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Storms

The 2018 hurricane season: storms are suddenly rolling in thick and fast

Until the end of August it looked as though this year’s hurricane season would be a quiet one. But then it was as if someone had turned the tap on, with storm activity in the tropical North Atlantic suddenly picking up at the beginning of September. Hurricane Florence is the first severe hurricane of the season to reach the US coastline, making landfall close to the border between North and South Carolina. Florence had weakened to a category 1 hurricane prior to reaching land, though still brought with it winds of 150 km/h – and even stronger gusts. In particular, the storm carried in widespread torrential rain. Before making landfall, Florence had been a category 4 hurricane at times, with wind speeds way over 200 km/h. What has caused this sudden change? Eberhard Faust, Leading Expert for climate risks and natural hazards at Munich Re, provides answers.

What has the hurricane season been like so far?

Eberhard Faust: Until the end of August, the tropical storm season in the North Atlantic was rather mild. Until then, there had been five tropical cyclones, just two of which reached hurricane force – that is, wind speeds of at least 119 km/h (74 mph). The only one to make landfall was tropical storm Alberto. The aggregate index of kinetic energy generated by the storm winds, ACE (accumulated cyclone energy), had reached just 57% of the 1981–2010 average by that point. But the first few days of September saw a flurry of storm activity. Within just a few days, four other storms formed, one of which was Florence. By 10 September, the ACE index had leapt up to 110% of the long-term average until this date.

The making of a hurricane

What factors facilitated this sudden emergence of hurricanes?

Over the previous months, the surface temperature of the tropical North Atlantic had been cooler than usual. The air in the atmosphere 5–6 kilometres above the sea was very dry, which had also prevented the formation of cyclones. In the past few weeks, the climatic conditions changed, with sea temperatures creeping up to above-average levels in southern parts in late August/early September, while the negative anomalies in the northern tropical areas disappeared. It was at this time that Hurricane Florence formed. In the meantime, temperatures in the eastern tropical part of the North Atlantic are back to slightly below average. The fact that the shear winds – winds travelling in different directions and at different speeds aloft and at sea level – also diminished in part of the main development region for tropical cyclones in the first week of September undoubtedly also played a role. A low level of vertical wind shear allows tropical cyclones to form more easily. Air pressure over the tropical Atlantic also fell, leading to more instability of the atmosphere. That, too, is conducive to tropical cyclones. Furthermore, the El Niño phase expected to begin in the equatorial Pacific – which would have had a dampening effect on hurricane activity in the Atlantic – has not yet begun. So a number of factors have played a role.

Should we expect further severe storms before the season ends in late November?

That’s hard to say. A lot depends on whether there really will be an El Niño phase in the Pacific Ocean and, if there is one, when it kicks in. It had originally been expected in August or September. Most recently, a number of significant forecasting models have pointed to October – after September, when most tropical cyclones tend to form. In practice, it is incredibly difficult to predict when an El Niño phase – a multi-year natural climate oscillation in the Pacific – will begin. It is entirely plausible that the 2018 hurricane season will end up somewhere near the long-term average (11.8 tropical storms, of which 6.4 hurricanes) – and, if El Niño does indeed experience a very late onset, we could even witness an above-average number of hurricanes. Incidentally, the northwest Pacific Ocean is experiencing an exceptionally high number of storms, with the ACE index at 10 September 21% above average for that time of year. This is caused in part by above-average water temperatures in the southern parts of the northwest Pacific. At almost exactly the same time as Florence, Super Typhoon Mangkhut – a category 5 storm, the highest possible, with wind speeds of over 250 km/h – approaced the north of the largest island in the Philippines, Luzon.

It is important to say that every severe hurricane making landfall is enough to bring about extreme losses. Whatever the forecasts say, it is essential to continue to take measures to prepare for storms and mitigate their impact, such as by improving the stability of buildings.

Further Information
Munich Re Experts
Dr. Eberhard Faust
Head of Research: Climate Risks and Natural Hazards
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