1. National Research Council
2B.7
New normal? Historical context of recent global tropical cyclone inactivity
Ryan N. Maue
National Research Council
AMS 30th Conference on Hurricanes and Tropical Meteorology
April 16, 2012

2. What is normal for global tropical cyclone (TC) activity?
Thoughts
What is normal for global tropical cyclone (TC) activity?
How do we quantify TC activity with our current best-track [quality]? What’s our record book?
For climatology (change), what is important? ratios, counts, integrated metrics, SST impacts…etc.
What are the relevant metrics ?
Landfalls
Normal: mean or seasonal, stratified by ENSO phase, counts or some other metric.
Measure TC activity: size, atm/ocean impact (SST imprint, wake), landfall or not, counts or ACE or PDI.
Is our record book sufficient – back to when – basin dependent?
Climatology: longer time series, integrated values vs. point values
To understand changes in TC activity, we need numerator and denominator of our metrics.
Many customers of TC climatology data. Climate change due to AGW very important for policy.

3. Global Tropical Cyclone Database
Best Track Data:
IBTrACS-All v03r03 includes global RSMCs + other centers
NHC HURDAT + JTWC best tracks & operational
Pre-1981, combination of available data, much less reliable, e.g. UCAR ds824.1
(clear underestimate?)
Frequency  Counts
Accumulated Cyclone Energy (Bell et al. 2000)
Only 34-knots + maximum one-minute sustained, tropical phase TCs
Analogous to Power Dissipation Index (V 3 PDI, Emanuel 2005)
Recipe
Usage of different datasets with discontinuous available data is subjective.
There are many datapoints prior to the 1970s that are just plain missing or simply TD, TS, H intensity without values, clear underestimate.
Not always information about the tropical or extratropical phase, may be pretty important for WPAC and NATL when long-track recurvers are present.
CONTINUED REANALYSIS
Knapp et al. (2010) BAMS

4. Historical Declines in TC Activity
Dramatic Downturns
Recent Marks
With many metrics, global TC activity has decreased markedly since 2006.
Frequency and integrated metrics such as ACE and PDI have reached near-record lows (Maue, 2011)
Has continued into 2012, with no end in sight?
Calendar Year 2010 saw 69 TCs globally, fewest since in 2011.
Northern Hemisphere in 2010 saw 46 TCs, fewest since 1977.
Global and Hemispheric TC Accumulated Cyclone Energy since 2007 has plumbed record lows
Just numbers or trivia without context.

5. 2011 Global TC Activity: Frequency
TS | Hurr | Major
Global: | | 21
Northern Hemisphere: | | 16
Southern Hemisphere: | | 3
October April 2012

6. Global Tropical Cyclone Frequency
12-month Running Sums TC (34 knots +) & HURR (64 knots +)
April 2011 – March 2012 = 74 TC | 38 HURR
Obviously the top line is the most sensitive to increased observation network density.
January 1, 2011 – December 31, 2011: 75 | 39
μ = 87 | 47
Linear Trends
[66,107], [28,63]

7. Global Tropical Cyclone Frequency
24-month Running Sums TC (34 knots +) & HURR (64 knots +)
April 2010 – March 2012 = 140 TC | 77 HURR TS
Hurr
Linear Trends
January 1, 2010 – December 31, 2011: 144 | 78
μ = 174 | 95

8. Global Tropical Cyclone Frequency
12-month Running Sums HURR (64 knots +) & MAJOR HURR (96 knots +)
Linear Trend

9. 24-month Running Sums | January 1970 - March 2012
[knots2]
Global Tropical Cyclone ACE
24-month Running Sums | January March 2012
Global NH

10. Global Tropical Cyclone ACE
[knots2]
Calendar Sums: Global & Northern Hemisphere – 2011
July-June offset yearly sums: Southern Hemisphere

11. Historical global tropical cyclone landfalls: 1970-2010
Good data for the Atlantic but who knows about elsewhere.

12. Historical global tropical cyclone landfalls: 1970-2010

13. Relevant metric: global major hurricane landfall$
…this subset of storms is a function of “climate”
Weinkle et al. (in press, J Climate)

14. Historical global tropical cyclone landfalls
Weinkle et al. (in press, J. Climate)
Over 1970 to 2010 the globe averaged about 15 TC landfalls per year
Of those 15, about 5 are intense (Category 3, 4 or 5) 
1978 had the fewest with 7
2011 tied for second place for the fewest global landfalls with 10 (and 3 were intense, tying 1973, 1981 and 2002)
1999 had the most intense TC landfalls with 9
1981 had the fewest intense TC landfalls with zero
There have been only 8 intense TC landfalls globally since 2008 ( ), very quiet but not unprecedented (two unique 3-year periods saw only 7 intense landfalls)
The US is currently in the midst of a record streak without an intense hurricane landfall (Wilma 2005)

15. ? ΔTC activity 2024 – 1970? “Normal”
Moving forward into the 2020s… Frequency
ΔTC activity 2024 – 1970?
“Normal” ?
Campaign slogan in Romney’s reelection slogan, or Hillary’s battle cry.

16. ? ΔTC activity 2024 – 1970? “Normal”
Moving forward into the 2020s… Integrated Metric (ACE)
ΔTC activity 2024 – 1970?
“Normal”
Campaign slogan in 2016. ?

17. ? Moving forward into the 2020s…
Best metrics to measure TC climatology – what is normal?
Best metrics to measure climate change impacts on TCs? ?
IPCC SREX (2012): “The present period of quiescence, as well as the period of heightened activity leading up to the high point in 2005, does not clearly represent substantial departures from past variability. (Maue, 2009)”
Global tropical cyclone frequency: annual mean ~ 87, range from
Is the recent downturn a regional or global phenomena -- represent a climate shift to a colder Pacific (-PDO)?
Active North Atlantic vs. inactive North Pacific. How long is this going to last?
Impacts on landfalls in the future?

18. IPCC, 2012: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change (Field et al. 2012)
In summary, there is low confidence that any observed long-term (i.e., 40 years or more) increases in tropical cyclone activity are robust, after accounting for past changes in observing capabilities.
The uncertainties in the historical tropical cyclone records, the incomplete understanding of the physical mechanisms linking tropical cyclone metrics to climate change, and the degree of tropical cyclone variability provide only low confidence for the attribution of any detectable changes in tropical cyclone activity
to anthropogenic influences.
There is low confidence in projections of changes in tropical cyclone genesis, location, tracks, duration, or areas of impact. Based on the level of consistency among models, and physical reasoning, it is likely that tropical cyclone related rainfall rates will increase with greenhouse warming.
It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged. An increase in mean tropical cyclone maximum wind speed is likely, although increases may not occur in all tropical regions.