Low-frequency variability of the North Equatorial Current bifurcation in the past 40 years from SODA
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Low-frequency variability of the North Equatorial Current bifurcation in the past 40 years from SODA MENG Qingjia1,2 , WANG Fan1∗ , LIU Na3 1
2 3
Key Laboratory of Ocean Circulation and Waves (KLOCAW), Chinese Academy of Sciences, Qingdao 266071, China Graduate School of Chinese Academy of Sciences, Beijing 100039, China National Marine Environmental Forecasting Center, Beijing 100081, China
Received 29 July 2010; accepted 5 November 2010 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2011
Abstract The low-frequency variability of the North Equatorial Current (NEC) bifurcation during 1958 to 2001 was investigated with the Simple Ocean Data Assimilation (SODA) 2.0.2 dataset. In agreement with recent observations, the NEC bifurcation latitude (NBL) shifted northward as depth increases, from about 12.7◦ N near the surface to about 17.1◦ N at depths around 500 m for the annual average. This study reveals that the interannual variations of NBL, with five years period, mainly focused on the upper 500 m with amplitude increasing as depth increased. The NBL shifted southward in the past 40 years, which was more significant in the subsurface at more than –0.02◦ /a. The NBL manifests itself in the transports of NMK (NEC-Mindanao Current (MC)-Kuroshio) system in strong relationship with MC (0.7) and Kuroshio (–0.7). The EOF analysis of meridional velocity off the Philippine coast shows that the first mode, explaining 62% of variance and 5 years period, was highly correlated with the southward shift of NBL with coefficient at about 0.75. The southward shift of NBL consists with the weakening of MC and strengthening of Kuroshio, which exhibited linear trends at –0.24Sv/a and 0.11Sv/a. Both interannual variation and trend of NBL were closely related to the variation of NMK system. Key words: North Equatorial Current bifurcation latitude, interannual variation, linear trend, NEC-Mindanao Current-Kuroshio system
1 Introduction In the Northern Hemisphere, the North Equatorial Current (NEC) bifurcates into the northward flowing Kuroshio and southward flowing Mindanao Current (MC) as it meets the Philippine coast (Schott, 1939; Tool et al., 1990). As an important indicator of the partition of the NEC mass, heat, and salt transport between the Kuroshio and MC, the NEC bifurcation latitude (NBL) plays a fundamental role in the water exchange between the subtropical and tropical gyres, which is involved in determining how much water flows to the equatorial region via the MC and how much returns to the subtropics via the Kuroshio. According to the Sverdrup theory, the NBL simply occurs at the zero line of zonally integrated wind stress curl. However, the surface wind changes in
both time and space, which highly impact on the NBL through the local effects and remote Rossby waves (Kim et al., 2004; Qiu et al., 1996). Thus the steady Sverdrup solution, totally ignoring the complex interactions and exchanges of the water masses between oceanic gyres, is not sufficient to describe the actual bifurcation latitude of the N
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