| dc.description.abstract | As global warming intensifies, the frequency of heatwaves is gradually increasing, and heatwaves are associated with high-pressure systems. During the summer, Taiwan is influenced by the Western North Pacific Subtropical High (WNPSH), which often leads to prolonged high-temperature events. Therefore, investigating the changes in WNPSH is crucial for analyzing heatwaves in Taiwan.
Through EOF analysis of the 850 hPa geopotential height and regression analysis of meteorological fields for each mode, the first three modes influencing the WNPSH are identified as tropical Atlantic warming, the negative phase of the Pacific-Japan (P-J) pattern, and ENSO. Under the influence of warming, the intensity of the WNPSH strengthens in the East Asia region, and under the impact of tropical Atlantic warming, its position shifts westward. In the negative phase of the P-J pattern, a cyclonic anomaly develops over Taiwan, and the regression of this anomaly leads to increased precipitation anomalies, which are unfavorable for the strengthening of WNPSH. During ENSO′s decay phase, an anticyclonic system strengthens in the Northwest Pacific, primarily related to the negative vorticity shear caused by the Kelvin wave triggered by the ENSO decay, resulting in the enhancement of WNPSH. In summary, conditions that favor WNPSH strengthening occur during tropical Atlantic warming, the positive phase of the P-J pattern, and the decay phase of ENSO.
After exploring the modes favorable for WNPSH development, this study analyzes observational data from the past 43 years, classifying weather patterns into four types: weak southwest monsoon (C1), strong southwest monsoon (C2), weak WNPSH (C3), and strong WNPSH (C4). The analysis shows that heatwave events are more likely to develop under conditions with less water vapor. Further analysis of the heatwave duration in the four categories under the three modes indicates that during tropical Atlantic sea surface temperature increases, the positive phase of the P-J pattern, and El Nino decaying years, the heatwave duration and frequency are higher than in normal years, while the intensity does not show significant differences. Therefore, Taiwan′s heatwaves are primarily caused by prolonged duration and high frequency. Finally, the correlation between the number of days in each weather pattern and the three modes is explored. In C4, the number of days shows a significant positive correlation with PC1, while C1 and C2 show significant negative correlations. This suggests that under warming conditions, the southwest monsoon weakens, and the number of WNPSH days increases, leading to an increase in heatwave days in Taiwan over the past 43 years. Through this analysis, the study integrates the influence of large-scale circulation changes due to warming and interannual variations on WNPSH and investigates how it affects Taiwan′s heatwave events. | en_US |