Two new papers have appeared in the peer reviewed literature on recent trends in major hurricanes. It will be informative to see if the media include these studies in their news on the upcoming hurricane season.
The articles are in Geophyiscal Research Letters
“Trends in global tropical cyclone activity over the past twenty years (1986–2005)” by Philip J. Klotzbach (May 20 2006). (This paper is also weblogged at Prometheus on May 1 2006).
The abstract reads,
“The recent destructive Atlantic hurricane seasons and several recent publications have sparked debate over whether warming tropical sea surface temperatures (SSTs) are causing more intense, longer-lived tropical cyclones. This paper investigates worldwide tropical cyclone frequency and intensity to determine trends in activity over the past twenty years during which there has been an approximate 0.2°–0.4°C warming of SSTs. The data indicate a large increasing trend in tropical cyclone intensity and longevity for the North Atlantic basin and a considerable decreasing trend for the Northeast Pacific. All other basins showed small trends, and there has been no significant change in global net tropical cyclone activity. There has been a small increase in global Category 4–5 hurricanes from the period 1986–1995 to the period 1996–2005. Most of this increase is likely due to improved observational technology. These findings indicate that other important factors govern intensity and frequency of tropical cyclones besides SSTs.”
The text included the statement,
“These findings are contradictory to the conclusions drawn by Emanuel  and Webster et al. . They do not support the argument that global TC frequency, intensity and longevity have undergone increases in recent years. Utilizing global ‘‘best track’’ data, there has been no significant increasing trend in ACE and only a small increase (~10%) in Category 4–5 hurricanes over the past twenty years, despite an increase in the trend of warming sea surface temperatures during this time period.”
The second paper is “Sea-surface temperatures and tropical cyclones in the Atlantic basin” by Patrick J. Michaels, Paul C. Knappenberger and Robert E. Davis (May 10 2006).
The abstract reads,
“Whereas there is a significant relationship between overall sea-surface temperature (SST) and tropical cyclone intensity, the relationship is much less clear in the upper range of SST normally associated with these storms. There, we find a step-like, rather than a continuous, influence of SST on cyclone strength, suggesting that there exists a SST threshold that must be exceeded before tropical cyclones develop into major hurricanes. Further, we show that the SST influence varies markedly over time, thereby indicating that other aspects of the tropical environment are also critically important for tropical cyclone intensification. These findings highlight the complex nature of hurricane development and weaken the notion of a simple cause-and-effect relationship between rising SST and stronger Atlantic hurricanes. ”
The text of the article includes the statement,
“Our results show that SST plays a relatively minor role in the observed characteristics of tropical storms and hurricanes in the North Atlantic Basin. As such, other factors must be involved in the increase in tropical cyclone activity recorded during the post-1994 Atlantic hurricane seasons. The full reason behind these observed changes remain an area of active scientific inquiry. We therefore recommend a cautious approach to assigning an underlying cause in this complex system.”
The criteria for tropical cyclone development and intensification were summarized in the following books:
Pielke, R.A., 1990: The hurricane. Routledge Press, London, England, 228 pp.
Pielke, R.A., Jr. and R.A. Pielke, Sr., 1997: Hurricanes: Their nature and impacts on society. John Wiley and Sons, England, 279 pp.
Chapter 3 of the second book discusses the following topics;
CHAPTER 3 Tropical Cyclones on Planet Earth 3.1 Life of a Hurricane 3.1.1 Birth and growth 3.1.2 Maturity 3.1.3 Decay 3.1.4 Criteria for development and intensification of a tropical cyclone 3.2 Special Cases of Development and Intensification 3.3 Geographic and Seasonal Distribution 3.3.1 Origin 3.3.2 Movement 3.3.3 Tropical cyclones in the Atlantic Ocean Basin.
A major conclusion from the analysis of major hurricanes that are summarized in these books is that vertical wind shear is the critical requirement to permit hurricanes to reach their maximum possible intensity for a given sea surface temperature. As easily seen on any analysis of tropical sea surface temperatures (e.g.see) very warm temperatures that permit major hurricanes are common over vast areas of the coean. That we seldom achieve such intensity is a result of the need for an optimal connection between low level moist inflow and the anticyclonic outward aloft. Even relatively weak vertical wind shear can disrupt this connection.
To focus on sea surface temperatures, rather then the entire synoptic environment of the tropical cyclone in multi-decadal trend assessments is another example of where the icon of the surface temperature as THE climate metric, unfortunately, persists.