When Tropical Storm Claudette formed Monday, it came well ahead of the average date of the third named storm of the Atlantic hurricane season — Aug. 14. That might seem surprising, because in every other way, this season has been fortuitously lackluster.
But what’s interesting is that the area that we look to for the development of the strongest hurricanes — the main development region just north of the equator in the Atlantic Ocean — has been very, very quiet. Not a single storm has formed in this area so far this year. Nothing has even hinted at it.
The lazy summer comes as no surprise to seasonal hurricane forecasters who all predicted a relatively inactive season before it began June 1. NOAA’s forecast in late May called for a 70 percent chance of a below-average season, with six to 11 named storms, three to six of which could become hurricanes, and up to two major hurricanes. It was the highest probability of a below-average season that NOAA had ever forecast. A month before that, Colorado State University research scientist Phil Klotzblach released what he said was the group’s lowest-ever April forecast.
So why the Atlantic Ocean crickets? There are four big reasons, and the ever-strengthening El Nino in the tropical Pacific spans nearly all of them, pushing conditions to be more and more unfavorable for hurricanes.
Wind shear is higher than it has been in decades
Wind shear is one of those things that can make or break a season. Even if everything else is working against hurricane formation — cooler than average ocean temperatures, few low-pressure waves — low wind shear could be the thing that tips the scales. El Nino’s most direct impact on the Atlantic hurricane season is increased wind shear.
Wind shear is defined as any change in wind speed or direction with height, or rather, as you go up in the atmosphere. As hurricanes develop from thunderstorms, they need to grow tall in the atmosphere as heat and moisture is concentrated in the middle of the storm. If winds are too strong at the upper levels, it can tear a young storm apart, or even prevent it from developing in the first place.
Such is the case this year, which has been exhibiting record-breaking wind shear. Klotzbach found that just in the Caribbean, wind shear has reached record levels since the beginning of the satellite era, nearly double what it was in the epic El Nino year of 1997.
Vertical shear in the Caribbean remains strong! Maxed at over 30 m/s on Saturday. 30-day avg well ahead of any year pic.twitter.com/EhCbZuNuZC— Philip Klotzbach (@philklotzbach) July 13, 2015
Cooler-than-average ocean surface temperature
Sea surface temperatures are the most straightforward way to determine if conditions in an ocean basin are conducive for hurricane development. At the very core of necessary conditions for hurricanes is warm ocean waters — generally 82 degrees or warmer — that serve as fuel for the storms to grow.
Coolest tropical Atlantic SSTs in ~20 years. #climate #tropics pic.twitter.com/ydpnEYb48T— Todd Kimberlain (@ToddKimberlain) July 6, 2015
So far this season, surface temperature in the tropical Atlantic has been running three to four degrees cooler than average, and the actual magnitude of the temperature is only marginally conducive to support hurricane formation, particularly in the main development region east of the Caribbean. Sea surface temperature is actually warmer in the Gulf of Mexico and off the east coast of Florida.
Klotzbach says the month of June in the main development region was the second-coldest on record since 1900, relative to the rest of the tropics.
High pressure and sinking air
Something that is closely related to the abnormally cool ocean temperatures is the pressure pattern between the Atlantic and the East Pacific. The East Pacific has been boiling over with record warm ocean water, fueling a record hurricane season there. That has led to a lot of hot, rising air west of Mexico and sinking air — or high pressure — over the Atlantic.
If there’s a place that storms are unlikely to form, it’s in a region of high pressure and sinking air, over an abnormally cool surface, and forecasts are calling for a continuation of this pattern into the fall.
LOTS of dust from the Sahara in Africa
The final nail in the hurricane season thus far is the copious amounts of dust blowing west off the coast of Africa. It helps that there hasn’t been much rain in the region (say, in the form of a tropical storm) to rinse the dust out of the atmosphere. In general, Saharan dust over the Atlantic is not in itself sufficient to totally destroy a season, but it does serve as the dry, sandy icing on the anti-hurricane cake.
Scientists don’t completely understand how dust may impact hurricanes, but there are three likely ways — two of which are inhibitors. The first is very straightforward: Dust from the Sahara tends to reduce the amount of sunlight that reaches the surface of the ocean (just like smoke from Canadian wildfires can dampen the sun in the Mid-Atlantic). The particles in the atmosphere reflect this light and back out into space, so it is unable to heat the ocean and the atmosphere. It means there’s less energy available for potential storms to draw from.
The second way is that the dust tends to also absorb solar radiation, which heats the air around it. Since the dust exists at the low to mid-levels of the atmosphere, it creates a temperature inversion — or increase in temperature with height — so that thunderstorms have a really hard time punching through to the upper levels of the atmosphere. In severe thunderstorm lingo, we call this a “cap,” because it can literally cap the thunderstorms at the level of the inversion.
The third and probably least understood way that dust may play a role is that it might actually create more clouds. Cloud droplets don’t just form out of thin air — there needs to be a tiny particle, like a piece of dust, for water vapor to condense onto. We call these particles condensation nuclei, and the Saharan dust could potentially serve as trillions of places where cloud droplets could form. However, this idea is still up for debate, and the final impact on hurricane formation is very hazy.