海水面變化對水深測量之影響

Impact of sea level variations on hydrographic survey

"本研究利用臺灣 33 個潮位站與 Jason-2 衛星測高資料對於 MOI.18v1、NAO.99b以及 DTU10 潮位模式(ocean tide model)進行精度評估。與潮位站資料相比， MOI.18v1 表現為最佳，差值 RMSE 的平均值為 0.090 m (0.047~0.153 m);而三個潮位模式改正測高資料之資料改善率(improvement percentage, IMP)為 68.2% (A 點)與88.2~89.2% (B 點)，並無顯著差異。利用潮位站、衛星測高以及 MOI.18v1 潮位模式分析在不同離岸距離或海底地形情況下，分別計算潮位變化與海水面變化之差異。成果顯示，臺灣周圍潮差變化非常顯著，潮差變化大的區域主要分布於淺海(≦ 200 m)，臺灣西岸潮差可達到 4 m，越往臺灣海峽潮差變化越大，最高可達 5 m 以上，但臺灣東岸與南岸約小於 1 m，因此在淺海區域進行水深測量必須進行潮位修正，才能提升水深測量之精度。此外，各海域潮位變化與距離最近潮位站資料相比較，差值標準偏差(standard deviation)在臺灣北部海域與台灣海峽等地區相對較大，部分區域差值標準偏差超過 50 cm，表示這些區域進行水深測量時必須結合潮位站以及潮位分區圖進行潮位修正，才可降低水深測量誤差量。 潮位模式僅包含潮位變化，無法反應實際海水面變化以及氣象造成的逆氣壓效應。臺灣周遭海域逆氣壓效應之年週期振幅為 6~10 cm，由南往北逐漸增大，馬祖可達 10 cm 以上; 臺灣周遭海水面年週期振幅為 3~15 cm，於中國大陸沿岸為最大，因此使用潮位模式修正水深資料時需考慮逆氣壓效應與海水面之季節性變化。此外，利用衛星測高計算 1993-2019 臺灣周圍海域的海水面速率，成果指出海水面呈現平均 3.04±0.22 mm/yr 的上升速率，與全球海水面平均值(~3 mm/yr)相近。"

"In the study, the accuracies of the ocean tide models, including MOI.18v, NAO.99b, and DTU10, are investigated by using 33 tide gauges and Jason-2 satellite altimetry around Taiwan. MOI.18v1 has the highest accuracy in near-shore by comparing to 33 tide gauge records, with the mean of difference RMSE of 0.090 m (0.047~0.153 m); and the improvement percentages (IMPs) of each ocean tide model correcting Jason-2 altimetry data are 68.2% (point A) and 88.2~89.2% (point B), with no significant difference between ocean tide models in offshore Taiwan. The differences of variations in ocean tide and sea level at different offshore distances or ocean bottom topography are analyzed using tide gauge records, satellite altimeter data and ocean tide models around Taiwan. Results show that the significant variations in tidal range around Taiwan are found and large tide range are mainly located in shallow waters (≦ 200 m), with the tidal range varying from less than 1 m in the eastern of Taiwan to 4 m in the west coast of Taiwan and larger than 5 m along the coast of China. Therefore, ocean tidal correction is necessary for bathymetry measurements in shallow waters to improve the accuracy of bathymetry data. In addition, the standard deviations (STD) of the difference between tide changes in height derived from MOI.18v1 and the nearest tide gauge records are relatively large in northern and western of Taiwan and could be even larger than 50 cm in some areas. The result indicates that the bathymetry measurements in these areas need to be corrected for the ocean tide using a combination of tidal gauges and tidal zones to reduce the uncertainty of bathymetry measurements. Ocean tide model only includes tidal change in height and cannot reflect the actual sea level change and inverted barometer effect caused by meteorological forcing. The annual amplitude of inverted barometer effect around Taiwan ranges from 6 to 10 cm, gradually increased from south of Taiwan to north of Taiwan, reaching more than 10 cm in Mazu; the annual amplitude of sea level change around Taiwan ranges from 3 to 15 cm, with the largest along the coast of China. Therefore, the seasonal effect in inverted barometer effect and sea level change have to be consider when only ocean tide model is used to correct bathymetry data. In addition, we investigate the sea level rise around Taiwan during the period 1993–2019 using satellite altimetry. The average sea level rise trend is 3.04±0.22 mm/yr, which is similar to global average trend of ~3 mm/yr. "