Science Behind Busy 2017 Hurricane Season

Ever wonder how hurricanes are named? The tradition can be traced back to the 1800s, when storms were named to honor Catholic saints, and evolved over the years.

(Published Wednesday, Sept. 20, 2017)

Not since 2005 has the United States been affected by so many hurricanes and tropical storms. As of Sept. 2, we are at the peak or mid-way point of Atlantic hurricane season and there have been 13 named storms.

Only 4 other seasons in the past 22 years have produced that many named storms by mid-September.

Why Hurricanes 1 NBC10
 

For this 2017 to make the top 15 list of busiest Atlantic hurricane years we would need two more named storms by the end of the season on Nov. 30 and it is very likely we will surpass that mark.

There have been busier Atlantic hurricane seasons since 2005, but not for the United States.

Hurricane Katia (l) Hurricane Irma (m) and Hurricane Jose (r) in the Atlantic Ocean on September 7, 2017 NOAA

Below is a list of the top 15 busiest Atlantic hurricane seasons (1851-present). Ten of the busiest seasons have been in the past 17 years.

1) 2005 - 28 storms

2) 1933 – 20 storms

3) 2012 – 19 storms

4) 2011 – 19 storms

5) 2010 – 19 storms

6) 1995 – 19 storms

7) 1987 – 19 storms

8) 1969 – 18 storms

9) 2008 – 16 storms

10) 2003 -16 storms,

11) 1936 – 16 storms

12) 2007 – 15 storms

13) 2004 – 15 storms

14) 2001 – 15 storms

15) 2000 – 15 storms

So, what is causing this busy season?

Why hurricanes 2 Map by NOAA Climate.gov, based on originals by Gerry Bell

1. Warm Water in the Atlantic and Less Wind Sheer

The main reason for so much activity this year can in part be attributed to the Atlantic Multi-Decadal Oscillation (AMO). The warm phase of the AMO leads to a lack of vertical wind shear in the atmosphere combined with very warm sea surface temperatures and a more active West African monsoon season, all resulting in frequent storm development and rapid intensification over the Atlantic basin.

We have been in the warm phase since 1995. A prolonged cold phase with lower activity was in place between 1971-1994.

Why hurricanes3 Map by NOAA Climate.gov, based on originals by Gerry Bell
 

2. Possible La Nina

Increased activity this season may be also be due to the recent cooling in the central and eastern equatorial Pacific Ocean waters indicating a possible La Nina weather pattern is building for the winter ahead.

La Nina years favor more hurricanes and tropical storms in the tropical Atlantic basin with less activity in the tropical Pacific basin. La Nina is the positive phase of the El Niño Southern Oscillation (ENSO).

During this phase the equatorial waters in the central and eastern tropical pacific are much cooler than average, leading to more stable atmospheric conditions and less tropical activity over the Pacific basin and less stable conditions over the Atlantic basin with higher tropical activity.

The opposite is true in El Nino years with warmer water piling up along the central and eastern side of the Pacific basin leading to unstable conditions over the Pacific with higher tropical activity and more stable conditions over the Atlantic basin.

3. Steering Winds

Finally, the atmospheric steering mechanisms have also played a part in storm tracks. There is an area of dominant sub-tropical high pressure known as the “Bermuda High” located over the Atlantic Ocean. Winds turn clockwise around this high and steer the tropical systems west toward the Caribbean, Gulf of Mexico and the east coast of the United States.

In the case of Irma the western edge of this high pressure was closer to the U.S. east coast forcing a northern turn over Florida.

In Maria’s case the sub-tropical high is father east over the Atlantic and Maria’s northward turn should be farther east than Irma’s was.

Hurricane Harvey was blocked from moving with no dominant steering mechanism for more than a week. The result was constant rain that resulted in historic flooding.

Jose has twice been stuck circling in place with no strong steering mechanism. However, in Jose’s case the constant heavy rain has remained offshore and not over the east coast.

What’s Unusual About 2017?

Hurricane Irma Heading Toward the Leeward Islands NOAA

Double Landfall on Northern Leeward Islands

Another distinction this year revolves around the northern Leeward Islands, which received a catastrophic hit from two major hurricanes, Irma and Maria. These islands had not experienced two major hurricanes in the same season since 1899 (118 years ago).

NASA-NOAA's Suomi NPP satellite captured this visible image of Tropical Depression One in the North Central Atlantic Ocean on April 20 at 15:12 UTC (11:12 a.m. EDT). The depression became Tropical Storm Arlene later in the day. NOAA/NASA

Rare April Hurricane

Also unusual was the first storm of 2017. Arlene was a rare off-season tropical storm in mid-April.

Strength of Irma's winds NOAA

Record-Breaking Hurricane Irma

This season has also produced the strongest hurricane on record for the Atlantic Basin, Irma (Excluding the Caribbean and Gulf of Mexico) and two of the top 20 most intense storms in recorded history, Irma and Maria.

Intensity is correlated with the lowest central pressure of a hurricane. The lower the pressure, the stronger the storm with higher wind speeds and 9 of the most intense storms have been in the past 20 years.

 

Hurricane Wilma had the lowest central pressure of any tropical cyclone ever recorded in the Atlantic--making it one of history's most intense hurricanes. NOAA

Most intense Atlantic Hurricanes on record

1) Wilma (2005) 882mb – 185 mph

2) Gilbert (1988) 888mb – 185 mph

3) Labor Day (1935) 892mb – 185 mph

4) Rita (2005) 895mb – 180 mph

5) Allen (1980) 899mb – 190 mph

6) Camille (1969) 900mb – 175 mph

7) Katrina (2005) 902mb – 175 mph

8) Mitch (1998) 905mb – 180 mph

9) Dean (2007) 905mb – 175 mph

10) Maria (2017) 906mb – 175 mph *

11) Hurricane #10 (1924) 910mb – 165 mph

12) Ivan (2004) 910mb – 165 mph

13) Irma (2017) 914mb – 185 mph *

14) Janet (1955) 914mb – 175 mph

15) Isabel (2003) 915mb – 165 mph

16) Cuba (1932) 915mb – 175 mph

17) Opal (1995) 916mb – 150 mph

18) Hugo (1989) 918mb – 160 mph

19) Gloria (1985) 919mb – 145 mph

20) Hattie (1961) 920mb - 160 mph