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