3:45 PM - 4:00 PM
[AOS18-02] Subtropical Mode Water in a recent persisting Kuroshio large meander period. Part I: Formation and advection over the entire distribution region
Keywords:Subtropical Mode Water, Kuroshio large meander, Kuroshio Extension
The current Kuroshio large-meander (LM) period, which started in August 2017, has exceeded three years, being second longest only to the 1975-80 occurrence. This study aims to examine how the current persisting LM has impacted the formation and advection of Subtropical Mode Water (STMW), based on observation data.
In the 137E section maintained by the Japan Meteorological Agency since 1967, the cross-sectional area of STMW had fluctuated decadally from mid-1970s to 2016 with an increase (decrease) during the stable (unstable) period of the Kuroshio Extension (KE; Oka et al., 2019). The KE transitioned from the stable state to the unstable state in mid-2016, following the start of a warm PDO phase in 2014, but unexpectedly switched back to the stable state in mid-2017 due to the LM occurrence (Qiu et al., 2020). Nevertheless, the cross-sectional area of STMW has continuously decreased since 2016, exhibiting a tendency different from the previous stable-KE periods. This might be due to the decrease of westward STMW advection from the region south of the KE to the region south of Japan in association with the LM (Suga and Hanawa, 1995), the decrease of STMW formation south of the KE, or both.
We next analyzed Argo float data during 2005-20. Out of STMW defined as low-potential vorticity water of 16-19.5C, a relatively cold variety of 16-18C, which is formed south of the KE, occupied more than 80% of the total volume. The formation volume of the 16-18C STMW was small during 2006-09 in the unstable KE period and large during 2010-15 in the stable KE period, leading to a decrease (increase) of its volume in the former (latter) period. In 2016, anomalously shallow and warm winter mixed layers were formed south of the KE, drastically decreasing the STMW formation volume. In 2018-20 after the current LM began, the formation volume of the 16-18C STMW has not restored to large amount despite the stable KE, being intermediate between the 2006-09 small and 2010-15 large values. In addition, since the current LM began, the 16-18C STMW has had to make a southern detour as indicated by previous studies, which led to the significant decrease of its westward advection from the formation region south of the KE to the region south of Japan. Due to such decline in the formation and advection, the total volume of STMW has gradually decreased since 2016.
In the 137E section maintained by the Japan Meteorological Agency since 1967, the cross-sectional area of STMW had fluctuated decadally from mid-1970s to 2016 with an increase (decrease) during the stable (unstable) period of the Kuroshio Extension (KE; Oka et al., 2019). The KE transitioned from the stable state to the unstable state in mid-2016, following the start of a warm PDO phase in 2014, but unexpectedly switched back to the stable state in mid-2017 due to the LM occurrence (Qiu et al., 2020). Nevertheless, the cross-sectional area of STMW has continuously decreased since 2016, exhibiting a tendency different from the previous stable-KE periods. This might be due to the decrease of westward STMW advection from the region south of the KE to the region south of Japan in association with the LM (Suga and Hanawa, 1995), the decrease of STMW formation south of the KE, or both.
We next analyzed Argo float data during 2005-20. Out of STMW defined as low-potential vorticity water of 16-19.5C, a relatively cold variety of 16-18C, which is formed south of the KE, occupied more than 80% of the total volume. The formation volume of the 16-18C STMW was small during 2006-09 in the unstable KE period and large during 2010-15 in the stable KE period, leading to a decrease (increase) of its volume in the former (latter) period. In 2016, anomalously shallow and warm winter mixed layers were formed south of the KE, drastically decreasing the STMW formation volume. In 2018-20 after the current LM began, the formation volume of the 16-18C STMW has not restored to large amount despite the stable KE, being intermediate between the 2006-09 small and 2010-15 large values. In addition, since the current LM began, the 16-18C STMW has had to make a southern detour as indicated by previous studies, which led to the significant decrease of its westward advection from the formation region south of the KE to the region south of Japan. Due to such decline in the formation and advection, the total volume of STMW has gradually decreased since 2016.