A multi-scale three-dimensional variational (MS-3DVAR) scheme is developed to assimilate high-resolution Himawari-8 sea surface temperature (SST) data for the first time into an operational ocean nowcast/forecast system targeting the North Western Pacific, JCOPE2. MS-3DVAR improves representation of the Kuroshio path south of Japan, its associated sea level variations, and temperature/salinity profiles south of Japan, the Kuroshio/Oyashio mixed water region, and the Japan Sea as compared to those of the products by the traditional single-scale 3DVAR. Validation results demonstrate that MS-3DVAR well assimilates the sparsely distributed in situ temperature and salinity profiles data by spreading the information over the large scale and by representing the detailed information near the measurement points. MS-3DVAR succeeds to assimilate the Himawari-8 SST product without noisy features caused by the cloud effects. We also find that MS-3DVAR is more effective for estimating oceanic conditions in regions with smaller mesoscale variability including the mixed water region and Japan Sea than in south of Japan.

The sea surface temperature (SST) of the Oyashio region in boreal summer abruptly increased in 2010 and high summertime SST repeated every year until 2016. Observations and an ocean reanalysis show that this marine heatwave occurred not only at the surface but also at deeper depths down to 200 m. Furthermore, salinity in summer also increased in parallel with the temperature. The rises in temperature and salinity indicate the strengthening of the Kuroshio water influence. The sea surface height and velocity show that the southward intrusion of the Oyashio near the coast in summer weakened from 2010 accompanied by an increase in anticyclonic eddies from the Kuroshio Extension. The much more frequent existence of anticyclonic eddies to the east of the first intrusion of the Oyashio in summer is closely associated with the weakening of the first intrusion and the strengthening of the second intrusion. It is suggested that the rise in the water temperature could increase a catch of yellowtail (Seriola quinqueradiata) in northern Japan.