Residents of the Atlantic island of Bermuda are waking up to relative calm this morning after what was a brutal night, compliments of Hurricane Gonzalo, the sixth named storm of the 2014 Atlantic Hurricane Season.
Wind gusted to 98 knots at Wade International Airport as the southern eyewall passed over late Friday night, but the storm is now weakening and accelerating northeastwards.
It is losing its tropical characteristics but will still bring strong winds to New Foundland on Sunday, and then Ireland and the UK on Monday night/Tuesday. By that stage, though, it will not be a hurricane, as it is impossible for this type of tropical storm to survive the trip over the cool seas and strong westerly jet stream of the North Atlantic.
Let’s take a look at what happened as the eye of Gonzalo passed right over Bermuda last night. It is rare to get such a direct hit and be able to record data at the same time, but that is what we have in this case.
I have plotted wind and pressure data from the airport METARs below (times in GMT, 3 hours ahead of Bermuda time). We can see easterly winds increasing steadily and pressure falling rapidly as the storm approaches from the southwest. Then things go almost dead calm as the eye passes over at about 00:30 GMT (21:30 local), pressure bottoming out at 953 hPa, before all hell breaks loose again as westerly winds from the southern eyewall pass through at 02:55 GMT (23:55 local).
Sustained windspeeds of 81 knots (93 mph, 150 kph) and gusts of 98 knots (113 mph, 182 kph) are not something that this island gets every day, but luckily, as of yet, no injuries have been reported. This comes just days after Tropical Storm Fay caused some damage to trees and powerlines, and 11 years after Hurricane Fabian caused $300 million worth of damage in 2003.
So what’s in store for Ireland? The remains of Gonzalo will interact with the westerly upper winds and race towards us during late Sunday and Monday. Latest model guidance has it as a depression of around 980 hPa over the Hebrides around midnight Monday, introducing a very tight pressure-gradient on its southern flank. Much of Ireland, but especially the northern half, will feel this in the form of very strong winds with violent gusts.
Remember, because the depression will be moving eastwards, its motion will add to the gradient winds to its south, meaning enhanced wind for Ireland. The low will continue eastwards, introducing a strong and gusty northwesterly gradient over Ireland Tuesday morning. Only later in the day will the winds ease as a transient ridge approaches from the west.
So why can’t we get hurricanes in Ireland? The answer is cold seas and strong upper winds.
Hurricanes start off as clusters of showers and thunderstorms at low latitudes, feeding off massive amounts of energy from the warm sea-surface temperatures (SSTs). As the warm and humid air rises, the water vapour condenses, releasing huge amounts of laten heat of vaporisation. This heat feeds the towering cumulonimbus clouds, which can reach heights of well over 50,000 ft (15 km). As long as the SST is above 26.5 °C then there is usually enough energy available to feed this gorging beast.
The whole cluster can start to rotate, with air now starting to descend in the centre. This descending air warms adiabatically, dissolving clouds and forming that famous clear eye.
This eye is warmer than the surrounding storm, which is why tropical storms are refered to as “warm-core” systems. This is one criterion for defining a system tropical in nature.
A second defining criterion is that the strongest winds are found in the eyewall surrounding the eye, not farther away from it, as in storms at our latitude.
A third defining criterion is that tropical systems are barotropic, meaning they do not feed off temperature gradients like our baroclinic storms do. Tropical storms have no fronts, whereas we all know the familiar warm, cold and occluded fronts of our storms.
So although we can get storms with hurricane-force winds, we do not get hurricanes as defined above. The long track over cold seas, the interaction with the polar front and the strong upper jet stream all transform tropical systems into extra-tropical baroclinic depressions long before they reach us here. Please, if anyone from the press is reading, remember this the next time you post your headlines!