19 Dec 2023
 | 19 Dec 2023

North Pacific Subtropical Mode Water Volume and Density Anomalous Decrease in a Recent Stable Kuroshio Extension Period from Argo Observations

Jing Sheng, Cong Liu, Yanzhen Gu, Peiliang Li, Fangguo Zhai, and Ning Zhou

Abstract. North Pacific subtropical mode water (NPSTMW) is formed as the low stratification water mass in the wintertime mixed layer south of the Kuroshio Extension (KE). In a recent period of 2018–2021, the KE jet is in a persisting stable dynamic state. But based on analysis of Argo observation, the mean volume of NPSTMW in ventilation region drop anomalously by ~21 % during 2018–2021 relative to 2012–2015 when the KE jet is likewise stable. Moreover, the NPSTMW volume in denser density range (approximately σθ >25.2 kg m-3) starts to decrease but the volume of lighter density of the NPSTMW increases since 2018. The decreasing of the NPSTMW subduction and formation rate are associated with anomalously shallow wintertime mixed layer depth (MLD) and weak heat loss in the NPSTMW formation region. The interannual variations of the NPSTMW subduction and formation reflect the variability of the overlying atmosphere which is correlated with Pacific decadal oscillation (PDO) shift in 2018–2021. When the PDO shifts from its positive to a negative phase in analysis period, the effects of local wind stress anomalies seem to play an evident role in driving the variability of NPSTMW on interannual time scales. The MLD and heat loss change during the cold season in 2018–2021 are strongly coupled with the poleward shift of the westerlies-which cause the weaker wintertime wind and the easterly wind anomalies over the NPSTMW formation region. The declines of heat loss and southward Ekman transport, owing to the wind stress anomalies, further prohibit the upper-ocean convection and mixed layer deepening and cooling. Additionally, the insufficient development of wintertime MLD in 2018–2021 may also be correlated with the significantly intensified preconditioning of near surface stratification (<150 m depth) due to the persisting near surface warming.

Jing Sheng, Cong Liu, Yanzhen Gu, Peiliang Li, Fangguo Zhai, and Ning Zhou

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2931', Anonymous Referee #1, 08 Jan 2024
  • RC2: 'Comment on egusphere-2023-2931', Anonymous Referee #2, 12 Jan 2024
  • RC3: 'Comment on egusphere-2023-2931', Eitarou Oka, 22 Jan 2024
  • EC1: 'Comment on egusphere-2023-2931', Bernadette Sloyan, 22 Jan 2024
Jing Sheng, Cong Liu, Yanzhen Gu, Peiliang Li, Fangguo Zhai, and Ning Zhou
Jing Sheng, Cong Liu, Yanzhen Gu, Peiliang Li, Fangguo Zhai, and Ning Zhou


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Short summary
The homogeneous water column, named as mode water, retaining atmosphere condition and biogeochemical elements from the deep winter mixed layer, became weaker and warmer in the North Pacific subtropical Ocean in 2018–2021, even the Kuroshio Extension is stable. Locally anomalous east wind transporting warm water to the north and enhanced near-surface stratification hinder the deepening of winter mixed layer. This study has broad implications for climate change and biogeochemical cycles.