So far this hurricane season has been crickets. Enhanced water vapor satellite image over the Atlantic from 11:45 am EDT today. Medium to darker blues indicate dry air in the mid-upper troposphere. (NRLMRY)

We’re nearly halfway through the typical Atlantic hurricane season, and yet not a single hurricane has formed. It’s not a totally abnormal season yet, but it is a testament to the intensity of the strengthening El Niño in the tropical Pacific, and the season is becoming more of an outlier with each passing day that we don’t see a hurricane.

As expected, hurricane activity has been notably suppressed thanks to El Niño. Much to the delight of many coastal residents, conditions across the typical formation zones have been hostile to hurricanes — a situation that does not look like it will be changing any time soon.

Despite having already seen three named storms this season — Ana, Bill and Claudette — they have only lasted for a combined five and a half days. Not one of these storms reached hurricane status, and none of them actually occurred in the tropics — they all formed farther north in the Atlantic Ocean. To date, the year is running at just 37 percent of average in terms of accumulated cyclone energy, a metric used to measure how active a hurricane season has been.

Could we go the entire year without a single hurricane? We just passed the median date for the formation of the first hurricane, so we actually still have plenty of weeks left in the season. In order to find the last season in which not one hurricane formed, we have to look all the way back to 1914, and then 1907 before that. But notably, neither of these seasons occurred during the modern record-keeping era, and there hasn’t been a single year without a hurricane since we launched weather-monitoring satellites into space in the 1960s.

What has been driving this exceptionally quiet season? For starters, since July 1, the air’s moisture content, or humidity, has been very low in the areas where we typically see hurricanes form — the Gulf of Mexico, the Caribbean Sea and the central tropical Atlantic. Any storms that might have a shot at developing in these areas would be quickly choked off by the lack of low-level moisture.

Relative humidity anomalies at 700 mb from July 1 through August 9. (NOAA)

Another factor is the vertical instability, or lack thereof, in the Caribbean and tropical Atlantic. This is a measure of how easy or likely it is for clusters of thunderstorms develop and stay strong. In these charts (Caribbean on left, tropical Atlantic on right), you see a seasonal time series of the area-averaged vertical instability compared to the climatological values. Values lower on the chart indicate less instability, which tends to suppress hurricane activity.

Seasonal time series of vertical instability averaged over the Caribbean (left) and tropical Atlantic (right) shown by the blue line. The black line is the climatoligical value. (CIRA/RAMMB)

Ocean temperatures across the tropical Atlantic have actually been close to average, which suggests there would be enough warmth to fuel hurricanes if they were able to form. But the tropical eastern Pacific is significantly warmer than average — the tell-tale signature of a strong El Niño.

Global sea surface temperature anomalies over the past month, in degrees Celsius. (NOAA)

In the early 1980s, atmospheric scientist Bill Gray at Colorado State University discovered the relationship between strong El Niño events and suppressed Atlantic hurricane activity. This image below, taken from his canonical 1984 paper, shows a simplified schematic of the upper-level wind anomalies during a moderate-to-strong El Niño.

Hatched areas depict areas with anomalously warm water, and dashed arrows depict anomalous upper-level winds. (Gray 1984, AMS)

Now, let’s take a look at the ACTUAL upper-level wind anomalies over the past five weeks. The arrows show the direction of the anomalous upper level wind — it’s a textbook representation of what Gray described in his research.

Upper-level wind anomalies from 1Jul-9Aug, in meters per second (multiply by 2.2 to get miles per hour). (NOAA)

Forecasts from agencies around the world continue to indicate that this El Niño could be among the strongest ever recorded in the months to come, which will have widespread implications, possibly including keeping the Atlantic hurricane season suppressed. In fact, NOAA just released a forecast that called for a 90 percent probability of a below normal season. It was the highest confidence forecast for a below-normal season that the agency has ever issued, since forecasts began in 1998.

[NOAA as certain as it gets about a quiet hurricane season]

But ‘suppressed’ does not necessarily mean ‘dead’.

I searched the years where the Oceanic Niño Index (ONI) was at least moderately positive (≥1.0) on the monthly triad centered on August since 1950. There have been seven such periods: 1957, 1963, 1965, 1972, 1982, 1987, and 1997. Then I plotted all hurricanes that formed during August on those years, and found just five, which are shown on this map:

In the seven months included in the search, the five hurricanes were all rather forgettable, with the exception of Betsy in 1965.

Hurricane Betsy was the nation’s first hurricane that inflicted over $1 billion in damage, and included a very significant landfall in south Florida and another in Louisiana. Betsy peaked as a category 4 with sustained winds of 155 mph, and killed over 80 people.

So the “it only takes one” mantra rings true — although the basin is very quiet now, we must not forget that it is indeed still hurricane season, and we still have about three months to go.