High concentration chlorophyll a rings associated with the formation of intrathermocline eddies

High concentration chlorophyll a (Chl a ) rings formed around anticyclones have been observed frequently in the Japan/East Sea. To study their formation, an anticyclone to intrathermocline eddy (ITE) transformation is examined using hydrographic observations, remotely sensed Chl a , sea level anomaly fields, and sea surface temperature images in the Japan/East Sea. Due to the winter cooling up to March, a cold homogeneous layer forms inside the anticyclone. In April, the East Korean Warm Current from the south merges with the anticyclone and encircles the clockwise‐rotating anticyclone. The wrapping warm current generates a geostrophic flow opposite to the clockwise direction of the anticyclone and effectively decelerates it. The geostrophic flow weakens along the periphery of the eddy as the warm current gradually cools down through horizontal mixing, resulting in an accelerating anticyclonic current around the transformed ITE. A high Chl a concentration area in the form of a ring forms around the ITE along the accelerating, thus horizontally divergent surface flow. The divergence around the ITE continues throughout the summer above the submerged homogeneous lens under the stratified surface layer. Based on observations, it is shown that the accelerating surface flow induced through the anticyclone‐to‐ITE transformation process causes an upward transport of nutrients, enhancing the Chl a concentration around the ITE.