Between 3 and 9 am GMT on Sun 29th Sep 2019, Hurricane Lorenzo achieved peak sustained wind speeds of 160 mph, making it a category 5 on the Saffir-Simpson scale.
Just how strong is that? Well, the scale I just mentioned describes the typical damage from category 5 winds as ‘Catastrophic: Most buildings destroyed. Vegetation destroyed. Major roads cut off. Homes flooded’.
We’re talking peak sustained winds on par with a severe tornado but spanning a much wider area (the widest tornado on record was few miles wide, while a hurricane eye is typically 10 to 100 miles wide).
Scarce Events, Strange Places
Category 5 hurricanes are rare beasts; between N. America & Africa there have been just 36 recorded since 1924, an average of one every 2.6 years. Over the North Atlantic, they’re even scarcer; only 10 since 1924 – one every 9.5 years!
Lorenzo was rarer still – in fact unprecedented. It’s category 5 peak was by far the furthest east on record in the North Atlantic (see graphic above).
This comes just 28 days after Hurricane Dorian became the furthest-north category 5 on record for the same ocean.
Times Are Changing…
To me, the unprecedented behaviour of these two hurricanes seems very fitting for 2019. This year has felt like a preview of what many guidance tools predict to be ‘normal’ some 40-80 years from now.
So, how was Hurricane Lorenzo able to achieve top-tier classification so far outside the historical stomping ground of category 5 hurricanes?
Warming Oceans Expand Horizons…
Sustaining a Cyclone
One of the best clues can be found by looking at a plot of sea surface temperature anomalies (SSTAs)…
The large area of +1 to +1.5°C SSTAs you can see in the above map equates to SSTs of 26-28°C. The typical minimum requirement to sustain a tropical cyclone (TC) is 26°C, with major hurricanes tending to need at least 27°C.
So normally, most of this area of ocean would be insufficiently warm to fuel a major hurricane (let alone a category 5!), but it wasn’t in late Sep 2019.
As the oceans warm as part of climate change, these areas of unusually supportive SSTs are being able to develop increasingly far from the regions where such warmth is normally expected.
Carousels & the Earth’s Spin
Now we come to a concept that’s a little less straightforward.
The Earth imparts spin onto weather systems, to a degree that increases from a minimum (of zero) at the equator to a maximum at the poles.
Think of it this way – if you ride a carousel in the way your supposed to (e.g. in a seat or on a model horse), you travel sideways and don’t become dizzy, but if you were to stand on top of it instead, you’d really notice yourself spinning around.
This change with distance from the Earth’s equator (i.e. latitude) is a big deal with tropical cyclones, for they’re all about spinning.
In fact, they rotate in the same direction as the Earth (anticlockwise in the northern hemisphere, clockwise in the southern hemisphere). So, the further a TC is from the equator, the more the Earth helps it to spin.
Now, this is only going to matter for a given TC if there is sufficiently warm ocean beneath it…
Here, we find a link back to climate change; areas of such ocean warmth are developing at increasingly high latitudes.
In these newly warmed areas further from the equator, TCs can exploit the Earth’s rotation to an unprecedented degree.
In the North Atlantic, 27°C+ SSTs typically reach furthest north in the far west, due to the Gulf Stream, which transports very warm water to the north-northeast from the Gulf of Mexico (hence ‘Dorian’ which impacted the Bahamas, holding the record for the most northerly category 5).
Major hurricanes there pose far more of an immediate threat to land, so what’s the big deal about ones developing unusually far north in the middle of the ocean?
Well, here we must look at the bigger picture. As the possible area of major hurricanes expands, so do those for hurricanes and tropical storms. This brings them closer to the European continent along with the Atlantic coastal stretch of north-western Africa (where a cool ocean current from the north has been keeping SSTs too low for TCs).
Tropical Tempests to Threaten Europe
With each passing decade, the risk is increasing that TCs will make landfall on the European continent. We’ve already seen one landfall in recent years; tropical depression Vince made landfall near Huelva, Spain on 11th Oct 2005. It brought so much rain that despite a record-breaking summer drought in the affected region beforehand, some minor flooding developed in the wake of the storm.
TC landfalls on mainland Europe should never become a regular occurrence, thanks to the frequent presence of strong upper-level winds which tear them apart. However, there are typically at least a few windows of opportunity each Aug-Nov for a TC approaching Europe to avoid those strong upper-level winds.
Currently, they tend to run out of oceanic support (i.e. heat) before they can make landfall, but if recent trends continue, that’s not going to be the case for much longer…
Of course, the North Atlantic isn’t the only ocean on Earth with a concerning warming trend. I’ll be watching out for any unusual TC behaviour, especially new records, in the coming weeks.
James Peacock MSc
Head Meteorologist at MetSwift