12:00 PM - 12:15 PM
[ACG33-11] Heat budget in the upper layer of the North Pacific north of 24°N
Keywords:ocean heat content, meridional heat transport, surface heat flux, Kuroshio, Pacific Meridional Mode, subarctic gyre
Meridional heat transport (MHT) of the ocean and atmosphere is one of the most important factors that determine earth’s climate, and its climatology have been examined. Basically it is not easy to capture temporal variations of the meridional transport across a zonal section in a wide ocean basin, but some studies showed them. A previous study indicated that the MHT across 24°N in the North Pacific (NP) increased in the 1980s and 1990s due to changes in the Kuroshio and Ekman transports (Kawai et al. 2008, JGR). These increases resulted in different heat distribution: ocean heat content (OHC) increased in the 1980s, and net surface heat release was strengthened in the 1990s. The reason for the differences, however, has yet to be explained. This study revisited the heat balance in the NP utilizing the latest datasets. The purposes of this study are (1) to interpret why the heat distribution in the NP was different between the 1980s and 1990s, (2) to extend the analysis of the heat balance to the 2010s and assess surface heat flux of several datasets.
The target region of this study was partitioned by the 24°N and 137°E lines. The OHC above 700 m north of 24°N and east of 137°E showed sharp increases around 1990 and in the 2010s, and was nearly stable between them. From the late 1980s to the early 1990s, OHC increased in the coastal area southeast off Japan and the Kuroshio Extension (KE), then the KE northern branch (KENB), and the anomaly propagated toward the Gulf of Alaska along the boundary of the subarctic gyre. Meanwhile, the Kuroshio recirculation was also warmed. The northward retreat of the subarctic gyre boundary coincided with spin-up of the subtropical gyre in the 1980s, letting the warm anomaly from the subtropics propagate northeastward. The concurrent weakening of the wintertime westerly resulted in the suppression of surface heat loss in the western NP. On the contrary, the southward shift of the subarctic front suppressed the OHC rise in spite of the MHT rise in the 1990s. The increase of the MHT was redistributed to the atmosphere through surface turbulence instead of being stored in the ocean.
In the 2010s, unprecedented warming occurred in the eastern NP, the Bering Sea, and east off northern Japan. The Pacific Meridional Mode was mainly responsible for the warming in the eastern part. The MHT estimation based on hydrographic observations indicates that net surface heating must have been strengthened, but the latest atmospheric reanalysis datasets failed to reproduce it.
The target region of this study was partitioned by the 24°N and 137°E lines. The OHC above 700 m north of 24°N and east of 137°E showed sharp increases around 1990 and in the 2010s, and was nearly stable between them. From the late 1980s to the early 1990s, OHC increased in the coastal area southeast off Japan and the Kuroshio Extension (KE), then the KE northern branch (KENB), and the anomaly propagated toward the Gulf of Alaska along the boundary of the subarctic gyre. Meanwhile, the Kuroshio recirculation was also warmed. The northward retreat of the subarctic gyre boundary coincided with spin-up of the subtropical gyre in the 1980s, letting the warm anomaly from the subtropics propagate northeastward. The concurrent weakening of the wintertime westerly resulted in the suppression of surface heat loss in the western NP. On the contrary, the southward shift of the subarctic front suppressed the OHC rise in spite of the MHT rise in the 1990s. The increase of the MHT was redistributed to the atmosphere through surface turbulence instead of being stored in the ocean.
In the 2010s, unprecedented warming occurred in the eastern NP, the Bering Sea, and east off northern Japan. The Pacific Meridional Mode was mainly responsible for the warming in the eastern part. The MHT estimation based on hydrographic observations indicates that net surface heating must have been strengthened, but the latest atmospheric reanalysis datasets failed to reproduce it.