Decadal changes in the basin-wide heat budget of the mid-latitude North Pacific Ocean

Research has indicated that the meridional heat transport (MHT) in the North Pacific Ocean (NP) across 24°N increased in the 1980s and 1990s, resulting in different heat distributions: the ocean heat content (OHC) increased in the 1980s, while the net surface heat release was strengthened in the 1990s; however, the reasons for these differences remained unclear. The authors revisited the investigation of the heat budget in the NP mainly using hydrographic observations to understand why the heat distribution was different between these 2 decades and extend the analysis to the 2010s. The OHC in the upper 700 m north of 24°N and east of 137°E exhibited sharp increases around 1990 as well as in the 2010s, while it was nearly stable in between. The northward retreat of the subarctic gyre boundary coincided with the spin-up of the subtropical gyre in the late 1980s, thereby allowing warm anomaly from the subtropics to propagate northeastward. Meanwhile, the concurrent weakening of the wintertime westerlies resulted in the suppression of the surface heat loss in the western NP. In contrast, the southward shift of the subarctic front suppressed the OHC rise, despite the MHT increase in the late 1990s. In the 2010s, unprecedented warming occurred in the eastern NP. The MHT estimation based on hydrographic observations indicates that the net surface release must have been suppressed since the MHT did not increase; however, the latest atmospheric reanalysis datasets failed to reproduce this.