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Modern and past processes of ocean-atmosphere-climate interactions in the low-latitude western Pacific and Indian Ocean
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Characteristic and forcing mechanism of the North Equatorial Undercurrent: a numerical modeling study
P-G2-03-S
Junlu LI* , Division of Environment, The Hong Kong University of Science and Technology
Jianping GAN, Division of Environment, The Hong Kong University of Science and Technology
Presenter Email: jliaw@connect.ust.hk |
| The North Equatorial Current-Kuroshio-Mindanao Current (NKM) circulation system in the Western Pacific Ocean (WPO) governs the water/energy exchange in the warm pool and greatly influences the conditions in China Seas. Beneath each component of the NKM, there exist the eastward North Equatorial Undercurrent (NEUC), southward Luzon Undercurrent (LUC) and northward Mindanao Undercurrent (MUC), respectively. Although this undercurrent system (NKMU) has been observed in the past decades, its physical characteristics and underlying dynamics remain largely unclear. The NEUC transports intermediate water eastward and links dynamically with the NKM, while the LUC and MUC may fuel the NEUC. We know little about the connections among the undercurrents in the NKMU as well as the linkage between the NKMU and NKM. Based on physics-oriented modeling studies, we find that the NEUC extends from the bottom of the NEC at ~600 m to deeper than 1500 m. It is characterized with the northern and southern branches, each with width of ~400 km. The net eastward transport of the NEUC reaches its peak intensity in the second half of a year but is replaced by a net westward transport in the first half of a year. Different dynamics origins lead to different seasonal variations in the northern and southern branches, which peaks in August and November, respectively. By decomposing the velocity of the NEUC into large-scale (>400 km) and meso-scale (<400 km) components according to the intrinsic width of each branch, we find that the large-scale variability has a stronger seasonality, which is significantly associated with the combined effect of the meridional barotropic pressure gradient force (PGF_BT) and the baroclinic PGF (PGF_BC). The meso-scale variability is spatiotemporally independent, which is attributed to the westward propagating meso-scale eddies. The NEUC is mainly fueled by the inflows of the adjacent LUC and MUC, and the variation and magnitude of the northern and southern branch of the NEUC is highly correlated with the LUC and MUC, respectively. The NEUC is largely offset by Sverdrup transport induced by wind stress curl, yet the atmospheric forcing contributes ~60% to the seasonal variation of the NEUC. |
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