shorter.
The impact of ENSO on the hurricane season is major and the evolution of our understanding of this climatic phenomenon will allow us to better anticipate the activity of the seasons and much earlier. In the meantime, we are subject to its changes, the forecasts of which often remain erratic even at 3 or 4 months, which can lead to large differences between the seasonal hurricane forecasts and the reality of the activity in the basin.
Information note : As always, these publications are intended for the understanding of these subjects by people completely neophytes. It is a question of popularizing as much as possible and in this context, certain approximations may exist to simplify as much as possible. I fully accept these approximations. For those who want to understand in more detail and in depth the operation of ENSO and its effects, there are many online publications by Météo France (here ou here among others) in French and by NOAA in English (here ou here among others).
Image credit: noaa
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