As I type this from a sunny southern England, a strong tropical-like storm is hammering western Greece with strong winds and very heavy rainfall.
The Head of Civil Protection in Greece, Nikos Hardalias, issued a warning of flooding, flying debris and possible outages to power, water and telecommunications.
Severe impacts are being observed. There are reports of the entire Jerusalem beach eroding away.
With no unified system in place, this storm has a confusing array of names. In Greece they call it ‘Ianos’, while the Italians call it ‘Cassilda’ and the Germains ‘Udine’!
Increasingly in recent years, media has been getting around this by referring to such a storm as a ‘medicane’. This is a merger of ‘Mediterranean’ and ‘Hurricane’, which is perhaps a little misleading, as only a few of these storms reach equivalent intensity to a hurricane. On the other hand, ‘Medical Storm’ wouldn’t have worked well, for obvious reasons!
In this blog entry, I take a brief look at what a ‘medicane’ is, and then explore the climatology and trend over time, including a look ahead to the coming decades.
Snapshots taken from https://en.sat24.com/en/gr/visual, showing the current ‘medicane’ at 10:40 UTC 17th Sep 2020 (left) and 10:00 UTC 18th Sep 2020 (right). Note the presence of an ‘eye’ feature, clearest while it was over open water on 17th.
Structure of a ‘Medicane’
Whatever you call them, these are powerful systems with tropical-like characteristics. Most notably, a ‘warm core’, which is a central column of rising air that’s warmer than the surroundings. This sets them apart from the much more common non-tropical cyclones (see comparison below).
The warm core rotates rapidly and can easily produce sustained winds of tropical storm force (over 40 mph). It also supports towering thunderstorms (deep convection). The more vigorous and sustained this is, the stronger the rising motion. Also, the faster the warm core spins, the more this rising motion and deep convection becomes focused toward the edges.
With enough uplift and rotation, medicanes can develop an ‘eye’ feature, with a partial or even full ‘eyewall’ around it. The rising motion is focused so much on the warm core edge that air can descend into the centre from above the storm. Sinking air suppresses cloud formation, potentially leading to a clearing of the eye, such as you can see in the snapshot from yesterday, above.
Climatology and Trend
As you can see below-left, there’s a clear pattern to the frequency of these storms across the year. The frequency peaks Sep-Jan in what may be considered the ‘medicane season’. This is largely down to the warmth of the Mediterranean Sea, which peaks Sep-Oct and hits a minimum Mar-Apr.
You see, warm core storms require a continual supply of warm, moisture-laden air from the surface. Land areas are usually too dry to provide that, so it’s down to the sea to be provider. The warmer the sea, the more likely there will be enough moisture and rising motion to support a warm core storm.
Derived from ‘List of storms, by decade’ and ‘List of storms, by month’ on this Wikipedia page. Base period for the monthly breakdown is 1947-2011.
Now we come to the biggest talking point of this blog piece: The trend over time, past and future.
The observation data presented above-right implies that the 21st Century has so far been more active than all preceding decades. The truth of this, though, is uncertain.
Prior to the late 1980s, satellite observations of the Mediterranean were scarce. This is a big deal, because sustained deep convection and overall storm structure are difficult to deduce from ground-level. It’s very likely that at least a few storms were missed each decade up to the 1980s. Perhaps even ten or more!
However, even if we account for that, it seems unlikely that the true numbers were more than observed in the 1990s. So, fewer storms than in the 2000s or 2010s.
Still – the 2010s saw seven less than the 2000s, so we don’t have a steady upward trend to cause us alarm. This has been despite a net warming of the Mediterranean Sea during this time. This is down to other important factors, the implications of which are explored now:
Climate Change Versus Complex Constraints
Broadly, climate models predict continued warming of the Mediterranean during the coming decades. That means an increasing number of occasions where sufficient warmth and moisture is available for a tropical-like cyclone to potentially develop.
The outlook is complicated by the fact that the Mediterranean Sea is a skinny, shallow being that resides well north of the tropics. It has an average depth of 1,500 m, which may sound like a lot, but is well short of the Atlantic Ocean’s 3,646 m, for example. Comparing with tropical seas, it fares better (Caribbean 2,200 m, Gulf of Mexico 1,615 m), but those receive stronger sunlight across the year, meaning they have much higher heat content per unit of water.
What all this means is that a warm-core cyclone over the Mediterranean Sea will quickly expend the available heat energy of the water beneath it. Merely being sluggish can be the end of a medicane.
Then there’s all that land poking in. The Mediterranean Sea is essentially a gigantic lake, a.k.a. ‘inland sea’. It’s akin to the Black Sea, but on a larger scale. As mentioned earlier, overland air tends to be too dry to sustain a warm-core cyclone. In fact, when it comes to medicanes, it tends to cause weakening or collapse. This is often rapid if the core moves fully overland.
As if that wasn’t enough, climate change is predicted to increase the overall stability of the atmosphere over the Mediterranean. This means a shift in favour of sinking, rather than rising, motion. That’s not friendly to medicanes.
The Outlook: Mostly Like Before – but Higher Peaks
Generally, climate projections show these competing factors leading to little overall change in medicane frequency or intensity during the coming decades.
Which means, we can expect to see 15-25 per decade, mainly peaking at the equivalent of mid to high-end tropical storm intensity. That equates to peak sustained winds of 40-60 mph, peak gusts 60-80 mph.
But there will always be vigorous exceptions – and some climate projections suggest these will be able to overcome increased atmospheric stability and utilise further-warmed waters to reach higher peak intensity than we’ve seen in the past. We’re talking peak sustained winds of hurricane force (over 74 mph) with gusts to over 100 mph. That, on top of copious rainfall – even a ‘typical’ medicane can easily deliver more than 100 mm where the warm core travels.
To put it into perspective, a typical non-tropical windstorm in the North Atlantic has peak sustained winds of 30-40 mph, peak gusts 50-60 mph, and puts down 20-60 mm of rain.
Bottom line is, medicanes can cause severe damage to infrastructure and disruption to services, which means that regardless of trend over time, they should always be watched out for.
James Peacock MSc
Head Meteorologist at MetSwift
Cover Photo is a MODIS image captured by NASA’s Terra satellite (- EOSDIS Worldview), which is licensed under CC BY-SA. It shows tropical-like cyclone ‘Numa’ with a distinct eye-like feature as it traversed the Ionian Sea on 18/11/2017.