Climate Mechanism for Stronger Typhoons in a Warmer World

Slide deck for talk at the 6th International Summit on Hurricanes and Climate Change

Transcript

1. Climate Mechanism for Stronger Typhoons in a Warmer World James B. Elsner (@JBElsner) & Nam-Young Kang1 Department of Geography, Florida State University Tallahassee, FL June 7, 2017 6th Hurricanes & Climate Change Summit 1National Typhoon Center, Korea Meteorological Administration
2. What is storm activity?
3. Intensity vs frequency FRQ INT INT is the annual mean intensity of TCs having lifetime-maximum winds (LMWs) exceeding a threshold quantile. The lowest threshold quantile (zero empirical probability level—no chance of a TC having weaker winds) is set as 17 m s−1. INT is computed at successively higher thresholds. FRQ is the annual number of TCs above each successive threshold. By definition the variation of FRQ is not affected by the probability level.
4. FRQ INT The positive diagonal provides an axis that captures the in-phase relationship between INT and FRQ that we denote ACT. ACT is a principal component computed as ACT = INT − µINT σINT + FRQ − µFRQ σFRQ / √ 2, where INT and FRQ are vectors of annual values. µ and σ denote their respective mean and standard deviations. ACT is comparable to TC energy as indicated by its high correlation (≥ .9) with Accumulated Cyclone Energy (ACE) and Power Dissipation Index (PDI).
5. FRQ INT The negative diagonal provides an axis that captures the out-of phase relationship between INT and FRQ. This variability is denoted as EINT, which expresses the efficiency of intensity. Alternatively the negative EINT can be understood as the efficiency of frequency. EINT is the other principal component and is computed as EINT = INT − µINT σINT − FRQ − µFRQ σFRQ / √ 2.
6. FRQ INT Owing to the presence of EINT, a continuous two-dimensional variability space is formed, where the center is the mean of each standardized set of FRQ and INT values. Now, an annual TC climate can be indicated by a single point in the continuous variability space. In this framework, variability in any direction can be defined as TCIθ = INT · cos θ + FRQ · sin θ, (1) where TCI denotes a directional variability (θ) away from INT (positive is counterclockwise), which represents the weighted linear combination of FRQ and INT. ACT and EINT are the special cases when θ is +45 and −45, respectively.
7. Correlations between global mean (JJASON) ocean temperature and TCIθ are calculated for θ’s in 1◦ intervals using data over the period 1984–2014). The correlations are plotted with a loop on a correlation screen. INT ACT FRQ EINT −INT −ACT −FRQ −EINT 0.5 1.0 q 0.75
8. 0.0 0.2 0.4 0.6 0.8 1.0 0 10 20 30 Wind speed quantile Trend [m s−1 per K] 24 31 37 50 Lifetime highest wind speed [m s−1]
9. Increasing EINT (fewer TCs, but stronger typhoons) is an empirical result that arises from synchronous changes to physical factors under the influence of global warming. A decrease in FRQ together with an increase in INT results from an increase in saturation deficit occurring in the tropical free atmosphere in concert with a decrease in upward mass flux. −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1 1000 925 850 700 600 500 400 300 250 200 150 Correlation Coefficient Pressure (hPa) Moist Static Energy Geopotential Height
10. −0.2 0 0 0 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.8 0.8 100 ° 120 ° E 140 ° E 160 ° E 180 ° 0 ° 10 ° N 20 ° N 30 ° N 40 ° N 50 ° N (a) Sea surface temperature 0 0.2 0.2 0.2 0.2 0.2 0.2 0.4 0.4 0.6 0.6 0.6 0.8 0.8 100 ° 120 ° E 140 ° E 160 ° E 180 ° 0 ° 10 ° N 20 ° N 30 ° N 40 ° N 50 ° N (b) Geopotential height at 500 hPa −0.2 0 0 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.8 0.8 0.8 0.8 100 ° 120 ° E 140 ° E 160 ° E 180 ° 0 ° 10 ° N 20 ° N 30 ° N 40 ° N 50 ° N (a) Sea surface temperature 0 0.2 0.2 0.2 0.2 0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 0.8 100 ° 120 ° E 140 ° E 160 ° E 180 ° 0 ° 10 ° N 20 ° N 30 ° N 40 ° N 50 ° N (b) Geopotential height at 500 hPa
11. 1900 1920 1940 1960 1980 2000 2020 −10 −8 −6 −4 −2 0 2 4 17.1 17.3 17.6 17.8 18.0 18.3 18.5 18.7 Year Standardized EINT (s.d.) Global SST (° C) The trend of standardized SST over the period is +2.8 ± 0.37 (s.e.) s.d./30 yr which compares with the trend in EINT of +2.0 ± 0.52 (s.e.) s.d./30 yr. The correlation between the two series is +0.75 [0.53, 0.87] 95 % CI (+0.57 [0.25, 0.77] 95 % CI after removing the linear trends).
12. Year Relative Magnitude above Below normal Activity Global warming influence
13. U.S. Tornadoes (1985-2015) INT ACT FRQ EINT 0.5 1.0 Western Caribbean SST Gulf of Alaska SST ENSO NH Temperature
14. Atlantic Hurricanes & U.S. Tornadoes Correlation U.S. Tornado Climate Atlantic Hurricane Climate EINT INT ACT FRQ −EINT EINT INT ACT FRQ −EINT −0.4 −0.2 0.0 0.2 0.4 Hedging against hurricane & tornado losses.
15. Summary Single metrics of storm activity can be misleading in climate-change studies. How often and how strong are the two canonical components of storminess. Analyzing frequency independently from intensity illuminates a broad theoretical space of storm climate variability. With typhoons in the Pacific and tornadoes in the U.S., the finger print of climate change appears to be fewer, but stronger. Questions?