ESSL Holland, Hawaii On the Changing Characteristics of Atlantic Hurricanes Summary: Atlantic Changes and Relationship to SST Natural Variability vs Climate Change Greg Holland Based on Holland and Webster, 2006, Proceedings of the Royal Society of London (revised)

ESSL Holland, Hawaii The times they are a changin! The data base is biased (Landsea et al), but Kossin et al (2006) Models and theory predict a much smaller trend, hence the data are wrong! (Landsea etal 2006), but Michaels et al 2006 state the opposite? All we are seeing is natural variability (NOAA) The impacts are minor compared to demographic and related changes (Pielke et al) Criticisms Webster et al 2005 (also Emanuel 2005) Bob Dylan

ESSL Holland, Hawaii Focus on the North Atlantic and Hurricane Frequency Eastern Atlantic Western Atlantic Gulf 25 o N 2005 Hurricane Season (NASA)

ESSL Holland, Hawaii Atlantic Tropical Cyclone Trend Satellites 1995 TC1 TC3 TC2 Aircraft Recce

ESSL Holland, Hawaii TC Number-SST Relationship Changes between TC climate regimes are accompanied by similar changes in eastern Atlantic SSTs; SST leads cyclone changes and explains >60% of the variance in TC numbers (due entirely to regime changes). TC1 TC3 TC2 9-year running mean

ESSL Holland, Hawaii TC Climate Regimes 5-y running mean ~150% increase in TC numbers 1 TC for each 0.1 o C increase in SST 1 Hurricane for each 0.2 o C increase in SST

ESSL Holland, Hawaii Hurricane Proportions Since 1900 and esp. since 1945 hurricane proportions have been remarkably steady with only a few % variation; High hurricane proportion prior to 1900 and occasional year with only hurricanes up to 1940 are indicative of a tendency to miss weaker systems in earlier years.

ESSL Holland, Hawaii Major Hurricane Variation Independent of Hurricane or Tropical Cyclone Variation Period before 1900 dominated by data issues; No evidence of any trend in proportions since 1900; Remarkable, out of phase oscillation in major and minor hurricanes with multi- decadal period.

ESSL Holland, Hawaii However While there is no trend in the proportion of major hurricanes, there is a distinct trend in the number of major hurricanes.

ESSL Holland, Hawaii Natural Variability vs Climate Trend? We address the causal relationship: SST signal Relationship between SST and Hurricanes Natural variability or Climate Trend (After Curry Webster and Holland 2006)

ESSL Holland, Hawaii SST-Hurricane Relationship Webster et al (2005), Emanuel (2005) and Hoyas et al (2006) noted a strong global relationship with climatological intensity changes. Gray (1990), Landsea et al (1999), Goldenberg et al (2001), Vitart and Anderson (2001) all find a strong relationship between eastern North Atlantic SSTs and tropical cyclones. We have shown that Eastern North Atlantic SSTs explain >60% of smoothed variance in Atlantic Tropical Cyclones since 1905 (due entirely to regime changes); The SST-TC relationship is not direct, but arises from related atmospheric environmental changes, such as vertical wind shear (Delworth, 2006).

ESSL Holland, Hawaii Surface Temperature Variability Volcano Ozone Sulfate Solar There is no known natural forcing mechanism that can explain the surface temperature increases since 1960 (Meehl et al 2004, 2006). Santer et al (2006) obtain similar result for the Atlantic tropical cyclone development region “Natural” Forcing Greenhouse Gases

ESSL Holland, Hawaii First Conclusion The strong relationship between increases in storm and hurricane numbers and increases in SSTs leads to the inescapable conclusion that the majority of enhanced current TC and hurricane activity is a direct result of greenhouse warming. Why the inconsistency with theoretical and climate model projections (Henderson-Sellers et al 1998, Knutson et al, several studies)? Answer: The formation and intensification regions have moved!

ESSL Holland, Hawaii North Atlantic TC Location Changes SST variations explain 69% of the variance in equatorial (easterly wave) developments. 9-y Mean

ESSL Holland, Hawaii Conclusion North Atlantic tropical cyclone (and major hurricane) frequency changes due to anthropogenic climate change is not in the future, a significant trend appears to have already occurred. This is consistent with the climate change modelling study by Oochi et al (2006).