2013 quiet so far for global tropical activity

There have been four tropical storms to date during the Atlantic Hurricane season, but, overall, in the Northern Hemisphere, the amount of tropical activity is lagging well behind the normal for this time of the year. One way to measure tropical activity is to count the number of named storms. Another measure, which may be a better representation of the overall tropical activity, is to factor in the wind energy generated by the storms. This gives more weight to stronger, longer lasting storms than the brief, relatively weak tropical storms that have formed so far this year. The Accumulated Cyclone Energy (ACE) index is used to help quantify how active the tropics really are. The following is from the NOAA Climate Prediction Center website.

The measure of total seasonal activity used by NOAA is the Accumulated Cyclone Energy (ACE) index. The ACE index is a wind energy index, defined as the sum of the squares of the maximum sustained surface wind speed (knots) measured every six hours for all named storms while they are at least tropical storm strength.

Here is a look at the current year-to-date ACE index compared to the normal yearly ACE and the normal year-to-date. You can see that in the Northern Hemisphere the ACE index is roughly half of what it typically is by this point in the season. The biggest deficit so far this year is in the Pacific Ocean. Because it’s still relatively early in the year, a few big storms can turn the numbers around in a hurry. Forecasters are predicting an active hurricane season, and the action usually picks up by mid to late August.

Data from policlimate.com/tropical/
Southern Hemisphere season run

s from October to June