Trends in large-scale circulations and thermodynamic structures in the tropics derived from atmospheric reanalyses and climate change experiments

In recent years, the frequency of natural disasters due to tropical cyclones tends to increase, which raises questions about possible changes in tropical cyclone activity under global warming. Although recent observational and modeling studies suggest an increased intensity and a decreased frequency in the future, there is considerable uncertainty about climatological behaviors of tropical cyclones including their large natural variability.

The objective of this study is to accumulate scientific knowledge about future changes in tropical cyclone activity based on tendencies in tropical large-scale fields, which presumably are dominant factors that affect tropical cyclone activity. As the first step toward this objective, trends in large-scale tropical circulations and thermodynamic structures are investigated using multiple climate data. To examine large-scale circulations, intensities of Hadley, Walker, and monsoon circulations are defined based on velocity potential in the upper troposphere, and their climatology and interannual variations are compared. Climate data are reanalyses including ongoing Japanese data, and historical and future climate change experiments for the upcoming IPCC Fourth Assessment Report.

Time-series of the tropical circulation intensities from the reanalyses show common tendencies, such as intensification of Hadley circulation in December-February season. However, these tendencies are not necessarily consistent with results from the numerical experiments. For example, the numerical experiments demonstrate increased stability in the tropics by global warming, which, in turn, leads to abatement of Hadley circulation. Moreover, there are large differences among the reanalyses in vertical profiles of tropospheric temperature trends in the late 20th century, any of which do not well represent a typical profile in the experiments.

Although tendencies in the tropical circulations and thermodynamic structures are basic information for the projection of tropical cyclone activity, consistent results have not been attained among multiple reanalyses and numerical experiments. Reliability assessment for various aspects of these data is in progress.