Tropical Storm Grace, We Hardly Knew Ye

It begin at 11:00 PM on Sunday and ended at 11:00 PM on Monday.  In 24 hours, TS Grace went from non-tropical to tropical to extratropical as the storm was swept up by a mid-latitude cyclone about 200 miles southwest of Cork, Ireland.

As for the rest of the tropics, there is an area of showers and thunderstorms associated with low pressure about 550 miles East of the Leeward Islands.  Atmospheric conditions are not conducive to tropical cyclone development, so there is only a small chance of strengthening in the next couple of days.

Since Hurricane Fred dissipated on September 12, there have been two tropical cyclones – Grace and TD Eight.  Both of these storms lasted only 24 hours, continuing the quiet year in the Atlantic.  According to Ryan Maue’s page at Florida State, the accumulated cyclone energy (ACE) in the North Atlantic currently stands at 42 (10^4 kt^2, if you’re interested in the units).  The climatological norm through October 5 is 88.  This only confirms what we know – the tropical Atlantic is not very active this year.

A look around the Northern Hemisphere and Globe shows that ACE is at its’ lowest in nearly 30 years.  From Ryan Maue’s page:

24-month running mean of ACE for Northern Hemisphere (blue) and Globe (green)

(Click for large image.) 24-month running sums of ACE for Northern Hemisphere (blue) and Globe (green)

Looking at this image, it’s pretty clear the current lack of activity is not limited to the Atlantic.  Also, it’s not unique to this year when an El Nino is forming in the Pacific.  ACE has been trending downward in the Northern Hemisphere, as well as the globe, for several years.  And keeping in mind the detection of weaker, short-lived tropical cyclones has improved dramatically in the last 40 years, the left side of this graph may be underestimating ACE in in the late 70s/early 80s.

It’s hard to say exactly why ACE is trending downward in recent years, but it’s pretty clear that it is.  Simply saying the development of an El Nino is the reason for decreased activity this year is lazy.  There are other factors in play.

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