Where are the oceans warm enough for hurricanes and typhoons to thrive?
As the northern hemisphere summer continues to heat up the land and oceans, it’s a time of year when the tropics become most fruitful in terms of delivering hurricanes, typhoons and tropical cyclones.
But the occurrence of such storms doesn’t happen randomly. There are a certain number of oceanic and atmospheric factors that need to come together to make things happen.
Whilst the Atlantic basin has yet to have any hurricane activity, the eastern Pacific hurricane season has got off to a flying start. So far, it has delivered the most powerful hurricane, Amanda, on record for so early in the season.
Although Hurricane Amanda stayed over open water and didn’t bother land, it reached category-four status, with sustained winds of 155mph.
One significant factor that drove this major hurricane to become such a beast so early in the season is the presence of very warm ocean water, referred to as sea surface temperature (SSTs).
Sea surface temperatures – fuel for a tropical storm’s engine.
One of the most important factors for these tropical storms to form is a source of very warm, moist air with a sea surface temperature of around 27C or more.
The supply of heat and moisture effectively acts as fuel for any potential tropical storms. This is why they die when they move over land, as their energy supply is cut off.
It’s the evaporation of this very warm water that adds moisture and potential (latent heat) energy to the tropical atmosphere, which if combined with other favourable conditions, causes storms to develop.
Where is the water warm enough at the moment?
On 30 May, Nasa produced a map of sea surface temperatures for the whole world, using a variety of satellites that have on-board sensors to observe emissions of microwaves from the sea surface. Those measurements capture the temperature of the top few millimeters of the water.
The image below has been filtered so that sea surface temperatures that support the formation of tropical storms appears red and those that don’t blue.
As would be expected, the tropical waters in the vicinity of the equator are the areas that are warm enough for tropical storms to thrive in.
However, the breeding ground for storms in the Atlantic ocean is marginal, with the zone of warm enough waters probably a fraction further south than would be ideal for something notable to form at this point.
What else is needed for storm formation?
Winds at the surface need to converge together. This forces the warm, moisture-laden air upwards where it subsequently condenses and forms stormy cumulonimbus clouds.
Another important balancing act is having winds that don’t change direction too much with increasing height in the atmosphere. This allows a potential storm to develop further without getting torn apart.
Finally, there needs to be some spin from the rotation of the earth to get storms to circulate. This is why they almost always form at least five degrees either side of the equator.
El Nino’s potential influence
NOAA has predicted that this year’s Atlantic hurricane season will produce fewer storms than average, due an El Nino event that’s expected to develop in the coming months.
El Nino is a naturally occurring ocean-atmosphere phenomenon that is characterised by unusually warm surface water temperatures in the central and eastern Pacific.
It is important because it has a significant impact on the climate in may parts of the world, with a warming influence on global temperatures.
However, El Nino is likely to have the opposite effect on the eastern Pacific hurricane season, with anomalously warm sea surface temperatures providing optimal conditions for tropical storms to thrive.
I’ll be keeping a close eye how this year’s hurricane season evolves and posting regular updates on Twitter – @liamdutton