Hurricanes 2018

There was a lot of press about the 2017 hurricane season. Hurricanes Harvey, Irma, and Maria caused hundreds of billions of dollars in damage, killed thousands, and left untold more injured. By themselves, these three storms caused an estimated $265 billion in damage to the U.S. That figure is from January 8, 2018 and is likely to be even higher by now. Official records show 64 deaths in Puerto Rico, but analysis indicates this number is extremely low and the real number of fatalities lies between 1000 and 5000 people.

The question that was immediately asked was how much manmade global warming had to do with all of this. The deniers in government responded that 'this wasn't the time to talk about that.' Of course not. They didn't want to be held accountable for their actions. But, that didn't stop the conversation and the analysis. The conclusion? Yes, AGW most certainly made these storms worse. Attribution science shows global warming made the storms stronger, slower, and wetter. They are also growing in strength faster. Three independent studies last fall found that human activity made the damage from Hurricane Harvey more intense. The disastrous rainfall from Harvey was three times as likely as from a storm a hundred years ago and 15% more intense. The seven day rainfall was as much as 40% greater than from a similar storm even a few decades ago. The lead researcher on one study, Geert Jan van Oldenborgh, stated,

This multimethod analysis, drawing upon both observed rainfall data and high-resolution climate models, confirms that heavy rainfall events are increasing substantially across the Gulf Coast region because of human interference with our climate system.

[Of course, much of the damage could've been avoided if not for the climate-change denying director for the Harris County Flood Control District, Mike Talbot, who insisted climate change was a hoax and allowed construction in known flood plains for many years. It would be wonderful if Talbot was arrested for his crimes, but it won't happen.]

The problem is that the average hurricane strength has remained pretty constant with the accumulated cyclone energy (ACE) remaining relatively steady. But, while the ACE has remained constant, there has been an increase in storms in recent years of such incredible strength they are being referred to as category 6 storms, which would constitute a new level in the Saffir-Simpson index used to categorize cyclonic storms and would consist of storms with constant winds exceeding 175 or 180 mph.

The science is clear - global warming is resulting in cyclonic storms that develop faster, become stronger, move more slowly, and dump more rain. And, at the same time, the total energy of storms has remained fairly constant. How do you reconcile this contradictory information? One way is that small storms and large storms are both becoming more common. This results in the average remaining nearly the same. Another way is that very active years (2017) alternate with relatively quiet years.

This year, 2018, appears to be one of those slow years. The July forecasts are for below-average activity with eleven named storms and four hurricanes. Despite the fact that we've already had three full-strength hurricanes, it appears this should be a slow year. There are several factors leading to this conclusion.

Take a look at this plot of the sea surface temperature (SST) anomaly:

Source: CCI

This image is showing us three really important things.

First, note the red area in the North Atlantic. The sea surface is hotter than usual here. That is causing a high pressure system in the North Atlantic. This creates a clockwise cycle of wind which goes down past the Iberian Peninsula and westward across the mid-Atlantic. Take a look at this graphic illustrating the wind patterns:

Source: Earth

The result of this is to cool the sea surface. This is indicated by the blue, lower-than-normal temperature region in our SST figure above and is the second thing to note. That area is known as the Main Development Area for hurricanes.

Hurricanes need warm ocean water. The threshold temperature is typically regarded as at least 79 degrees Fahrenheit (about 26 degrees Celsius). The winds blowing across the mid-Atlantic are keeping the sea surface temperature down, which is bad for hurricane formation.

The third thing to note in the image above is the red area west of equatorial South America indicating hotter than usual SSTs. This is an El Nino possibly forming. The July ENSO Diagnostic Discussion estimates a 65% chance of an El Nino this fall and a 70% chance for this coming winter. Even if it doesn't turn into a full-scale El Nino, it has the effect of creating high-altitude wind shear across the Caribbean Sea. Hurricanes gain strength by forming a rising column of air in the eye, resulting in a low-pressure area and sucking in hot, moist air to replace it. This air rises, condenses, releases its energy and then cycles around to continue fueling the storm. High-altitude shear winds crossing over the top of this column will essentially chop it off, robbing the cyclone of its reinforcing wind pattern. This is also bad for hurricane formation.

So, two bad things for hurricane formation. But, there's a third. Take a look at this satellite image:

Source: NOAA/RAMMB, via Weather Underground

What you can see here is a big cloud of dust blowing off Northern Africa across the mid-Atlantic and into the Caribbean. This dust is contributing to the lower SST for the region by shading the sunlight.

So, we have cool SSTs caused by wind and dust, and high-altitude wind shears. This is a bad combination for the formation of tropical cyclones. So, despite having three hurricanes already, the estimate remains that it will be a slow hurricane season in the Atlantic.

The interesting thing is all of those factors are made worse by AGW. The high-SST area in the North Atlantic is the result of ocean warming, as is the El Nino. Manmade global warming makes both of these events more likely and more extreme. The dust blowing off Africa is due to more extreme drought conditions, again made worse by global warming.

So, does AGW make tropical hurricanes worse? Or, better? Apparently, depending on the year, it can be either.

"This multimethod analysis, drawing upon both observed rainfall data and high-resolution climate models, confirms that heavy rainfall events are increasing substantially across the Gulf Coast region because of human interference with our climate system."

Read more at: https://phys.org/news/2017-12-climate-harvey-rainfall-percent-intense.html#jCp

"This multimethod analysis, drawing upon both observed rainfall data and high-resolution climate models, confirms that heavy rainfall events are increasing substantially across the Gulf Coast region because of human interference with our climate system."

Read more at: https://phys.org/news/2017-12-climate-harvey-rainfall-percent-intense.html#jCp

"This multimethod analysis, drawing upon both observed rainfall data and high-resolution climate models, confirms that heavy rainfall events are increasing substantially across the Gulf Coast region because of human interference with our climate system."

Read more at: https://phys.org/news/2017-12-climate-harvey-rainfall-percent-intense.html#jCp