Update: Global Tropical Cyclone Energy nearing 50-year lows

Figure: Using a longer-database of hurricane tracks for the globe, the recent downturn in global TC energy is nearing record low levels of inactivity - the lowest in 50-years. Full details forthcoming. [updated June 26, 2009]
See Guest Climate Audit blog posting June 30, 2009

Global Tropical Cyclone ACE valid July 1, 2009

BASIN	2009 Current	YEARLY	Thru June 30	Thru July 31	Avg July
Northern Hemisphere	32.655	563	64.0	131.7	67.7
North Atlantic	0	106	1.4	7.6	6.3
Western Pacific	25.49	309	41.6	75.2	33.6
Eastern Pacific	2.9475	132	13.5	41.4	27.9
Northern Indian	4.22	17	7.5	7.5	0.02
Southern Hemisphere	107	229	Out of	Season	---

North Atlantic Hurricane Season Predictions

Figure: Previous Accumulated Cyclone Energy statistics for the past 3/4 century. The predicted ACE for 2009 by the UK Met office is indicated (60), which is well below the average (~100). The little box-and-whisker inset shows the distribution of ACE for the NATL since 1938.

July climatological activity

Figure. WPAC ACE for July from 1979-2008.
Other Figures -->June WPAC ACE August September

Tropical Cyclone ACE Update

Figure: 24-month running sum of tropical cyclone accumulated cyclone energy for the entire globe (top black squares / time series) and the Northern Hemisphere only (bottom green squares / time series). The difference between the two time series is the Southern Hemisphere total. Data is shown from June 1979 - June 29, 2009 mainly because intensity estimates of SH cyclones are often missing in the JTWC best-tracks prior to 1980. See notes.

Note: climatology is based upon the past 30-years of tropical cyclone activity (1979-2008). Historical tropical cyclone tracks are obtained from two sources: National Hurricane Center (NHC) for Eastern Pacific and North Atlantic basins and the Joint Typhoon Warning Center (JTWC) for the Western Pacific, Northern and Southern Indian Oceans, and the South Pacific including the Australian region. Best-track data when cyclones are in an extratropical phase are disregarded, where this is included in the datasets.

While there are several other sources of best-track hurricane data for the different basins around the globe, it is not apparent which source of tropical cyclone intensity estimates is the best, most correct, or most consistent throughout the past 30-years. Until that research is completed, it is my policy to use NHC and JTWC data for global tropical cyclone data. I will attempt to use RSMC (Tokyo) and the IBTrACS merged database for comparison purposes in the near future. (Update: June 24, after calculating ACE using the IBTrACS mean intensity, which can include from 1-4 different reporting centers, the differences are negligible. Therefore, it is appropriate to use the NHC+JTWC for global studies on yearly time scales. Naturally differences crop up when examining storm by storm and observation by observation differences. This is a disasterous complication when doing count/frequency studies such as Webster et al. (2005) but is mitigated with accumulated cyclone statistical studies (i.e. Emanuel 2005; Maue 2009)).

Duration component of ACE

ACE is the convolution or combination of the frequency, intensity, and duration of tropical cyclones in a given time period such as a season. The duration component is the dominating term in the calculation when seasons or years are chosen as the time integration metric. Put simply, the yearly global ACE time series during the last 30-years is very highly correlated with the number of hurricane days. This is a product of the relationship between individual storm's intensity and duration: stronger storms tend to last longer.

Figure. Two time-series are plotted based upon best-track datasets. The underlying calculations are very simple: cyclone data is binned according to category (Saffir-Simpson scale) and organized in 24-month running sums. Then, as in the case of Category 1+ / Category 3+, a simple ratio is created (1-ratio for figure purposes).
From the figure, a statistically significant increase in the ratio of major (3+) and Category 4+ cyclones has occurred during the past 30-years, with the trend resulting from a lull in the early 1980s and a maximum from 2003-2008. The period between 1988 and 2003 (~16 years) saw very stable major and Category 4+ ratios, globally.

Northern Hemisphere subset of 24-month ratios of hurricane days of various intensity
During the past 2 to 3-years, even though ACE has been at record low levels, the ratio of the various categories have been at relatively high levels, compared to the past 30-years.

Figure. 24-month running sums of global TC hurricane (Category 1+), major hurricane (Cat 3+), and intense hurricane (Cat 4+) days. There is a significant upward trend in the number of major and intense hurricane days since 1979, but not overall hurricane days. Indeed, global hurricane days have dropped to record low levels, not seen in at least 30-years, by far.

Additional figure: zoomed in view of Category 3+ and Category 4+ global TC durations (1979-2009).
Category 3+ Duration Figure

Duration component of ACE relationship to ENSO phase

Figure. 24-month running sum of global TC ACE from 1989-2009 and NINO 3 SST (both normalized to the 20.5 year period). There is clearly a close relationship (r > 0.7) on interannual time-scales between global TC ACE (duration = track locations) and ENSO indices, such as NINO 3, as plotted here. It is perhaps coincidental that the period from 1997-2003 closely matches the recent 2004-2009 global TC ACE activity.

Frequency component of ACE: Northern Hemisphere

Figure: Northern Hemisphere Minor, Major, and total hurricanes from 1979-2008. Minor hurricanes are defined as Category 1 & 2 (64-95 kt) and major hurricanes are Category 3+ (96 kt). No significant trend in any quanitity.