Accumulated Cyclone Energy

This is not a weather prediction. It is not a climate rant. It is a look at some interesting data of potential impact to cruisers.

Accumulated Cyclone Energy (ACE) is an index combining the numbers of tropical systems, their duration and their intensity as a function of time. It is calculated by squaring the maximum sustained surface wind in a system every six hours and summing across the storms of the season. It is expressed in 104kt2.  In many ways it is more useful than numbers and classification of individual storms, This is because it captures hurricane season as an energy redistribution cycle rather than a season of storms. ACE is indifferent to the when and where of storm genesis, path and landfall. The following graphic shows ACE, by year since 1851.*

The colored sections are parsed by solar cycle. The black curve is a leading-trailing 10 year moving average of ACE — to smooth things a bit and 10 years correlates roughly with these solar cycles. The straight black line is the trend for the leading trailing average.

The red arrows capture the gross behavior of the moving average — generally a steady increase to a high and then a slightly exponential fall to a low and so on.

So we have a climb from 1859-1891, 1909-1953, and 1972-2000 — three to four decades each.

But the first climb was essentially linear, while the second two are more exponential (the black circles).

The last decade of shows the beginnings of a more or less predictable decrease based on the prior two cycles’ behaviors — Again, this is ACE, not number of storms nor the violence of any one storm, nor the damage etc, etc.

These data indicate since 1851, the energy redistributed by (Atlantic) tropical weather has increased by 34%. However, I wasn’t around in 1851, and so it’s better to baseline the year I started  sailing, 1974, and since then the ACE has increased by ~70%.

So what? Well, since hurricane season is a fixed period (so far), and even though declining, the moving average ACE is still higher that the 1891 and 1953 peaks, it is driving us to consider tropical storm issues — how long offshore, harbors of refuge, hurricane holes — in more detail than in the past.

So I’m a data junkie….

*References:

Landsea, C.W., G.A. Vecchi, L. Bengtsson, and T.R. Knutson, 2010: “Impact of duration thresholds on Atlantic tropical cyclone counts.” J. Climate, (in press).

McAdie, C. J., C. W. Landsea, C. J. Neuman, J. E. David, E. Blake, and G. R. Hamner, 2009: Tropical Cyclones of the North Atlantic Ocean, 1851-2006. Historical Climatology Series 6-2,Prepared by the National Climatic Data Center, Asheville, NC in cooperation with the National Hurricane Center, Miami, FL, 238 pp.

Sheets, R.C., 1990: “The National Hurricane Center – Past, present, and future.”,Wea. Forecasting,5, 185-232.

Vecchi, G.A. and T. R. Knutson, 2008. “On estimates of historical North Atlantic tropical cyclone activity.”, J. Climate21, 3580.

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