Coupled atmosphere–ocean phenomena known as northeast Pacific marine heatwaves (MHWs) simulated in a climate model with an eddy-permitting ocean model were examined. During the analyzed 270-years of the preindustrial control run, 13 events of MHW (defined here by warm annual-mean SST anomalies with over 1.5 times of the standard deviation) were detected. The simulated MHWs are linked to the decadal-scale climate modes of PDO, inverted NPGO (IV-NPGO), and the central Pacific El Niño (CP-El-Niño): IV-NPGO and then PDO changed the signs from negative to positive a few years before the MHWs at around which PDO took the maxima, when CP-El-Niño occurred. Air–sea interactions between subtropical-tropical and within mid-latitudes suggest playing crucial roles in the evolution of the MHWs.

Hydrographic observations and two-year current measurements revealed the flow around the Erimo Seamount between the Japan and Kuril trenches. Northward currents were observed to flow both around the seamount and on the eastern flank of the trench. Since the dissolved oxygen concentration of the former is higher than that of the latter, these currents correspond to the western and eastern branch currents of the deep western boundary current in the North Pacific, respectively. The western branch current forms a Taylor cap over the seamount, although during certain periods, it joins the eastern branch current. During such periods, a strong jet forms around the seamount and the Taylor cap is washed away. Such alternately intensified currents are consistent with previous observations of the Japan Trench. On the western flank of the trench, a southward current flows near the seafloor, while an eastward current is formed at a depth of 3360 m due to the confluence of the currents from north and south.