A research team led by Dr. Chia Chou at the Research Center for Environmental Changes (RCEC) recently reported observational evidence that the difference in precipitation between wet and dry seasons has increased worldwide over the past three decades. The team analyzed global precipitation data spanning 1979–2010 for changes in seasonal precipitation. They found a trend towards wet seasons becoming wetter and dry seasons becoming drier — in accordance with some climate models — and also that the trend is largely defined by an increase in precipitation during wet seasons. The study was published online in Nature Geoscience.
Global temperatures have risen over the past few decades. The water vapor content of the atmosphere has increased as a result, strengthening the global water cycle. This has led to wet regions getting wetter and dry regions getting drier. Climate model simulations suggest that similar intensification of existing patterns of rainfall may also apply to the seasonal cycle of rainfall.
In their study, the team from the RCEC analyzed regional and global trends in seasonal precipitation extremes between 1979-2010 using a number of observational data sets. They found that globally, the annual precipitation range has increased, mainly due to wetter wet seasons. Although the magnitude of the shift was not ascertainable, largely owing to limitations inherent in the data sets used, the signs of the tendency were robust. The results were consistent with climate model simulations for the same period including the results of the team’s own previous study of climate model simulations. On a regional scale, the tendency for wet seasons to get wetter occurs over climatologically rainier regions and the tendency dry seasons to get drier is seen in drier regions. This implies a more extreme seasonal cycle of rainfall, occurs not only in as a global average but also on a regional scale.
Unlike global warming, changes in rainfall vary across regions, with increases in precipitation in some locations and decreases in others. This strong spatial variation means that it is difficult to accurately project future precipitation changes on a regional basis. Understanding the patterns of change in rainfall and the underlying mechanisms is a way to improve the accuracy of projection. A more extreme seasonal precipitation cycle in the future could have implications for drought and flood frequency, even if the overall amount of precipitation remains steady. These changes may have a potentially severe impact on local hydrology, and further influence the ecosystem and human society. As precipitation changes are usually evaluated from annual mean changes, seasonal changes in precipitation may have been overlooked until now.
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