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What's Up With All These Hurricanes?

The Atlantic hurricane season has been off to an explosive start and there are currently three tropical systems in the Atlantic Basin, Irma in the Caribbean, Jose in the Atlantic and Katia in the Gulf of Mexico. It's not the first time three tropical cyclones have formed at the same time across the Atlantic Basin. A similar pattern was in place in September 2010 when Igor, Julia, and Karl were nearly identical spots.

With all of these storms developing less than two weeks after Hurricane Harvey devastated Texas, many folks are asking "What's up with all these hurricanes?"

The Ingredients

The month of September is the peak of the Atlantic Hurricane Season. So, to have tropical cyclones develop in September is nothing unusual. There are three primary ingredients that are needed for tropical systems low wind shear, warm water temperatures and high atmospheric humidity. 

Low Wind Shear - In order for tropical systems to thrive, they need to be in an environment with weak wind shear. If a tropical low starts to organize in an environment where the winds are blowing one direction at the surface and a different direction aloft, the hurricane will tilt and be sheared apart preventing storms from becoming organized around the center of the tropical low. 

Warm Water Temperatures - Hurricanes thrive when they move over warm water that extends deep underwater. Water temperatures typically need to be above 85° and currently sit between 86° and 89° in the Caribbean, which is more than enough to generate strong hurricanes.

High Atmospheric Humidity - The more moisture in the air the better. Dry air impeding a tropical storm will lead to evaporation within the storm. As air evaporates, the air cools and will begin to sink towards the surface. Since tropical systems thrive off of rising warm air....sinking cool air can kill off or severely weaken the storm. 

All three of these ingredients are in place across the Atlantic Basin making it the perfect set up for storm development. 

Why the Sudden Uptick in Tropical Development in 2017?

The 2017 Atlantic hurricane season has been off to an explosive start. There have been 11 named storms so far which is about a month and a half ahead of schedule. However, since 2005 the number of landfalling hurricanes has decreased significantly. This was due in large part to a low-pressure trough that was located along the east coast. This trough would help steer storms back out to sea and away from the east coast of the U.S. So, even though the tropics have been producing near average tropical activity since 2005, many of these storms weren't making it to the United States, therefore, they were not in the news. 

The past few years haven't been quite as active, in large part due to El Niño. During El Niño, sea surface temperatures in the eastern Pacific are above average. This pushes stronger winds across the Atlantic Basin, creating higher wind shear, and limiting the overall development of tropical storm systems near the United States. This year, the El Niño Southern Oscillation has switched to more of a neutral phase which means lower wind shear over the Gulf of Mexico, Caribbean Sea, and the Atlantic Ocean creating better condiitons for tropical development. 

What is Steering These Storms in to the United States?

Tropical cyclones are driven by the locations of neighboring high and low-pressure systems. Hurricane Harvey was pushed in the Texas and then stalled as it ran in to a blocking area of high pressure to the west and a trough of low pressure to the north. With no Jet Stream winds moving in from the west to carry Harvey out of Texas, the storm remained in place. 

For Hurricane Irma, the driving force in Irma's track is an area of high pressure known as the Bermuda High. When the Bermuda high is weak, storms that develop west of Africa will be carried back out to sea. In this case, the Bermuda High remains very strong allowing high pressure to build all the way to the east coast steering Irma west towards the U.S. 

Could Climate Change Be Playing a Role?

You can't blame a single storm or a series of storms on climate change. A lot of folks were quick to blame global warming for the catastrophic flooding from Hurricane Harvey but, Professor Cliff Mass from the University of Washington wrote a great blog, 'Global Warming and Hurricane Harvey', countering that argument.  The effects that climate change will have on the frequency and strength of tropical cyclones will be observed over a long period of time. We simply don't have a large enough sample size to know.  

The modern age of weather observations began in the mid-70s with the launch of Satellites. Prior to the age of satellites, tropical storms and hurricanes had to be observed by landfalls and aircraft. Naturally, storms that never made landfall or were never flown threw were not recorded. The National Oceanic and Atmospheric Administration (NOAA) has adjusted the numbers of storms in an attempt to account for the lack of aircraft and satellite observations in the early years of storm observation. Looking at Chart 1, it's hard to identify any clear trend in hurricane activity since the late 1800's. If any thing, there's a slight downward trend. 

Another useful index to monitor tropical cyclone activity is called the Accumulated Cyclone Energy (ACE) Index (Chart 2) which accounts for cyclone strength, duration, and frequency. This index shows near to above average values in the 1950's and 1960's, near to below average activity from the 1970's through the mid-1990's, and then spiked back to above average in the early 2000's. In recent years, the ACE Index has been trending downward again. 

Even though there was a sizeable uptick in tropical cyclone activity in the early 2000's, changes in observation methods make it difficult to know whether tropical storm activity has actually shown an increase over time. 

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