environmentalresearchweb, October 18, 2011
As the Earth becomes warmer, tropical cyclones or typhoons are expected to behave differently. For example, increased water vapour in the air and warmer sea-surface temperatures lead to more intense tropical cyclones. However, most climate models predict that the total number of tropical cyclones, including both weak and intense, will decrease.
While much attention has been focused on how strongly and how often tropical cyclones will occur, when they will form is relatively little studied. In our new work, our team from Academia Sinica, Chinese Culture University, and Chinese Academy of Sciences has tried to examine changes in the seasonal variation of intense typhoons (Category 4 and 5) over the western North Pacific, and particularly in the number of intense typhoons in early summer.
Most intense typhoons form between July and November, with the maximum occurring in September and October (see figure a in ‘Seasonal cycles’). They are rare in May. Before 2000, intense typhoons seldom occurred in May – only about one per decade was recorded. Since 2000, however, they have appeared more frequently – almost one per year (figures b and c in ‘Seasonal cycles’).
Intense typhoons usually cause severe damage to the local economy and a massive loss of human life. That intense typhoons could now occur in early summer suggests that people ought to start preparing for such natural disasters much earlier in the year, so as to reduce potential damage and suffering. The shift to early summer for intense typhoons could also affect the local ecosystem – for instance, rice heading usually occurs in May.
Typhoon activity is strongly influenced by the large-scale environment. Our study shows that the atmosphere has become more favourable for intense typhoons since 2000, something that is consistent with the increase in measured tropical cyclone “genesis index,” which is used to estimate the impact of the large-scale environment on the formation of typhoons. Changes in the large-scale environment include warmer sea-surface temperatures, higher sea-surface height, larger upper-ocean heat content, weaker vertical wind shear, more water vapour in the troposphere, and higher mid-troposphere relative humidity. Of these changes, those associated with water vapour are more important than the others.
Is the change in the large-scale environment related to global warming? This is an interesting question that we would like to answer in the near future.