Seoul National University Changes in Tropical-Cyclone Lifetime-Maximum- and landfall- Intensity over the western North Pacific Doo-Sun Park Seoul National University 1

Notable Increase of SST in the western Pacific Time series of sea surface temperature in the western North Pacific Linear trend of sea surface temperature during the period 1982‒2010 (ºC decade‒1) The western North Pacific shows significant increasing tendency of sea surface temperature (SST) during the period 1982-2010. The increase of SST is observed over the almost entire western North Pacific basin. An increase in tropical cyclone (TC) intensity can be easily expected. Page  2

Tropical Cyclone Became Stronger? These conflicting results mainly come from the different best-track datasets used [Wu et al., 2006; Song et al., 2010]. Thus, in this study, to find consistent trends between the different best-track datasets, regional intensity trends are considered. The TC intensity and the number of intense TCs show significant increasing trends with tropical ocean warming [Emanuel, 2005; Webster et al., 2005]. decadal variations are more dominant than specific trends [Chan and Liu, 2004; Chan, 2006, 2008]. VS. Page  3

Inconsistent Trends among the Best-track Datasets Time series of average power dissipation index for the period 1977‒2010 (m3 s‒3 hour) Power Dissipation Index (PDI) [Emanuel, 2005]. Vmax: maximum wind speed t: total lifetime of tropical cyclone Year Page  4

Tropical Cyclone Became Stronger? These conflicting results mainly come from the different best-track datasets used [Wu et al., 2006; Song et al., 2010]. Thus, in this study, to find consistent trends between the different best-track datasets, regional intensity trends are considered. The TC intensity and the number of intense TCs show significant increasing trends with tropical ocean warming [Emanuel, 2005; Webster et al., 2005]. decadal variations are more dominant than specific trends [Chan and Liu, 2004; Chan, 2006, 2008]. VS. Page  5

Grid Analysis A 5º square box was set up, and average of intensity and occurrence of lifetime maximum intensity were calculated in the box. By shifting the box at a 1° interval in latitude and longitude (far less than the size of the window), gridded data with a 1º horizontal resolution were constructed, Page  6

Best-track Datasets RSMC JTWC HKO CMASTI Record period 1951‒present 1945‒present 1961‒present 1949‒present Sustained time 10 minutes 1 minute 2 minute Multiplicative factor ‒ 0.88 [Knapp and Kruk, 2010] 0.871 Time interval of record 6 hours (3 hours near Japan) Unit Knot m s ‒1 In this study, all available best-track datasets were utilized. Some differences among the datasets were standardized. The analyses were confined to the period 1977–2010. Page  7

Tropical Cyclone Intensity Change Contour: averages of trends Color shades: number of datasets that showing statistically significant trend Dots: All datasets represent same signs of trends Consistent intensity increase in the subtropical and mid-latitude ocean while consistent decrease in the tropical ocean. (m s‒1 decade‒1) (m s‒1 decade‒1) Mean UW/NCDC RSMC JTWC HKO STI SJ +0.97 +1.39* +0.64 +1.54* +1.05 +0.66 TP ‒2.35* ‒0.81 ‒2.28* ‒2.00* ‒2.75* ‒2.36* * denotes that the trend is statistically significant at the 90% confidence levels. Page  8

Change of Location of Maximum Wind Occurrence Contours: climatological distribution Colors: averages of trends Dots: three of datasets show statistically significant trends The location of maximum wind occurrence has been shifted toward the East Asian continent. This can make stronger tropical cyclones strike the East Asia if maximum intensity does not weaken. (decade‒1) Page  9

Changes of Maximum and Landfall Intensity (m s‒1 decade‒1), asterisks indicates the trend is statistically significant at the 95% levels RSMC JTWC HKO STI UW /NCDC East Asia Maximum intensity before landfall −0.04 +2.42* −0.77 −0.62 +0.01 Landfall intensity +0.58 +1.81* +0.92 +0.79* +0.56 Linear trends of TC maximum intensity do not represent notable decreasing tendencies. Thus, because the maximum wind occurrence has approached East Asia and its intensity rarely changed, much more intense TCs can strike East Asia during a recent decade. All of the TC datasets display consistent positive trends. However, some of them are not statistically significant at the 95% confidence levels. This may be because of the genesis shift. Page  10

Changes of Genesis Location Contours: climatological distribution Colors: averages of trends Dots: three of datasets show statistically significant trends The increase of genesis over the northern South China Sea may be a factor to reduce maximum intensity due to the shortened lifetime over ocean. (decade‒1) Page  11

Changes of Intensification Rate Contour: averages of trends Color shades: number of datasets that showing statistically significant trend Dots: All datasets represent same signs of trends Although genesis location shifted toward land, enhanced intensification rate could rapidly strengthen TC and help TC to sustain its intensity longer. Page  12

Changes of Maximum and Landfall Intensity (m s‒1 decade‒1), asterisks indicates the trend is statistically significant at the 95% levels RSMC JTWC HKO STI UW /NCDC Southeast Asia Maximum intensity −1.22 −0.51 −2.16* −1.56* −0.99 Landfall intensity −0.43 +0.63 −0.45 +0.08 −0.40 Northeast Asia +0.24 +3.55* +0.45 −0.54 +0.52 +1.29* +2.35* +2.04* +1.01* +1.43* Here, we divide East Asia into Northeast and Southeast Asia based on 25˚N. In the Southeast Asia where main genesis region is very close, the shifted genesis location may strongly affect the maximum intensity, but not in the Northeast Asia. In Southeast Asia, the maximum intensity trends are consistently negative although they are not significant. Otherwise, the landfall intensity trends are less negative or even slightly positive. In the Northeast Asia, there are no significant negative trends in maximum intensity. However, all of the landfall intensity trends are significantly positive. Page  13

Recent Changes of Thermodynamic and Dynamic Conditions (ºC decade‒1) (m s‒1 decade‒1) Along the coastal seas The east of the Philippine Sea Thermodynamic condition Favorable Weakly Favorable Dynamic condition Unfavorable Page  14

Tropical Convection and Wave Propagation Page  15

Model results: Tropical Convection and Wave Propagation Page  16

Summary : Recent maximum wind occurrence : Old maximum wind occurrence Favorable Unfavorable La-Nina-like warming  Enhanced Walker Circulation & PNA-like wave propagation  Changing Dynamic Conditions within the basin. More TCs reach their maximum intensity closer to the East Asian continent, which leads to stronger landfall intensity, particularly, over the Northeast Asia. Over the Southeast Asia, because of genesis effect, landfall intensity has not been intensified. Page  17

Thank you Park, D.-S. R., C.-H. Ho, J.-H. Kim*, and H.-S. Kim (2013) Spatially inhomogeneous trends of tropical cyclone intensity over the western North Pacific for 1977‒2010, Journal of Climate, 26, 5088–5101, doi:http://dx.doi.org/10.1175/JCLI-D-12-00386.1. Park, D.-S. R., C.-H. Ho*, and J.-H. Kim (2014) Growing threat of intense tropical cyclones to East Asia over the period 1977–2010, Environmental Research Letters, 9, 014008, doi:10.1088/1748-9326/9/1/014008. [Research highlight in Nature on 23 Jan 2014 - Climate change: Strong storms shift landwards, Nature, 505, 457, doi:10.1038/505457c]. Page  18