Double fronts in the Yellow Sea in summertime identified using sea surface temperature data of multi-scale ultra-high resolution analysis

Fronts are ubiquitous phenomena in oceans, and they play a significant role in marine hydrodynamics and ecology. During the stratified season of a shelf sea, the coastal front is usually considered as a single front, i.e., the tidal mixing front. However, using high resolution (similar to 1 km) Multi-scale Ultra-high Resolution (MUR) analysis sea surface temperature (SST) data, this study observes persistent double fronts along the Yellow Sea coast in summertime. The double fronts comprise the well-known offshore tidal mixing front and a nearshore front, and the nearshore front has not been previously reported. The climatological (2002-2017) monthly mean result shows that the double fronts with two SST gradient peaks exceeding similar to 2 degrees C/100 km and opposite SST gradient directions basically remain unchanged from June to August, whereas the frontal spacing decreases in September. Analyses based on a two-layer concept model suggest that a topographic slope along with tidal mixing could induce the pattern of double fronts. The frontogenesis of nearshore thermal front could be associated with the different responses of the water column of different water depths to insolation. The offshore movement of the nearshore front in September could be related to the fast cooling of nearshore water and intensified offshore wind, and the topographic slope is important for determining the pattern of double fronts (loose or tight). This study shows a new pattern of coastal fronts in the stratified season, and indicates the significance of high resolution satellite data. The discovery of the double front pattern implies the influence of coastal fronts during the stratified season on marine ecology and environment in a shelf sea might be underestimated.