We know the ingredients that came together to make this happen: weak ambient winds in the Atlantic, very warm ocean temperatures and the fact that the storm’s core has been able to stay out over the open water without making landfall on a major coast.
The “wind shear” over the Atlantic Ocean is low this year, compared with previous years. This term describes a change in wind speed or direction with height. Wind shear tends to tear a hurricane apart. When conditions are relatively tranquil, “cyclogenesis” can occur — which basically means a hurricane can form.
With exceptionally low wind shear over the Atlantic and Caribbean, the stage was set months ago for an exceptionally active season.
The image below shows estimated wind shear over the Atlantic Ocean on Thursday. Green lines mean the wind shear is weak, red means it’s high. Note the areas of green on the right-hand side of the image posed just off the African coast. This marks a region of weak winds aloft, making it particularly conducive to tropical cyclone development and intensification.
Bathlike ocean water provided a lot of fuel for Hurricane Irma to strengthen from. The map below shows how much warmer the water is than normal — about 1 to 3 degrees above average for this time of year. As ocean temperature continues to rise from climate change, this will probably influence how strong Atlantic hurricanes can become.
The pronounced area of blue in the Gulf of Mexico near Texas is because of the cooling effects of Hurricane Harvey. It can be attributed both to the heat that the storm extracted as well as the upwelling of cold water from the depths.
The final reason Irma has stayed so strong is because the storm has not hit a major land area. The hurricane ravaged islands in the Caribbean — including Barbuda, St. Martin and the Virgin Islands — but so far it hasn’t come into contact with a body of land that could potentially slow it down.
Importantly, the storm’s eye has stayed over water, which allowed it to maintain its strength as the outer winds and rain bands lashed the Lesser and Greater Antilles. Until the core reaches Florida, this storm will continue to maintain strength.
Bringing it all together is the Bermuda High, which is why the storm went where it did.
The Bermuda High is a ridge of high pressure centered over the central Atlantic. This crest of calm weather and clear skies is a semi-permanent feature this time of year and shifts east in the wintertime. The high acts as a guardrail for hurricanes, forcing them as far west as its boundary exists.
This year, the Bermuda High is more intense and farther west than normal. As such, storms that ordinarily would have a chance of turning out to sea are now able to target the United States.
In addition to steering the hurricane, the high pressure also contributes to its strong winds. Wind speed is caused by a change in pressure. If the Bermuda high is exceptionally high, then the pressure difference between that and the hurricane is much larger, which leads to stronger winds.
This pressure trace measured during Irma’s direct pass over Barbuda illustrates this concept quite well. A drop of nearly 90 millibars, equating to roughly 10 percent of the atmosphere’s weight, occurred within a six-hour window. This stark contrast and enormous pressure gradient is what makes Irma’s winds so strong.
Angela Fritz contributed to this story.