Hurricane-Climate Research of Relevance to RPSEA NCAR Earth System Laboratory National Center for Atmospheric Research NCAR is Sponsored by NSF and this work is partially supported by the Willis Research Network and the Research Program to Secure Energy for America Greg Holland 1

The WMO Committee of Experts Established by WMO to provide an expert update on the IPCC and a previous WMO assessment; Membership – Tom Knutson and John McBride (joint chairs) – Johnny Chan (Hong Kong), Kerry Emanuel (MIT), Greg Holland, Chris Landsea (NHC), Isaac Held (GFDL), James P. Kossin (Wisconsin), A. K. Srivastava (India) and Masato Sugi (Japan). 2Holland Lit Survey RPSEA 0310

WMO Main Findings It remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes; Future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100; Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%; Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre; For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies. 3Holland Lit Survey RPSEA 0310

East Atlantic Warming Causes and Consequences Holland Lit Survey RPSEA Genesis Locations 25%

Eastern Atlantic Warming Holland Lit Survey RPSEA Our simulations predict eastern Atlantic warming exceeding the global average over the next years.

Eastern Atlantic Warming Causes Amato et al Science 2009: – Suggested that 69% of the recent upward trend in eastern tropical Atlantic Ocean temperatures is the mixed layer’s response to regional variability in aerosols. Climate models consistently get the eastern Atlantic warming without aerosol influence; – CF Santer et al (2006) who found that 67% of the warming was due to anthropogenic effects. Conclude that this is most likely due largely to global warming, but there is uncertainty. Modis 6Holland Lit Survey RPSEA 0310

Western Atlantic-GOM SST For western Atlantic and GOM, a number of studies have indicated a strong impact of the AMO (Goldenberg et al 2001; Knight et al 2005). However Mann and Emanuel (2006) suggest that the major factor may be anthropogenic aerosol emissions (e.g. SO2), which temporarily countered the greenhouse warming. Probable that the lack of increase over GOM and western Atlantic substantially due to anthropogenic aerosols. The Eastern Atlantic warm anomaly is projected to increase in our simulations (cf Vecchi et al 2008 on hurricane impact). 7Holland Lit Survey RPSEA 0310

East Atlantic Warming Consequences (Holland 2007) (Wu et al 2010) Several studies have claimed the response is due entirely to improved observing systems (e.g. Landsea 2007) 8Holland Lit Survey RPSEA 0310

East Atlantic Warming Consequences ctd Vertical Wind-Shear Changes SST Changes Circulation Changes (Wu et al 2010) 9Holland Lit Survey RPSEA 0310

Summary: Hurricane Changes (WMO) “It remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes” (Knutson et al 2010); We consider that there has been a recent increase and that this has a substantial anthropogenic signal. Holland Lit Survey RPSEA

Summary: Hurricane Changes (WMO) “Future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100”; “….higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre”; We concur with both statements based on the growing amount of physical process evidence that is emerging. Holland Lit Survey RPSEA

Summary: Hurricane Changes (WMO) “Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%”; At least some of the model results appear to be due to model bias. For now, we suggest keeping Atlantic activity steady at current rates, but there is likely to be important changes in location and impact. “For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies”. We agree, and this becomes even more pronounced for smaller regions such as the GOM. However, there is still useful information in the current predictions. Holland Lit Survey RPSEA

Finally: Relationship between EA and Landfall Linear (rank) correlations between SST and hurricane numbers are around 0.5 (0.5) for the basin, and around 0.15 (0.2) for landfalling hurricanes (Laepple et al 2007); There is a strong relationship between MDR SST and basin hurricanes, but only a weak relationship for total hurricane numbers and no relationship at all for intense hurricane numbers (Binter et al 2007, our work). (Laepple et al 2007) Conclude that SST increases in the Atlantic are not related well to landfall statistics….but for GOM and eastern Atlantic…??? Genesis Locations 25% 13Holland Lit Survey RPSEA 0310

WMO Main Findings It remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes; Future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100; Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%; Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre; For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies. 14Holland Lit Survey RPSEA 0310