Friday, June 30, 2017

Global Tropical Cyclone Activity Paltry So Far, Despite Cindy and Dora

Jon Erdman
Published: June 29,2017

Tropical cyclone activity so far in 2017 is running well below average through late June, despite a recent uptick in both the Atlantic and eastern Pacific basins.
(MORE: Hurricane Central)
According to Colorado State University atmospheric scientist Dr. Phil Klotzbach, the northern hemisphere's ACE (Accumulated Cyclone Energy) index, a measure of tropical cyclone activity, through June 28 is less than one-quarter of the average year-to-date total.
The ACE index is calculated by adding each tropical storm, hurricane or typhoon's wind speed through its life cycle.
Long-lived, intense hurricanes/typhoons have a high ACE index. Short-lived, weak tropical storms, a low ACE index. The ACE of a season is the sum of the ACE for each storm and takes into account the number, strength and duration of all the tropical storms in the season.
(MORE: 10 Most Extreme Atlantic Hurricane Seasons)

Atlantic, East Pacific: Many, But Short-Lived

You can't blame the Atlantic basin for the slump.
There have already been three tropical storms – Arlene, Bret, and Cindy – before the end of June.
(MORE: Strange Start to the Atlantic Hurricane Season)
Tracks of Atlantic and eastern Pacific basin tropical cyclones in 2017, through June 28.

An average Atlantic hurricane season doesn't see the first named storm form until the second week of July, according to the National Hurricane Center.
June typically doesn't feature intense, long-lived Atlantic hurricanes, but rather short-lived tropical depressions or storms. This is partly due to a more restricted region of development, stronger wind shear and drier air compared to later months.
The eastern Pacific basin has also seen more named storms than average through late June, punctuated by the season's first hurricane, Dora.
However, three of the four named storms in this basin were short-lived tropical storms, and Dora only maxed out as a Category 1 hurricane for a little over 24 hours.
When computing the ACE index, again combining both intensity and longevity, eastern Pacific tropical activity has been less than 50 percent of average through late June, while Atlantic activity has been above the relatively low season-to-date average.

The Real Drought Source: The Western Pacific Basin

If you haven't heard the word "typhoon" in a while, it's because there hasn't been one yet in 2017.
Tracks of northwest Pacific tropical cyclones in 2017, through June 28. Tracks of three tropical depressions are not shown.
Through late June, there have been only two tropical storms – Muifa in late April and Merbok in mid-June – in 2017, so far in the western Pacific basin.
According to Dr. Klotzbach, this is about half the average named storms-to-date (4 to 5). Typically, two to three typhoons have developed by late June of an average year, Klotzbach calculated. One to two of those typhoons would have reached at least Category 3 intensity in a typical year, as well.
Given only two tropical storms lasting a combined three days, the western Pacific's ACE index was a scant 5 percent of average.
"One of the reasons for suppression of the NW Pacific TC season to date is that we've generally had sinking motion across most of the basin since early May," said Klotzbach.
Stronger than average trade winds, blowing east-to-west near the Philippines, Taiwan, and the South China Sea, have also been in place since May, Klotzbach said. This increases wind shear which tends to either prevent tropical cyclones from forming or rip apart those that have formed.
(MORE: Which Countries Get Hit Most by Tropical Cyclones?)

2016 Déjà Vu?

Interestingly, we were at roughly this same point of inactivity last June.
"Climatological northern hemisphere ACE-to-date is about 60, and this year we're at a measly 14. Last year, we were at 18 on June 28," Klotzbach said.
But then the western Pacific exploded in early July, as Super Typhoon Nepartak grew to Category 5 intensity before slamming into Taiwan.
That was the first of nine typhoons of Category 3 intensity or stronger in 2016, ending with Super Typhoon Haima hammering the northern Philippines in mid-October.
The eastern Pacific also roared to life last July with major hurricanes Blas and Darby.
The Atlantic hurricane season's peak months of August and September still lie ahead. If it hadn't been for January oddball Alex, all Atlantic hurricanes in 2016 would've formed from August on, including Matthew.
The bottom line is that it's still very early.
"We're only about 11 percent of the way through the Northern Hemisphere tropical cyclone season from a climatological perspective," Klotzbach said.
Despite the slow start, 2016's tropical cyclone activity ended up a tad above average, according to Klotzbach.
In recent days, several meteorologists have noted the strength and procession of tropical waves marching westward off west Africa resembles what you typically see during the heart of the hurricane season, not late June.
(MORE: Why Tropical Waves are Important During Hurricane Season)
All in all, don't be fooled by a relatively benign start to any tropical cyclone season.  After all, in June 2015, we were tracking a record active start of northern hemisphere tropical cyclones.
Jonathan Erdman is a senior meteorologist at and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.

MORE: Atlantic Basin Retired Hurricanes and Tropical Storm Names

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