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Haiyuan Yang, Lixin Wu, Sun Shantong, and Chen Zhaohui

Abstract

The interannual variability of the upper-ocean circulation forced by seasonally varying monsoonal wind is investigated in a two-layer quasigeostrophic (QG) model, with the aim to understand the low-frequency variability of the South China Sea (SCS) circulation. It is demonstrated that the seasonally varying monsoonal wind can force the upper-ocean circulation with significant internal variability, which is mainly associated with the intrinsic nonlinear dynamics of the summer double-gyre system. This arises from the fact that the intrinsic variability, characterized by the Rossby wave adjustment in the winter single-gyre system, is much weaker than that in the summer double-gyre system driven by the intergyre eddy potential vorticity flux through barotropic instability.

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Haiyuan Yang, Lixin Wu, Shantong Sun, and Zhaohui Chen

Abstract

The role of the South China Sea (SCS) in regulating the North Pacific circulation is investigated using a two-layer quasigeostrophic (QG) model. The double-gyre circulations in the North Pacific with and without the SCS are compared and analyzed. It is found that the SCS acts as a sink of both potential vorticity (PV) and energy in the Pacific–SCS system for the mean state. Consequently, the Kuroshio and the upstream Kuroshio Extension (KE) are weaker in the presence of the SCS. Moreover, the eddy activity is also lower in the North Pacific Ocean because the barotropic instability is suppressed for a weaker circulation. In terms of low-frequency variations at interannual to decadal time scale, the presence of the SCS is found to enhance the variability of the latitudinal position and intensity of the KE jet. This is explained by a positive feedback process that is associated with the negative correlation between the inertia of the Kuroshio and its intrusion into the SCS.

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Haiyuan Yang, Lixin Wu, Shantong Sun, and Zhaohui Chen

Abstract

The response of the South China Sea (SCS) circulation to intraseasonal variability of the summer monsoon is studied with both observations and a 1.5-layer reduced-gravity model. Intraseasonal variability of the SCS summer monsoon is characterized by evolution of the wind jet intensity in the midbasin with typical amplitude of 6 m s−1 and several peaks on its power spectrum between 10 and 60 days. However, this study finds that intraseasonal variability of the sea surface height (SSH) in the SCS presents significant variability to the southeast of Vietnam with amplitude of 6 cm and a period only between 40 and 60 days. This implicates the frequency selectivity of oceanic response to wind forcing. Numerical experiments suggest that the intrinsic variability of the SCS circulation accounts for this phenomenon. Based on the Rossby basin mode theory, this is explained by the interaction between the long, westward-propagating Rossby waves and the short, eastward-propagating Rossby waves.

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Ruichen Zhu, Zhaohui Chen, Zhiwei Zhang, Haiyuan Yang, and Lixin Wu

Abstract

Subthermocline eddies (STEs), also termed intrathermocline eddies or submesoscale coherent vortices, are lens-shaped eddies with anomalous water properties located in or below the thermocline. Although STEs have been discovered in many parts of the World Ocean, most of them were observed accidentally in hydrographic profiles, and direct velocity measurements are very rare. In this study, dynamic features of STEs in the Kuroshio Extension (KE) region are examined in detail using concurrent temperature/salinity and velocity measurements from mooring arrays. During the moored observation periods of 2004–06 and 2015–19, 11 single-core STEs, including 8 with warm/salty cores and 3 with cold/fresh cores, were captured. The thermohaline properties in their cores suggest that these STEs may originate from the subarctic front and the upstream Kuroshio south of Japan. The estimated radius of these STEs varied from 8 to 66 km with the mean value of ~30 km. The warm/salty STEs seemed to be larger and rotate faster than the cold/fresh ones. In addition to single-core STEs, a dual-core STE was observed in the KE recirculation region, which showed that the upper cold/fresh cores stacked vertically over the lower warm/salty cores. Based on the observed parameters of the STEs, their Rossby number and Burger number were further estimated, with values up to 0.5 and 1, respectively. Furthermore, a low Richardson number O (0.25) was found at the periphery of these STEs, suggesting that shear instability-induced turbulent mixing may be an erosion route for the STEs.

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Haiyuan Yang, Ping Chang, Bo Qiu, Qiuying Zhang, Lixin Wu, Zhaohui Chen, and Hong Wang

ABSTRACT

Using the high-resolution Community Earth System Model (CESM) output, this study investigates air–sea interaction and its role in eddy energy dissipation in the Kuroshio Extension (KE) region. Based on an eddy energetics analysis, it is found that the baroclinic pathway associated with temperature variability is the main eddy energy source in this region. Both the air–sea heat flux and wind stress act as eddy killers that remove energy from oceanic eddies. Heat exchange between atmosphere and oceanic eddies dominates the dissipation of eddy temperature variance within the surface layer and accounts for 36% of the total dissipation in the upper 350-m layer. Compared to the heat exchange, the role of wind power in damping the eddy kinetic energy (EKE) is relatively small. Only 18% of EKE dissipation in the upper 350 m is attributed to eddy wind power. Misrepresentation of the damping role of mesoscale ocean–atmosphere interaction can result in an incorrect vertical structure of eddy energy dissipation, leading to an erroneous representation of vertical mixing in the interior ocean.

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Haiyuan Yang, Lixin Wu, Ping Chang, Bo Qiu, Zhao Jing, Qiuying Zhang, and Zhaohui Chen

Abstract

Using eddy-resolving Community Earth System Model (CESM) simulations, this study investigates mesoscale energetics and air–sea interaction at two different time-scale windows in the Kuroshio Extension (KE) region. Based on an energy budget analysis, it is found that both baroclinic and barotropic pathways contribute to eddy energy generation within the low-frequency window (longer than 3 weeks) in this region, while both air–sea heat fluxes and wind stresses act as prominent eddy killers that remove energy from the ocean. In contrast, within the high-frequency window oceanic variability is mainly fed by baroclinic instability and regulated by turbulent thermal wind (TTW) processes, while the positive wind work is derived primarily from ageostrophic flow, i.e., Ekman drift, and along with air–sea heat fluxes has little influence on geostrophic mesoscale eddies.

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