我国南海位于全球台风最频发地区的西太平洋, 在全球变暖导致台风频率和强度增加的背景下, 南海灰沙岛的稳定性在未来台风增强情况下将面临严峻挑战。但目前尚不明确南海灰沙岛对台风的响应特征及导致其响应差异的影响因素。针对这一问题, 本文基于Sentinel-2卫星多光谱遥感影像, 使用GVF-Snake(gradient vector flow-snake)模型提取南海西沙北岛、中岛和南岛的海滩基部线(toe of beach, ToB)和植被边界线, 以获取灰沙岛及其海滩面积。根据灰沙岛各区域的地貌特征与在礁盘上的分布位置, 将灰沙岛划分为北端、南端和向海区。采用突变检验法, 判别出2016—2022年造成研究区岛屿月均面积发生突变的台风“杜苏芮”、“环高”和“奥鹿”。通过统计台风事件中灰沙岛归一化植被指数 (normalized difference vegetation index, NDVI)和各分区海滩面积的变化, 并利用数字化海岸线分析系统(digital shoreline analysis system, DSAS)计算ToB线和植被边界线的移动距离, 分析了西沙灰沙岛受台风影响后的时空变化特征及其影响因素。结果表明：1)台风导致灰沙岛植被以轻度受损为主, 中度及以上受损植被分布在岛屿沿岸, 造成植被边界线的后退, 而沉积物随受损植被剥脱, 并随风浪冲刷至海滩, 造成ToB线的向海推进。受台风路径方向影响, 迎风面沿岸首先遭受冲击, 灰沙岛迎风面沿岸植被重度受损, 因而造成ToB线和植被边界线在迎风面具有更大的移动距离; 2)在台风影响末期, 由于灰沙岛沿岸植被受损, 植被边界线的后退和ToB线的向海推进, 使得海滩面积扩大, 而在台风过后, 因失去植被的固定, 在海滩堆积的松散沉积物在风浪作用下从岛屿运离至礁坪, 使得海滩面积缩减。因此, 具有固定与缓冲作用的沿岸植被在维持灰沙岛的稳定性方面具有重要作用; 3)台风风速增大和台风路径与灰沙岛之间距离的缩短, 都将增大灰沙岛严重受损植被的面积, 进而造成更大的岸线移动的距离和海滩面积的变化。此外, 台风可引起沉积物在灰沙岛及其周围环境中的运移, 进而直接改变灰沙岛海滩面积及其形态。台风过后灰沙岛上大量松散沉积物的流失, 导致了灰沙岛面积突变减小, 加之全球变暖造成生物碳酸盐产率下降, 以及来自周围环境的沉积物供应不足, 可能是南海西沙灰沙岛面积长期呈下降趋势的原因。

Global warming has led to an increase in the frequency of strong typhoons, posing a challenge to the stability of lime-sand islands. The South China Sea(SCS) is located in the western Pacific Ocean, the most typhoon prone regions in the world, and the stability of the lime-sand islands in SCS will also be challenged by future typhoon intensification. However, the response characteristics of the SCS lime-sand islands to typhoons and the factors leading to the differences in their responses remain poorly understood. In this study, we explored the changing characteristics of spatial patterns of lime-sand islands in the Xisha Islands under the impacts of typhoons and their driving factors, which are of great significance to the ecological protection and sustainable development of lime-sand islands in the SCS. First, to extract the area of the BeiDao, ZhongDao and Nandao islands of Xisha Islands and their beaches, the toe of beach line (ToB) and the boundary of vegetation were extracted using the gradient vector flow-snake(GVF-snake) model based on Sentinel-2 remote sensing images. According to the geomorphological characteristics of the various areas of the islands and the distribution on the reef flats, the islands were divided into the north tip, the south tip and the oceanward section.Afterwards, the movement of boundary lines affected by typhoons was calculated using the digital shoreline analysis(DSAS) system. The monthly average area of the study islands from 2016 to 2022 was examined using the change point detection method, and the Typhoon Doksuri in 2017, Typhoon Vamco in 2020, and Typhoon Noru in 2022 were identified caused significant changes in the size of the study islands. Finally, the spatial and temporal characteristics of islands affected by typhoons were analyzed based on the changes in lime-sand islands area, vegetation area, beach area and normalized difference vegetation index(NDVI) during typhoon events. The results show that: 1) The vegetation of the islands was mainly slightly stressed by the typhoon, and the severely stressed areas were mainly located along the shorelines. The typhoons caused a significant retreat of the vegetation boundary of the islands, and the sediments were stripped off with the damaged vegetation and washed to the beach with the wind and waves, resulting in the seaward advancement of the ToB lines. The windward coasts were the first to be impacted by the direction of the typhoon path, and the vegetation along the windward side of islands was heavily damaged, resulting in more significant changes in vegetation, vegetation boundary, and ToB lines on the windward side. 2) As the vegetation along the lime-sand islands was damaged, the retreat of the vegetation boundary line and the seaward advancement of the ToB line resulted in the expansion of the beach area, whereas after the typhoon, due to the loss of vegetation fixation, the loose sediment accumulated on the beach was transported away from the islands to the reef flats by the wind and wave action, resulting in the reduction of the beach area. Therefore, coastal vegetation that buffers and fixes sand was critical to the stability of lime-sand islands. 3) Increased typhoon wind speeds and shorter distances between typhoon paths and islands can lead to an increase in the area of severely damaged vegetation on the lime-sand islands, resulting in greater shoreline movement and changes in beach area. In addition, typhoons can cause sediment transport between the lime-sand islands and their surroundings, which can directly change the beach area and morphology of the lime-sand islands. The loss of large amounts of unconsolidated sediments on the lime-sand islands after the typhoon, which led to an abrupt decrease in the size of the islands, together with the decrease in the rate of biogenic carbonate production due to global warming and the insufficient supply of sediments from the surrounding environment, may be the reasons for the long-term decreasing trend in the size of the lime-sand islands in the Xisha Islands.