Recent intense hurricane response to global climat

Authors: Greg Holland Cindy L Bruyère

Publish Date: 2013/03/15

Volume: 42, Issue: 3-4, Pages: 617-627

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Abstract

An Anthropogenic Climate Change Index ACCI is developed and used to investigate the potential global warming contribution to current tropical cyclone activity The ACCI is defined as the difference between the means of ensembles of climate simulations with and without anthropogenic gases and aerosols This index indicates that the bulk of the current anthropogenic warming has occurred in the past four decades which enables improved confidence in assessing hurricane changes as it removes many of the data issues from previous eras We find no anthropogenic signal in annual global tropical cyclone or hurricane frequencies But a strong signal is found in proportions of both weaker and stronger hurricanes the proportion of Category 4 and 5 hurricanes has increased at a rate of ~25–30  per °C of global warming after accounting for analysis and observing system changes This has been balanced by a similar decrease in Category 1 and 2 hurricane proportions leading to development of a distinctly bimodal intensity distribution with the secondary maximum at Category 4 hurricanes This global signal is reproduced in all ocean basins The observed increase in Category 4–5 hurricanes may not continue at the same rate with future global warming The analysis suggests that following an initial climate increase in intense hurricane proportions a saturation level will be reached beyond which any further global warming will have little effectRecent community consensus Knutson et al 2010 Intergovernmental Panel on Climate Change—IPCC 2012 has concluded that it is likely that the frequency of intense hurricanes will increase with future anthropogenic climate change The increases are substantial approaching a doubling in frequency of Cat 4 and 5 hurricanes based on the Saffir–Simpson classification Simpson and Rielh 1981 for each °C in global warming Bender et al 2010 IPCC 2012 Done et al 2012 IPCC 2007 also concluded that the current ‘warming of the climate system is unequivocal’ If we accept these two statements then it logically follows that there should already be an observable increase in intense hurricanes Yet IPCC 2012 concluded that ‘There is low confidence in any observed longterm ie 40 years or more increases in tropical cyclone activity’ based largely on potential errors in the observed data Here we investigate this apparent anomaly and find that there has been an increase in the proportion of intense hurricanes relative to all hurricanes and that is strongly related to an Anthropogenic Climate Change Index ACCI The index is developed in Sect 2 Sect 3 examines the recent hurricane signal and discusses some of the consequences and our conclusions are in Sect 4Ensemble simulations of annualmean global surface temperature with red and without blue anthropogenic gas forcing together with the observed global surface temperatures black thin lines indicate ensemble extremes a Based on CMIP3 adapted from Fig 2d of Meehl et al 2004 b based on CCSM4 from Fig 2 of Meehl et al 2012a ACCI calculated from the differences between the ensemble annual means in Fig 1 with the solid line being a cubic fit to the annual values b Relationship between the ACCI and annual global tropical SST anomalies ±30° Lat together with linear trends both of the indicated variances have p  001The ACCI data in Fig 2a indicate that consideration of the impact of anthropogenic climate change should be limited to the last four decades This is quite different to a number of other studies that have assumed an essentially linear warming over the past 100 years or so see eg Knutson et al 2010 for hurricane implications If greenhouse gases alone are considered then an extended period of nearly linear warming is justified but anthropogenic impacts on the climate also arise from a range of other contributors that have counteracted the pure greenhouse gas warming until recently see Meehl et al 2012 for a detailed discussionFor the global hurricane analysis we choose to focus on the slightly shorter period from 1975 to 2010 as this is a reasonably homogeneous period of global satellite data going back to 1960 does not affect our conclusions CMIP5 Coupled Model Intercomparison Project 5 data were not available at the analysis time so to enable use of the full tropical cyclone record we use the CMIP3 ACCI up to 2000 and extend this to 2010 using the annual changes in CCSM4 A test using linear extrapolation of the CMIP3 data to 2010 produced essentially the same resultsFigure 2b compares the ACCI with the observed annual tropical 30° north and south sea surface temperature SST anomalies relative to the mean for 1975–2010 from the merged Hadley Centre and NOAA’s optimum interpolation OI SST data set Hurrell et al 2008 They track each other closely with the ACCI explaining 50  of the observed tropical SST variance p  001 Note that the slope in Fig 2b is 1 since the ACCI values have an additional component arising from the net cooling in the natural forcing simulations Fig 1 compared to the observationsFocusing on the relationship with ACCI from 1975 removes much of the hurricane analysis difficulty that has been experienced with issues such as changing observing systems and analysis approaches prior to 1975 eg Landsea et al 2006 Knutson et al 2010 Holland and Webster 2007 The approach of using a global index also is consistent with the findings of Grinsted et al 2012 who showed that North Atlantic landfalling hurricane variations are more closely related to global mean SST than they are to local SST or climate indices We shall show that using a shorter period of record is not a concern as the signal is large and significant

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