May 10, 2006

Major Hurricanes: More, but not Stronger

Filed under: Hurricanes

We have a new paper in Geophysical Research Letters in which we find that it is plausible that a future rise in sea surface temperatures (SST) in the tropical North Atlantic Ocean will lead to a greater number of major hurricanes (categories 3 through 5), but that it should not lead to an increased intensity of these storms (Michaels et al., 2006).

The ultimate intensity that a storm achieves is less dependent on the underlying SST than it is on a myriad other environmental factors, such as vertical wind shear and atmospheric stability—which that are less clearly related to anthropogenic greenhouse-related changes than is SST. Our results call into question how significantly future global warming will impact Atlantic tropical cyclones and whether or not such an impact is currently detectable.

In our paper, entitled “Sea-surface temperatures and tropical cyclones in the Atlantic basin” (pdf available here ), we matched up the underlying SST with the 6-hr location and intensity of each named tropical cyclone (tropical storm or hurricane) in the North Atlantic basin (the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico) from 1982 through 2005. In this way, we could examine how the actual SST encountered by Atlantic tropical cyclones may or may not be related to the storm intensity. This direct examination is an element that has been missing from the collection of recent proclamations that rising basin-wide, seasonal-average SSTs are giving rise to stronger storms (e.g. Emanuel 2005; Webster et al., 2005; Hoyos et al. 2006).

We found several interesting things:

1) Tropical cyclones do not typically reach their maximum intensity over the water with the highest temperature. Instead, by the time the effects of a higher SST work their way into the storm’s dynamics, forward motion has usually carried it further north, over cooler waters. Figure 1 shows this. It is the contemporaneous SST and intensity reported by the National Hurricane Center for each 6-hourly tropical cyclone position. Notice that the most intense storms do not occur at the absolute highest SST.


Figure 1. The relationship between wind speed and the underlying SST for the complete set of 6-hourly observations from all Atlantic basin tropical systems from 1982 to 2005.

2) A better indicator of how strong a storm will become is the maximum SST that the storm encounters prior to reaching its maximum intensity. This relationship is shown in Figure 2. Over the full range of SSTs, the relationship between maximum storm strength and previously encountered SST is statistically significant and indicates that for each additional degree (Celsius) of warming, storms add about 6.25 mph (2.8 meters per second) to their maximum wind speed reached. But an interesting difference arises once the water reaches a bit over 83˚F. This is the threshold SST that must be met or exceeded before a storm has the potential to reach major hurricane status. Only two tropical cyclones in the past 24 years became major hurricanes without encountering water of this temperature


Figure 2. Maximum wind speed attained by the 270 named Atlantic tropical cyclones from 1982 to 2005 plotted against the maximum SST encountered prior to (or concurrent with) the maximum wind speed. Storms with sustained surface winds of at least 50 m/s (111 mph) are categorized as major hurricanes (category 3, 4, or 5 storms on the Saffir-Simpson hurricane scale). The regression line through all the data points is statistically significant.

3) Once the 83ºF threshold is met or exceeded, there is no longer any influence of SST on ultimate storm intensity. This can be seen in Figure 3. In other words, hurricanes don’t continue to get stronger as the SST increases above the threshold. Consequently, a rise in SST (whether from natural variations or increasing greenhouse gases) initially acts to increase the number of storms that encounter SSTs above the threshold (we say initially because after about 2ºC of warming, all storms will develop in a world where they encounter water above the threshold), but does nothing to influence the strength of major hurricanes.


Figure 3. Maximum wind speed and the highest SST encountered prior to (or concurrent with) reaching the maximum wind speed at temperatures greater than or equal to 28.25ºC (83˚F). The relationship is statistically insignificant.

This is an important point because of recent claims that as SSTs continue to rise (because of global warming), hurricanes will continue to grow stronger and stronger. We show, based upon observation rather than theory, that this does not happen.

How much influence have rising SSTs had on the number of major hurricanes in the Atlantic during the past 24 years? To find this, we can we divide our dataset up into the periods 1982-1994 and 1995-2005 (the time before and after tropical cyclone activity picked up in the Atlantic). During the first 13-year period, there were 71 named storms that encountered SST that exceeded the 28.25ºC threshold, and 16 of them (or 22.5%) went on to become major hurricanes. In the last 11 years, 124 storms encountered waters exceeding the threshold and 42 (33.8%) became major hurricanes. If SSTs alone where responsible to the number of major hurricanes, we would have expected that only 22.5% of the 124 storms, or 28 total storms, to have become major hurricanes. Instead, we witnessed 42. Thus, the difference between the number we expected (28) and the number we observed (42), 14 storms in all, must arise from factors other than simply SST changes.

A recent study by Hoyos et al. (2006) provides some clues as to what else may be going on. Hoyos et al. (2006) found that the amount of vertical wind shear (change in speed and direction) over the Atlantic basin has been declining. Vertical shear tends to blow storms apart, keeping them from strengthening. Also, the degree of atmospheric stability (temperature change with height) has also been declining. The less stable the atmosphere is, the more favorable it is for the development of strong storms.

Climate models run with increasing levels of greenhouse gases project that atmospheric stability should be increasing (less favorable conditions for strong storms) and they are pretty equivocal concerning vertical wind shear (most predict only small changes).

The current climate in the tropical North Atlantic is one in which one environmental variable has evolved according to climate model projections (SST) while another has changed contrary to model projections (atmospheric stability) and another (vertical wind shear) is predicted equivocally by the models. But, all three of these variables have evolved in such as way as to provide better conditions for the development of major hurricanes.

So while it is certainly possible that anthropogenic activities are playing some role in current behavior of Atlantic tropical cyclones, there are at least two other factors involved.

We emphasize this in the conclusions to our paper, where we write:

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.

We can only hope that our colleagues, as well as the press, take this advice to heart.

References:

Emanuel, K., 2005a. Increasing destructiveness of tropical cyclones over the past 30 years. Nature, 436, 686-688.

Hoyos, C.D., et al., 2006. Deconvolution of the factors contributing to the increase in global hurricane intensity. SciencExpress, March 16, 2006.

Michaels, P.J., et al., 2006. Sea-surface temperatures and tropical cyclones in the Atlantic basin. Geophysical Research Letters, 33, doi:10.1029/2006GL025757.

Webster, P. J., et al., 2005. Changes in tropical cyclone number, duration, and intensity in a warming environment. Science, 309, 1844-1846.




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