High-Resolution Sea Surface Temperatures Derived from Landsat 8: A Study of Submesoscale Frontal Structures on the Pacific Shelf off the Hokkaido Coast, Japan

Coastal and offshore waters are generally separated by a barrier or “ocean front” on the continental shelf. A basic question arises as to what the representative spatial scale across the front may be. To answer this question, we simply corrected skin sea surface temperatures (SSTs) estimated from Landsat 8 imagery with a resolution of 100 m using skin SSTs estimated from geostationary meteorological satellite Himawari 8 with a resolution of 2 km. We analyzed snapshot images of skin SSTs on 13 October 2016, when we performed a simultaneous ship survey. We focused in particular on submesoscale thermal fronts on the Pacific shelf off the southeastern coast of Hokkaido, Japan. The overall spatial distribution of skin SSTs was consistent between Landsat 8 and Himawari 8; however, the spatial distribution of horizontal gradients of skin SSTs differed greatly between the two datasets. Some parts of strong fronts on the order of 1 °C km−1 were underestimated with Himawari 8, mainly because of low resolution, whereas weak fronts on the order of 0.1 °C km−1 were obscured in the Landsat 8 imagery because the signal-to-noise ratios were low. The widths of the strong fronts were estimated to be 114–461 m via Landsat 8 imagery and 539–1050 m via in situ ship survey. The difference was probably attributable to the difference in measurement depth of the SST, i.e., about 10-μm skin layer by satellite and a few dozen centimeters below the sea surface by the in situ survey. Our results indicated that an ocean model with a grid size of no more than ≤100–200 m is essential for realistic simulation of the frontal structure on the shelf.

沿海和近海水域通常被大陆架上的一道屏障或“海洋锋”所分隔。一个基本问题是，锋面的代表性空间尺度可能是多少。为了回答这个问题，我们简单地用分辨率为2公里的地球静止气象卫星“向日葵8号”估算的海表皮肤温度校正了分辨率为100米的“陆地卫星8号”影像估算的海表皮肤温度。我们分析了2016年10月13日的海表皮肤温度快照图像，当时我们同时进行了一次船舶调查。我们特别关注了日本北海道东南海岸外太平洋大陆架上的亚中尺度热锋。“陆地卫星8号”和“向日葵8号”的海表皮肤温度总体空间分布是一致的；然而，这两个数据集的海表皮肤温度水平梯度的空间分布差异很大。“向日葵8号”低估了约1°C/公里量级的一些强锋部分，主要是因为分辨率低，而“陆地卫星8号”影像中约0.1°C/公里量级的弱锋则不明显，因为信噪比低。通过“陆地卫星8号”影像估计强锋的宽度为114 - 461米，通过现场船舶调查估计为539 - 1050米。这种差异可能是由于海表温度测量深度的不同，即卫星测量的是约10微米的表层，而现场调查是在海面以下几十厘米处。我们的结果表明，网格尺寸不超过100 - 200米的海洋模型对于大陆架上锋面结构的真实模拟是必不可少的。