“Given this state of affairs, projections of changes in [tropical cyclone] intensity due to future global warming must be approached cautiously.”
This is the concluding sentence of a just-published article by University of Wisconsin-Milwaukee’s Kyle Swanson in which he carefully examined the historical relationship between sea surface temperatures and tropical cyclone intensity in the Atlantic and western Pacific ocean.
Swanson justified his research efforts, well summarizing the current state of things (including references):
Recent studies have found an apparent increase in the proportion and number of tropical cyclones (TCs) that become intense [Webster et al., 2005] along with links of this increase to positive sea surface temperature anomalies [Emanuel, 2005; Hoyos et al., 2006] and possibly global warming [Trenberth, 2005]. However, the sensitivity of TCs to changes in sea surface temperature (SST) remains controversial [Landsea et al., 2006; Shapiro and Goldenberg, 1998], as modeling and theoretical studies suggest only small changes to TC intensities given the observed 0.5ºC SST warming that has occurred since the 1970s [Emanuel, 1988; Knutson et al., 2001]). Further, satellite reanalysis suggests no increase in the fraction of intense TCs outside the North Atlantic basin [Kossin et al., 2007]. Trends in TC intensity are difficult to discern, as statistics are inherently noisy due to fluctuating storm numbers and life spans. As the theory underlying TC intensities specifically predicts only the maximum potential intensity, it is necessary to control for these other factors if the response of the TC intensity to changes in SST is to be understood.
In looking for the primary drivers of tropical cyclone intensity, Swanson found that tropical cyclones do not always react the same way to changes in local sea surface temperatures (SSTs). During some periods, like the mid-1970s through the present, an increase in the percentage of stronger hurricanes has accompanied rising SSTs, but during other earlier periods, the apparent relationship was not so clear. In fact, overall, Swanson found no statistically significant correlation between SSTs and average tropical cyclone intensity in either ocean basin during the 1950 to 2005 period of his study.
Consequently, here is what Swanson had to say about recent papers claiming to have found an definitive link between rising SSTs, tropical cyclone intensity (and anthropogenic global warming):
[T]he period 1975–2004 examined by Webster et al.  is fortuitous; it captures the minimum of [tropical cyclone, TC] intensities during the 1970s and the subsequent increase in TC intensities. However, the post-1975 upward intensity trend over this period does not appear to mark a fundamental shift in TC intensity behavior; this behavior is still within the upper bound set during the 1950s in both the NATL and WNPAC basins.
Instead of a relationship with local SST variability, Swanson found that tropical cyclone intensity was much more closely linked to local SST anomalies—that is, the difference between the SST in the primary tropical cyclone formation regions in the Atlantic and the western Pacific, and that of the average SST in the tropics as a whole. For instance, during the times when the central tropical Atlantic SST were higher than the average SST across the entire tropics, Atlantic hurricane activity and intensity levels were above normal, conversely, when the Atlantic SSTs were below the tropical average, hurricane activity was diminished. Figure 1, taken from Swanson (2007), depicts this relationship. Average hurricane intensity was as high in the 1950s and early 1960s as it has been recently (Figure 1b and c), despite the fact that SSTs were more than 0.5ºC lower in the main cyclone development regions in the Atlantic and western Pacific in the 1950s than presently (Figure 1a). But, when compared to the average SST in all the world’s tropical areas, the 1950s and early 1960s were relatively warm in the cyclone development regions in both the Atlantic and western Pacific.
Figure 1. (a) Sea surface temperature (SST) anomalies for the Atlantic and western north Pacific main development regions (MDR), along with the tropical mean SST anomaly. (b) Tropical cyclone intensity anomaly for the Atlantic along with the Atlantic relative MDR SST anomaly. (c) Same as (b), but for the western north Pacific. (from Swanson. 2007).
Swanson suggests that this type of behavior “is consistent with the tendency for regions of anomalously warm SSTs to cannibalize moist convection in the tropics, most apparent in the global-scale reorganization of convective behavior that occurs during El Niño events.” In other words, warm pools of water, rather than uniformly warm water, are more conducive to firing up thunderstorm complexes that can lead to tropical cyclone formation.
Here is how Swanson sums up this finding, including its implication for predictions of global warming-induced changes to tropical cyclone intensities:
Finally, the apparent sensitivity of TC intensity to relative MDR SST anomalies is itself troublesome. How these relative SST anomalies will change under global warming scenarios is unclear, as modeling relative SST anomalies is a much more difficult task than modeling SST anomalies for the tropics as a whole. As such, it is unclear whether the coincident increase in MDR SST anomalies and relative MDR SST anomalies since the mid-1970s shown in [our Figure 1, above] will continue. Given this state of affairs, projections of changes in TC intensity due to future global warming must be approached cautiously.
Swanson’s conclusions are similar to those recently reported by some folks working out of the University of Wisconsin’s more westerly Madison campus. Jim Kossin and colleagues conducted a research project (for more details of the Kossin et al., study, see here) in which they carefully constructed a homogenous tropical cyclone dataset for all the world’s ocean basins for the past 23 years. After examining their new record for trends, they concluded:
Using a homogeneous record, we were not able to corroborate the presence of upward trends in hurricane intensity over the past two decades in any basin other than the Atlantic. Since the Atlantic basin accounts for less than 15% of global hurricane activity, this result poses a challenge to hypotheses that directly relate globally increasing tropical SST to increases in long-term mean global hurricane intensity.
It certainly is beginning to seem that the more and more people look, the less and less they can find any clear relationship between rising SSTs and increased activity and intensity levels of tropical cyclones. Further, the lack of a clear understanding of the past and present relationships serves to cloud our ability to see into the future when many aspects of the tropical environment are projected to change, not simply sea surface temperatures (for more information about how these other projected changes may impede tropical cyclone development, see here).
Kossin, J.P., et al., 2007. A globally consistent reanalysis of hurricane variability and trends. Geophysical Research Letters, 34, L4815, doi: 10.1029/2006GL028836.
Swanson, K.L, 2007. Impact of scaling behavior on tropical cyclone intensities. Geophysical Research Letters, 34, doi:10.1029/2007GL030851.