Earlier this year, we posted two discussions on the association between climate change and storms: Storms and Climate Change and Some recent updates. I will use the abbreviation TC here in the loose meaning of a tropical cyclone. These posts discussed the high number of TCs during the previous hurricane season and an essay on the relation between TCs and climate change. The uncertainty surrounding trends in storminess was underlined, and a point was being made about this subject being controversial.
Whereas some meteorologists believe that there is not a link between gobal warming and storms, there has also been a recent model study on how the storm statistics can be affected by a global warming that we failed to mention in the previous discussion. Furthermore, a new analysis published in Nature by Kerry Emanuel (‘Increasing destructiveness of tropical cyclones over the past 30 years’) points to a high correlation between the power of the TCs and the sea surface temperature (SST). This paper is also discussed in The New Scientist. He suggests that a likely consequence of a future warming is an upward trend in the destructive potential of TCs. Emanuel argues that a net power dissipation is a better indicator of TC threat than their frequency or intensity alone.
The conclusion of Emanuel at first sight appears to contrast with the a recent statement from NOAA that a 20-30-year reoccurring cycle is the dominant climate factor that controls Atlantic hurricane activity and that any potentially weak signal associated with longer-term climate change appears to be a minor factor. But, in fact, Emanuel acknowedged the lack of a trend in the frequency of the TCs, but finds a trend in their their destructive power which is related to their intensity and duration.
There are also some further differences. Chan and Liu (2004) argue that current models are not yet sufficiently good for addressing the question regarding global warming and typhoons (A typhoon is technically the same as a hurricane, the difference being that they form over the western Pacific or the Indean Ocean). But the GFDL study is based on a state-of-the-art high-resolution model that is more appropriate for hurricane studies and provides important evidence suggesting that climate change may have an effect on the TCs. Chan and Liu (2004) point to a lack of positive correlation between SSTs in the western part of the tropical pacific and the typhoon activity in the western North Pacific. They argue that the typhoon activity is related to El Ninos and that higher moist static energy provides conditions favourable for TCs. The atmospheric flow will also have an influence, as strong vertical wind shear can inhibit cyclogenesis (spawning of cyclones). I will not draw their analysis into doubt other than that I believed that they incorrectly framed the question. Therefore, I believe that there could be a different interpretation of their results. The SSTs they examined were from the ‘warm pool’ – the region with the highest temperatures in the world (~30degC) and well above the critical threshold of ~27 degC. SSTs in this region are not strongly positively correlated with ENSO. However, the region of high temperatures expands into central and eastern parts of the tropical Pacific during an El Nino resulting in an increased area of SST higher than the critical threshold value, and this aspect is in my opinion crucial to the interpretation. The tropical Typhoon frequency may be sensitive to the area with high SST (above ~27deg C). The clear seasonality in TCs (“hurricane season”) with highest activities during the summer is one of the strongest pieces of empirical evidence that higher temperatures give more favourable conditions for tropical cyclones (After all, TCs only form in the warm tropics…).
One argument is as follows: as the globe warms, the area with high temperatures will increase, increasing the area on which tropical cyclones can spawn. However, climate model studies differ in their account on the tends in TC frequencies. I’m not aware of of any study which correlates the TC activity with the area of high temperatures, but such an analysis would perhaps be more appropriate than just correlating with the SST within one specific region. The notion of more intense TCs with higher temperatures is nevertheless supported by model studies from GFDL.
There have been TCs already in this hurricane season. According to Gerry Bell from NOAA’s, seven TCs during June and July is a new record. NOAA also forecasts a high hurricane activity for the remainder of the season, and time will show if this season will match the 2004 in terms of number of tropical cyclones. But we do not yet know if this constitutes an emerging trend – only hindsight after decades will tell.
Thus, the bottom line so far seems to be that SSTs play a role for the TC statistics (destructive power). The most dominant change in TC activity is related to natural cycles (on inter-annual and inter-decadal time scales), which we would call ‘noise’ in a trend analysis in which the trend would be the ‘signal’. There is little evidence of a trend in the TC frequency, however there are some indications of a trend in the destructive capability of the TCs. Conlusions can differ depending what you look at: frequency, intensity, duration of storms (life times) or destructive potential. Emanuel suggests that the recent upturn in the tropical cyclones’ destructiveness can have two explanations: increased level of intensity (consistent with discussions on Storms and Climate Change and Some recent updates) or longer cyclone life times.
Note that when it comes to mid-latitude storms, there is a different story because they are affected by different types of instabilities (baroclinic) and different conditions.
Chan and Liu (2004), Global Warming and Western North Pacific Typhoon Activity from an Observational Perspective, J.Clim.,
Kerry Emanuel (2005), Increasing destructiveness of tropical cyclones over the past 30 years, Nature, online publication; published online 31 July 2005 | doi: 10.1038/nature03906