An increase in extreme weather events is often associated with climate change. However, according to a new study, the number of tropical cyclones declined in the 20th century. The research team also presents a possible explanation for the effect.
The number of tropical cyclones such as hurricanes and typhoons decreased by about 13 percent in the 20th century compared to the second half of the 19th century. This is the result of a recent study in which researchers combined weather data with computer simulations. However, the resolution of the simulations is not high enough to make statements about the development of the intensity of the storms, they write in the journal “Nature Climate Change”. The trend can be explained with climate change, which has become particularly apparent since around 1950.
Because there are large natural fluctuations in the currents in the oceans and in the atmosphere, it is generally difficult to identify trends in tropical cyclones, emphasize the scientists led by Savin Chand from Federation University in Ballarat, Australia. Therefore, previous studies have shown either a tendency towards more or fewer tropical cyclones, depending on the calculation model. ‘Before the advent of geostationary weather satellite monitoring in the 1970s, historical global records of tropical cyclones were more prone to disruption and sampling problems,’ the scientists write. They are therefore considered problematic for trend analyzes of climate change.
Chand’s team focused on a unified measurement, namely the air pressure near the sea surface. Using a new hurricane detection and tracking scheme, the researchers developed a unified data set for the period 1850 to 2010 to identify trends spanning more than a century and a half. They discovered that in four out of seven tropical sea regions the number of tropical cyclones increased from 1850 to 1900. During this period, numbers remained about the same in the South Pacific, but decreased in the western North Pacific and North Atlantic.
The North Atlantic is the one exception to the 20th-century trend: while all oceanic regions saw tropical cyclone numbers decrease from 1900 to 2010, researchers in the North Atlantic recorded increasing numbers of these cyclones over the same period. If you include the period 1850 to 1900, however, it becomes apparent that during this period there were an average of around 15 hurricanes per year, while in the first half of the 20th century the value was more like ten to eleven and thus unusually low. The upward trend since about 1960 therefore corresponds more to an approximation to the level of 1850 to 1900 than to an increase in times of climate change.
Chand’s team also has an explanation for the declining trend in tropical cyclones: the weakening of large-scale air circulation between the tropical and subtropical climate zones. According to the researchers, higher temperatures at the sea surface cause more evaporation, which leads to less buoyancy of the water-saturated air masses. As a result, more dry air is entrained upwards with vertical air movements, which makes the formation of tropical cyclones less likely; because the hurricanes need moist, warm air.
“These results place the observed global decline since 1990, which is dominated by North Pacific trends, in a longer-term context,” writes Alexander Baker of the University of Reading (UK) in a comment, also in Nature Climate Change. The authors of the study contrasted a period of exclusively natural fluctuations (1850 to 1900) with a period of natural fluctuations and the influence of increased greenhouse gas emissions (1900 to 2010). In doing so, they showed the influence of man-made climate change on the frequency of tropical cyclones.
