11:15 〜 11:30
[ACG32-09] Increasing wintertime sea surface temperature around the subarctic front region in the western North Pacific
キーワード：亜寒帯前線、SSTトレンド、OGCM、寒気容量
Two parallel sea surface temperature (SST) fronts lie in the western North Pacific: the subarctic front (SAF) along 40°N and a front associated with the Kuroshio Extension (KE) along 36°N. Previous studies showed that the SAF has decadal-scale (about 10 years) variability (e.g., Frankignoul et al., 2011). Qiu et al. (2017) pointed out that SST variations in the SAF region are associated with the meridional movement of the KE.
We investigated long-term SST changes in mid-latitude North Pacific in winter (January–March) using satellite-derived data and in-situ temperature/salinity profiles. We found that SST has increased remarkably after 1982 around the SAF region, which reached +3.0 °C per century. To investigate an influence of the meridional shift of the SAF on the SST increase, we extracted the SAF location as the latitude at which the southward meridional SST gradient was the largest from 35°N to 45°N, at each longitude of 145°E–170°E. The result showed no northward shift of the SAF. To provide additional evidence for no shift in the SAF location, we explored repeat hydrographic sections along 165°E for 1997–2021 by the Japan Meteorological Agency. The KE path indicated the northward shift, while the SAF had no shift. A cross section of temperature showed an increase in both temperature and salinity in the SAF zone, the positive anomalies of which are distributed until a depth of 300 dbar. To investigate whether or not the northward shift of KE influences the warming around the SAF, we explored outputs of numerical experiments driven by the raw-atmospheric forcing, including trend and de-trended atmospheric forcing, based on a high-resolution ocean general circulation model, which was produced by Kawakami et al. (2023). The results showed no difference in the latitudinal position of SAF between the two experiments. Interestingly, the de-trended atmospheric forcing run indicated cooling of about minus 2 °C around the SAF. These results show that the warming around the SAF zone could not be explained by the oceans. Therefore, we examine this point from an atmospheric perspective.
Atmospheric reanalysis data showed the weakening of the Aleutian Low after 1982. We used negative heat content (NHC; Iwasaki et al., 2014), defined as cold air below 290 K, to indicate strength in cold-air outbreaks from the large continent. We found that NHC has decreased over Japan due to a decrease in the inflow of cold air.
Our study focused on long-term SST changes around the SAF region in winter. It is known that decadal-scale variability of the SAF affects large-scale storm-track activity (e.g., Nakamura et al., 2004; Taguchi et al., 2012). Moreover, SST in the SAF zone is now attention because of its much higher value in 2023–2024, i.e., marine heatwave. To predict ocean warming and its devastating impact on society, it is necessary to understand the mechanism of the increasing trend.
We investigated long-term SST changes in mid-latitude North Pacific in winter (January–March) using satellite-derived data and in-situ temperature/salinity profiles. We found that SST has increased remarkably after 1982 around the SAF region, which reached +3.0 °C per century. To investigate an influence of the meridional shift of the SAF on the SST increase, we extracted the SAF location as the latitude at which the southward meridional SST gradient was the largest from 35°N to 45°N, at each longitude of 145°E–170°E. The result showed no northward shift of the SAF. To provide additional evidence for no shift in the SAF location, we explored repeat hydrographic sections along 165°E for 1997–2021 by the Japan Meteorological Agency. The KE path indicated the northward shift, while the SAF had no shift. A cross section of temperature showed an increase in both temperature and salinity in the SAF zone, the positive anomalies of which are distributed until a depth of 300 dbar. To investigate whether or not the northward shift of KE influences the warming around the SAF, we explored outputs of numerical experiments driven by the raw-atmospheric forcing, including trend and de-trended atmospheric forcing, based on a high-resolution ocean general circulation model, which was produced by Kawakami et al. (2023). The results showed no difference in the latitudinal position of SAF between the two experiments. Interestingly, the de-trended atmospheric forcing run indicated cooling of about minus 2 °C around the SAF. These results show that the warming around the SAF zone could not be explained by the oceans. Therefore, we examine this point from an atmospheric perspective.
Atmospheric reanalysis data showed the weakening of the Aleutian Low after 1982. We used negative heat content (NHC; Iwasaki et al., 2014), defined as cold air below 290 K, to indicate strength in cold-air outbreaks from the large continent. We found that NHC has decreased over Japan due to a decrease in the inflow of cold air.
Our study focused on long-term SST changes around the SAF region in winter. It is known that decadal-scale variability of the SAF affects large-scale storm-track activity (e.g., Nakamura et al., 2004; Taguchi et al., 2012). Moreover, SST in the SAF zone is now attention because of its much higher value in 2023–2024, i.e., marine heatwave. To predict ocean warming and its devastating impact on society, it is necessary to understand the mechanism of the increasing trend.