Ming-Cheng Yen

Seasonal variation of the rainfall over Taiwan

Two different approaches [empirical orthogonal function (EOF) analysis and rainfall regime identification] were applied to analyse the seasonal variation in Taiwan rainfall using 15 coastal stations administrated by the Central Weather Bureau of Taiwan, along the coastal zone of this island during the period 1981–1997. The major findings are as follows: Using the EOF analysis, the major seasonal variations in Taiwan rainfall may be shown as a seasonal seesaw in rainfall between the southern and northern Taiwan modes following the alternation of the two East Asian monsoon components. The former mode is associated with the summer southwest monsoon, while the latter with the winter northeast monsoon. Four (spring, summer, summer–fall and fall–winter) rainfall regimes are identified by the second approach: the four regimes cover northwest, southwest, east, and northeast Taiwan, respectively. The southern Taiwan mode is primarily formed by the second rainfall regime, while the northern Taiwan mode by the fourth. The seasonal variation in Taiwan rainfall is formed by the transition of these four rainfall regimes; this is reflected by a counter-clockwise rotation in these rainfall regimes around the island. Copyright

Impact of equatorial and continental airflow on primary greenhouse gases in the northern South China Sea

Four-year ground-level measurements of the two primary greenhouse gases (carbon dioxide (CO2) and methane (CH4)) were conducted at Dongsha Island (DSI), situated in the northern South China Sea (SCS), from March 2010 to February 2014. Their mean mixing ratios are calculated to be 396.3 ± 5.4 ppm and 1863.6 ± 50.5 ppb, with an annual growth rate of +2.19 ± 0.5 ppm yr–1 and +4.70 ± 4.4 ppb yr–1 for CO2 and CH4, respectively, over the study period. Our results suggest that the Asian continental outflow driven by the winter northeast monsoon could have brought air pollutants into the northern SCS, as denoted by significantly elevated levels of 6.5 ppm for CO2 and 59.6 ppb for CH4, which are greater than the marine boundary layer references at Cape Kumukahi (KUM) in the tropical northern Pacific in January. By contrast, the summertime CH4 at DSI is shown to be lower than that at KUM by 19.7 ppb, whereas CO2 is shown to have no differences (<0.42 ppm in July) during the same period. Positive biases of the Greenhouse Gases Observing Satellite (GOSAT) L4B data against the surface measurements are estimated to be 2.4 ± 3.4 ppm for CO2 and 43.2 ± 36.8 ppb for CH4. The satellite products retrieved from the GOSAT showed the effects of anthropogenic emissions and vegetative sinks on land on a vertical profiling basis. The prevailing southeasterly winds originating from as far south as the equator or Southern Hemisphere pass through the lower troposphere in the northern SCS, forming a tunnel of relatively clean air masses as indicated by the low CH4 mixing ratios observed on the DSI in summer.