Reduction in sunshine duration over Taiwan: Causes and implications

Long-term records (about 100 years) of meteorological parameters of Taiwan have been analyzed to study causes and implications of decreasing trends in sunshine duration observed during Taiwan's rapid economic developing period that started in the 1960's. Although the largest decreasing trends in sunshine duration are found in major urban centers, in-depth analysis shows that the reductions are surprisingly uniform and well correlated among stations throughout Taiwan and her surrounding islands, implying that direct scattering by aerosols is not the major cause of the reductions. Similar reductions occur at two high altitude stations, namely Alishan (2.4 km) and Yushan (3.85 km), suggesting that the cause of the reductions is at altitude above 3.85 km. After eliminating other potential causes, we hypothesize that the trends are most likely caused by an increase in regional clouds and/or cloud albedo as a result of increased anthropogenic aerosols. The extent of the region depends on the season. Strong westerlies above 3.85 km in winter and spring imply that southern China, Southeast Asia, India and regions further upwind may suffer similar reductions and could be the major source of anthropogenic aerosols. In summer and fall, the wind pattern indicates Taiwan's own emissions of aerosols can play a significant role in the reduction. Potential ramifications of the increase in clouds/cloud albedo to regional climate change are serious. The increase in clouds/cloud albedo obviously has lead to a decrease in the diurnal temperature range. These changes may also be linked to an increase in precipitation intensity and other observed changes in some key climate parameters in Taiwan.