In this paper, we attempt to divide the global divergent field at the upper troposphere in contributions from the Hadley, Walker and monsoon circulations, using a monthly mean velocity potential field at 200‐hPa level. First, the zonal mean of the velocity potential is analysed to represent the Hadley circulation. The deviation from the zonal mean is then divided into its annual mean and the seasonal cycle parts, which are considered to represent the Walker and monsoon circulations, respectively. The intensities of each circulation are measured by their peaks in the velocity potential field separated in each component. According to this separation, the mean intensities of the Walker, monsoon and Hadley circulations appear to be 120: 60: 40 (× 105 m2 s−1) in January and 120: 90: 45 (×105 m2 s−1) in July, respectively. Based on this simple definition, interannual variabilities of each circulation are then examined quantitatively using the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. The time series of the intensity of the Walker circulation coincides with the Southern Oscillation index (SOI), and the intensity has weakened in recent decades. That of the Hadley circulation indicates intensifying trend in boreal winter. Finally, the same analysis is applied for the model atmosphere by the Meteorological Research Institute (MRI) coupled atmosphere–ocean general circulation model (CGCM1) with a gradual increase in CO2 at a compound rate of 1% yr−1 for 150 yr. It is shown that the Hadley circulation intensifies by 40% and the monsoon circulation decays by 20% in boreal summer when the global warming has occurred in a century later. The result demonstrates that the proposed simple separation of the tropical circulation in the Walker, monsoon and Hadley components is useful, although it is not rigorous, for the initial assessment of the model response to the global warming.

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Bias corrected climate scenarios over Japan were developed using two distinct methods, namely, the cumulative distribution function-based downscaling method (CDFDM) and Gaussian-type Scaling approach (GSA). We compared spatial distribution, monthly variation, and future trends. The seasonal distribution of bias-corrected data using CDFDM closely followed the original general circulation model (GCM) outputs. GSA overestimated the amount of precipitation by 12-18 % in every season because of an unsuitable assumption on the probability distribution. We also examined the contributions of each source of the uncertainty in daily temperature and precipitation indices. For daily temperature indices, GCM selection was the main source of uncertainty in the near future (2026-2050), while different Representative Concentration Pathways Ishizaki et al., Evaluation of Two Bias-Correction Methods for Climate Scenarios over Japan 2 (RCPs) resulted in large variability at the end of the 21st century (2076-2100). We found large uncertainty using the bias-correction (BC) methods for daily precipitation indices even in the near future. Our results indicated that BC methods are an important source of uncertainty in climate risk assessments, especially for sectors where precipitation plays a dominant role. An appropriate choice of BC, or use of different BC methods, is encouraged for local mitigation and adaptation planning in addition to the use of different GCMs and RCPs.

In this study, intensities and trends of Hadley, Walker, and monsoon circulations are compared for the IPCC 20th Century simulations and for 21st Century simulations, using the upper tropospheric velocity potential data. As a result, we showed significantly weaker biases in Walker and monsoon circulations for the JJA climate in the IPCC 20th Century simulations. The dispersal in the scatter diagram of the model biases is considerably large. The same analyses are applied for the IPCC 21st Century simulations to investigate the trends of these tropical circulations in response to the projected global warming. As a result, it is anticipated that Hadley, Walker, and monsoon circulations are weakened by 9, 8, and 14%, respectively, by the late 21st Century, according to the ensemble mean of the IPCC model simulations. Considering the large model biases demonstrated for the IPCC 20th Century simulations, further studies are needed to quantify those trends.

The climatological features of warming events over Toyama Plain and the heating mechanisms are investigated by using observation and the non-hydrostatic regional climate model NHRCM. Warming events are frequently observed in March and April. The 20km-NHRCM can reproduce the seasonal variation of the frequency of warming events. In spite of the large diversity of heating mechanisms in the warming events, typical warming factors are apparent, and the ratio of contribution differs case by case. The results of the 10km-NHRCM and backward trajectory analysis indicate that these factors are moist-adiabatic processes, dry-adiabatic processes, and sunshine insolation. The frequency of the typical insolation event is quite low, but it often accelerates warming with other factors. The NHRCM demonstrates high potential for the application of local climate studies as a dynamical downscaling tool.