Optics of the offshore Columbia River plume from glider observations and satellite imagery Journal of Geophysical Research: Oceans

The Columbia River (CR) is the largest source of freshwater along the U.S. Pacific coast. The resultant plume is often transported southward and offshore forming a large buoyant feature off Oregon and northern California in spring-summer—the offshore CR plume. Observations from autonomous underwater gliders and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery are used to characterize the optics of the offshore CR plume off Newport, Oregon. Vertical sections, under contrasting river flow conditions, reveal a low-salinity and warm surface layer of ∼20–25 m (fresher in spring and warmer in summer), high Colored Dissolved Organic Matter (CDOM) concentration, and backscatter, and associated with the base of the plume high chlorophyll fluorescence. Plume characteristics vary in the offshore direction as the warm and fresh surface layer thickens progressively to an average 30–40 m of depth 270–310 km offshore; CDOM, backscatter, and chlorophyll fluorescence decrease in the upper 20 m and increase at subsurface levels (30–50 m depth). MODIS normalized water-leaving radiance (nLw(λ)) spectra for CR plume cases show enhanced water-leaving radiance at green bands (as compared to no-CR plume cases) up to ∼154 km from shore. Farther offshore, the spectral shapes for both cases are very similar, and consequently, a contrasting color signature of low-salinity plume water is practically imperceptible from ocean color remote sensing. Empirical algorithms based on multivariate regression analyses of nLw(λ) plus SST data produce more accurate results detecting offshore plume waters than previous studies using single visible bands (e.g., adg(412) or nLw(555)). Grant no. NA11NOS0120036

哥伦比亚河（Columbia River, CR）是美国太平洋沿岸最大的淡水来源。其形成的羽状流常向南及近海方向输送，在春夏季于俄勒冈州与加利福尼亚州北部近海形成大型浮力特征——近海哥伦比亚河羽状流。本研究利用自主水下滑翔机与中分辨率成像光谱仪（Moderate Resolution Imaging Spectroradiometer, MODIS）卫星影像的观测数据，对俄勒冈州纽波特近海的哥伦比亚河近海羽状流的光学特性进行表征。不同径流条件下的垂直剖面显示，存在约20~25米厚的低盐暖表层（春季盐度更低，夏季水温更高），同时伴随高浓度有色溶解有机物（Colored Dissolved Organic Matter, CDOM）与后向散射信号，且在羽状流底部伴有高叶绿素荧光信号。羽状流特征随近海距离发生变化：暖淡水表层厚度逐渐增加，在距岸270~310公里处平均厚度可达30~40米；上层20米内的CDOM浓度、后向散射强度与叶绿素荧光信号均出现下降，而在30~50米的次表层中上述参数则有所升高。针对哥伦比亚河羽状流案例的MODIS归一化离水辐亮度（normalized water-leaving radiance, nLw(λ)）光谱显示，在距岸约154公里范围内，绿光波段的离水辐亮度较无羽状流案例显著增强。再往近海区域，两类案例的光谱形状极为相似，因此通过海洋颜色遥感几乎无法分辨低盐羽状流水体的特征颜色信号。基于nLw(λ)与海表温度（Sea Surface Temperature, SST）数据的多元回归分析构建的经验算法，相较于此前仅使用单一可见光波段（如adg(412)或nLw(555)）的研究，能更精准地检测近海羽状流水体。本研究受资助编号NA11NOS0120036支持。