The Features Of Bifurcate Line And Transportation About The North Equatorial Current In The Pacific

The equatorial current in the North Pacific is an upper layer westerly ocean current, it flows to the west boundary of the ocean, east of Philippine, and bifurcate into the northerly Kuroshio and the main body of the southerly Mindanao current. Thus, NEC is both the south branch of Subtropical Circulation and the north branch of the Tropical Circulation. In this paper, we call the subtropical circulation branch as the north branch of NEC, and the tropical circulation branch as the south branch of NEC. The junction of the two branches extended to the west boundary to connect the bifurcation points therefore form the bifurcation line. The resent study shows that there are three branches of NEC in the thermocline, in addition to Kuroshio and Mindanao current., there is a current flow directly from inner ocean to the equator. Thus, in each layer, there are two bifurcation line of NEC, they are west boundary bifurcation and inner bifurcation respectively. The position North Pacific Equatorial Current bifurcation line of the surface determines the exchange and distribution between subtropical and tropical circulation. The interannual and interdecadal change of North Equatorial Current bifurcation line can affect the he heat distribution and thermal structure changes of tropical and subtropical Pacific, thus affecting the local or global climate. Therefore, the study of North Equatorial Current bifurcation line and the change of channel is really a key point for in-depth understanding of sub-tropical ocean circulation, tropical ocean circulation and heat transport, it has indicative significance or the understanding of ocean circulation and climate change itself.In paper, University of Maryland School of Marine Science Atmospheric latest version of SODA (Simple Ocean Data Assimilation) data (0.5°* 0.5°,1958-2007) was used to divide the Pacific North Equatorial Current, a new identification method to track line and the bifurcation channel was used in this study, we focus on the climatological characteristics of the western boundary of the North Equatorial Current bifurcation line and the inner area, from the point of view to the western boundary the bifurcation line drifted polar ward with depth, inside the area from the branch line down to 180°E showed a clockwise rotation, for seasonal variation, the average position of western boundary of the bifurcation line is most south in June, and is most north in December, while the inner zone Pitchfork back south east there are the spring and summer, autumn and winter west northward trend, while the initial signal source of the two bifurcation lines'latitude change are different. NEC bifurcation of the western boundary north to the channel (that is, the Kuroshio channel), NEC bifurcation of the western boundary of the south channel and the NEC three channels within the area channel characterization are mainly achieved by the main wall of the six channels to achieve: the black curve from top to bottom from left to right were: NEC North boundary, NEC bifurcation to the south boundary of the Kuroshio, NEC bifurcation flow to the northern boundary of Mindanao, NEC bifurcation flow to the southern boundary of Mindanao, NEC bifurcation zone of the western boundary inward , NEC bifurcation zone of the eastern boundary of the total inward six lines, as each channel wall, resulting in the change of variable along the channel characteristics and laws. NEC bifurcation of the western boundary north-south channel and the pattern of channel width and the average speed were in reverse phase, the volume of transport in the north is consistent with the seasonal changes, for the summer it is larger and smaller in the winter, smallest in December and biggest in June. Seasonal variation of transport volume in south channel is on the contrary to that of in the north channel, and they have a complementary relationship, but the seasonal change by only about half of the north channel, the transportation of seasonal change is less than the North Channel. At the same time changes in the channel near the western boundary and near the inner zone is different, variation of transportation have obvious differences between channel to the east of 140°E and west of 140°E.From sprint to winter in the years when ENSO is developing, the position of the west boundary bifurcation of NEC is relatively north (south) in El Nino(La Nina) compared to normal years. The inner bifurcation extended to a deeper layer which means the clear of the inner pathway, while the inner pathway is blocked in the El Nino years. The total volume transportation of the north and south branch of NEC in El Nino years is larger than that of in La Nina years, and the north branch pathway is more obvious. According to the analysis of the main modal of flow pattern, the flow field strength has become weak with time, and there is abrupt change, mutation testing time is the 20th century, late 70s early 80s. The analysis of decadal and weak-strong year variation shows that, when the flow pattern has an abnormal weakening trend, the meridianal width of pathway increase, and the depth of pathway extend, in each layer the north and south branch of NEC shows a complementary effect. According to the analysis of monthly data about 50 years, high response of NEC current is the most significant to wind stress of the tropical region in season, NEC current focus behind the wind stress curl 3 to 4 months is best relevant. The phenomenon is confirmed by a 1&1/2 Model. Wind stress-driven one and a half model results also confirmed the following features of data analysis : The line of bifurcation at the western boundary move to north(south) during the enhanced El Nino (La Nina), and move to south (north) during the weakened El Nino (La Nina). The wind stress vorticity zero line plays a decisive role in decide the position of west boundary bifurcation, the increase (decrease) of wind stress vorticity in the field causes the strengthen (weaken) of Mindanao Vorticity and the eastward (westward) of low latitude central and east Pacific circulation by the propagate of Rossby Wave and coastal Kenvin Wave, and therefore caused the decrease (increase) of inner pathway.