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Rui Xin Huang and Bo Qiu

Abstract

The structure of the wind-driven circulation in the subtropical South Pacific is studied using simple diagnostic and analytical models. The diagnostic calculation is based on the Levitus climatology. The analytical model is forced by observed winter mixed layer density and depth calculated from the Levitus climatology and by the surface wind stress data from the Hellerman and Rosenstein climatology. The wind-driven gyre in the South Pacific is relatively deep, reaching 2.4 km along the southern edge of the gyre. The gross feature of subduction obtained from both the data analysis and the analytical model is similar, with an annual ventilation rate of 21.6 Sv (Sv ≡ 106 m3 s−1), including 18.1 Sv from vertical pumping and 3.5 Sv from lateral induction. Although the annual subduction rate in the South Pacific is comparable to that in the North Atlantic, lack of localized subduction leads to relatively weak mode water formation in the region where the East Australian Current separates from its western boundary. In addition, results from the analytical model indicate the existence of an isopycnal slope reversal in the southeastern Pacific.

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Bo Qiu and Rui Xin Huang

Abstract

Ventilation in the North Atlantic and North Pacific is examined by analyzing the Levitus climatological data and the Hellerman and Rosenstein wind stress data. Ventilation between the permanent pycnocline and the overlying seasonal pycnocline and mixed layer consists of two physical processes: subduction and obduction. Subduction takes place mainly in the subtropical basin where surface water is irreversibly transferred into the permanent pycnocline below. Obduction takes place in the subpolar basin where water from the permanent pycnocline is irreversibly transferred into the mixed layer above.

Veatilation in the North Atlantic and North Pacific can be clarified into four physically different regions: the subductive region, the obductive region, the ambiductive region where both subduction and obduction take place, and the insulated region where neither subduction nor obduction occurs. Although the total subduction rates in these two oceans are comparable, the total obduction rates are considerably different. In the North Atlantic, obduction is strong (23.5 Sv), consistent with the notion of the fast thermohaline circulation and the relatively short renewal time of the subpolar water masses in the Atlantic basin. Obduction is weak in the North Pacific (7.8 Sv), this is consistent with the sluggish thermohaline circulation and the slower renewal process of the subpolar water masses there. Accordingly, the water mass renewal time based on the subduction/obduction rate is calculated and compared with previous estimations.

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Rui Xin Huang and Bo Qiu

Abstract

The subduction rate is calculated for the North Pacific based on Levitus climatology data and Hellerman and Rosenstein wind stress data. Because the period of effective subduction is rather short, subduction rates calculated in Eulerian and Lagrangian coordinates are very close. The subduction rate defined in the Lagrangian sense consists of two parts. The first part is due to the vertical pumping along the one-year trajectory, and the second part is due to the difference in the winter mixed layer depth over the one-year trajectory. Since the mixed layer is relatively shallow in the North Pacific, the vertical pumping term is very close to the Ekman pumping, while the sloping mixed layer base enhances subduction, especially near the Kuroshio Extension. For most of the subtropical North Pacific, the subduction rate is no more than 75 m yr−1, slightly larger than the Ekman pumping. The water mass volume and total amount of ventilation integrated for each interval of 0.2σ unit is computed. The corresponding renewal time for each water mass is obtained. The inferred renewal time is 5–6 years for the shallow water masses (σ = 23.0–25.0), and about 10 years for the subtropical mode water (σ = 25.2–25.4).

Within the subtropical gyre the total amount of Ekman pumping is 28.8 Sv (Sv ≡ 106 m3 s−1) and the total subduction rate is 33.1 Sv, which is slightly larger than the Ekman pumping rate. To this 33.1 Sv, the vertical pumping contributes 24.1 Sv and the lateral induction 9 Sv. The maximum barotropic mass flux of the subtropical gyre is about 46 Sv (cut of 135°E). This mass flux is partitioned as follows. The total horizontal mass flux in the ventilated thermocline, the seasonal thermocline, and the Ekman layer is about 30 Sv, and the remaining 16 Sv is in the unventilated thermocline. Thus, about one-third of the man flux in the wind-driven gyre is sheltered from direct air–sea interaction.

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Zhongbin Sun, Zhiwei Zhang, Bo Qiu, Xincheng Zhang, Chun Zhou, Xiaodong Huang, Wei Zhao, and Jiwei Tian

Abstract

Based on long-term mooring-array and satellite observations, three-dimensional structure and interannual variability of the Kuroshio Loop Current (KLC) in the northeastern South China Sea (SCS) were investigated. The 3-yr moored data between 2014 and 2017 revealed that the KLC mainly occurred in winter and it exhibited significant interannual variability with moderate, weak, and strong strengths in the winters of 2014/15, 2015/16, and 2016/17, respectively. Spatially, the KLC structure was initially confined to the upper 500 m near the Luzon Strait, but it became more barotropic, with kinetic energy transferring from the baroclinic mode to the barotropic mode when it extended into the SCS interior. Through analyzing the historical altimeter data between 1993 and 2019, it is found that the KLC event in 2016/17 winter is the strongest one since 1993. Moored-data-based energetics analysis suggested that the growth of this KLC event was primarily fed by the strong wind work associated with the strengthened northeast monsoon in that La Niña–year winter. By examining all of the historical KLC events, it is found that the strength of KLC is significantly modulated by El Niño–Southern Oscillation, being stronger in La Niña and weaker in El Niño years. This interannual modulation could be explained by the strengthened (weakened) northeast monsoon associated with the anomalous atmospheric cyclone (anticyclone) in the western North Pacific during La Niña (El Niño) years, which inputs more (less) energy and negative vorticity southwest of Taiwan that is favorable (unfavorable) for the development of KLC.

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Zhiwei Zhang, Xincheng Zhang, Bo Qiu, Wei Zhao, Chun Zhou, Xiaodong Huang, and Jiwei Tian

Abstract

Although observational efforts have been made to detect submesoscale currents (submesoscales) in regions with deep mixed layers and/or strong mesoscale kinetic energy (KE), there have been no long-term submesoscale observations in subtropical gyres, which are characterized by moderate values of both mixed layer depths and mesoscale KE. To explore submesoscale dynamics in this oceanic regime, two nested mesoscale- and submesoscale-resolving mooring arrays were deployed in the northwestern Pacific subtropical countercurrent region during 2017–19. Based on the 2 years of data, submesoscales featuring order one Rossby numbers, large vertical velocities (with magnitude of 10–50 m day−1) and vertical heat flux, and strong ageostrophic KE are revealed in the upper 150 m. Although most of the submesoscales are surface intensified, they are found to penetrate far beneath the mixed layer. They are most energetic during strong mesoscale strain periods in the winter–spring season but are generally weak in the summer–autumn season. Energetics analysis suggests that the submesoscales receive KE from potential energy release but lose a portion of it through inverse cascade. Because this KE sink is smaller than the source term, a forward cascade must occur to balance the submesoscale KE budget, for which symmetric instability may be a candidate mechanism. By synthesizing observations and theories, we argue that the submesoscales are generated through a combination of baroclinic instability in the upper mixed and transitional layers and mesoscale strain-induced frontogenesis, among which the former should play a more dominant role in their final generation stage.

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