24 HEADLINE MISSION: WATER HURRICANE ZONE Q: And of course we also felt the impacts of Maria. What are the drivers of such extreme storms and of these extreme hurricane seasons? Phil: There is a recipe for a perfect storm or perfect season. You need to have a few fundamental factors in play. These were first laid out in a 1968 paper written by my predecessor, Dr. Gray. You need warm waters, because hurricanes are basically a heat engine, they live off of warm ocean water. So, if the waters are cool, or the hurricane goes over land, it weakens rather quickly. Typically, a hurricane needs waters of about 80°F (26.7°C) to form, and basically, the warmer the waters, the more fuel there is for the storm. So, this past year, the waters were running about one to two degrees [Fahrenheit] warmer than normal, which certainly may not sound like a lot. If water is 84 or 86 degrees [Fahrenheit] and you jumped in, you probably wouldn’t even feel the difference. However, when it comes to a hurricane, it does provide a lot more fuel for the storm. Now, you can have really hot water and still have a quiet hurricane season. You also need upper-level winds to be fairly weak near the storm. Strong upper-level westerly winds tend to tear apart hurricanes as they're trying to develop. Coupled with the low-level trade winds that blow out of the east, it's called vertical wind shear. So if you have a very strong change in wind direction in the storm, it tears apart the hurricane. When [the National Hurricane Center] talks about individual storms, they always reference vertical wind shear because it's such a critical factor. You also need lots of moisture in the atmosphere. Dry conditions squelch the development of the thunderstorms, which are the building blocks of a hurricane. And, even with all of these conditions met perfectly, there still must be a disturbance. There are about 60 to 70 deep thunderstorm complexes, known as easterly waves, that move off of West Africa every year. And basically, when those disturbances move into an environment that's conducive, then they can intensify. Storms like Irma, Maria, and Harvey all formed from these waves moving off of Africa. And so, in a year when conditions are conducive, you can see, say, six, eight, even ten of those thunderstorm complexes become hurricanes. Whereas in other years, you may only get one or two of those waves becoming hurricanes in the Atlantic. There's approximately the same number of easterly waves moving off Africa every year. The water temperature, wind, and moisture levels in the Atlantic determine whether you have an active season or not. So, in the case of 2017, it was a perfect storm. You had very warm waters. If you look in the tropical Atlantic, it was the third warmest on record. Then we had low levels of vertical wind shear, so the winds were fairly weak. There was also a lot of moisture at mid-levels in the atmosphere. We had the perfect recipe for an active season. And, if you look back at active seasons in the past like 2004 or 2005, in general, the large scale conditions were fairly similar. There is a recipe for a perfect storm...You need to have a few fundamental factors in play. In this colorized infrared image, taken on September 20th , 2017 at 6:15 UTC, the well-defined eye of Hurricane Maria can be seen as it skirts the island of St Croix. (Image Credit: NOAA/NASA Suomi NPP satellite) Puerto Rico St. Croix Virgin Islands