Relationship Between The North Equatorial Current Transport And El Ni?o In The Tropical Pacific Ocean

In the North Pacific Ocean, the North Equatorial Current(NEC) driven by the trade wind flows from the east to the west and bifurcates into the northward Kuroshio Current(KC) and southward Mindanao Current(MC) near the Philippine coast, comprising the NEC-MC-KC(NMK) current system, which is a protruding feature in the western Pacific Ocean. As the core of the NMK current system, the NEC transports a huge amount of water from the eastern Pacific to the western Pacific Ocean, which plays an important role in the variation of the western Pacific warm pool and the global thermohaline circulation, and further influences ENSO(El Ni?o and Southern Oscillation) cycle and global climate change. So to study the relationship between the NEC transport and El Ni?o has an important significance. The NEC transport and El Ni?o have close and complex realtionship. As we know,El Ni?os have been categorized into two types: East Pacific El Ni?o(EP-El Ni?o) and Central Pacific El Ni?o(CP-El Ni?o). The characteristics of the NEC transport variations during the two types of El Ni?os are different. But nowadays there are little studies on their relationship, and many problems are not clear.In this paper, the variations of the NEC transport during the two types of El Ni?os are investigated by composite analysis with the long-time data of ECMWF Ocean Analysis/Reanalysis System 3(ORA-S3). In addtion, absolute geostrophic currents in the North Pacific Ocean are calculated by using P-vector method from ocean objective analysis data MOAA GPV, and the relationship between the NEC and El Ni?o during 2001-2013 is examined. The dynamic mechanisms in these two sections are also discussed, respectively. The main research results are summarized as follows.(1) During the two types of El Ni?os, the interannual variations of the NEC transport are significantly different. During EP-El Ni?o, the NEC transport shows positive anomalies from ten months before the Ni?o peak month to six months after that, and the largest anomalies appear from three months before the Ni?o peak month to one month after that in the west of 160°E. In addition, the time of the NEC transport maximum anomaly gradually lags westward along with the longitude. During CP-El Ni?o, however, the NEC transport shows positive anomalies only before the Ni?o peak month and the largest anomalies appear from nine months to eight months before the Ni?o peak month in the area of 140°E~150°E. Though the time of the NEC transport maximum anomaly also gradually lags westward along with the longitude, but this phenomenon is not obvious. Additionally, the strength of the NEC transport anomalies is much weaker than those during EP-El Ni?o.The NEC transport variations are strongly associated with variations of the gyre and wind forcing in the tropical North Pacific Ocean. During EP-El Ni?o, the strong westerly wind anomalies and positive wind stress curl anomalies in the tropical North Pacific Ocean cause upward Ekman pumping velocity anomalies, which induce negative sea surface height anomalies and westward propagating upwelling Rossby waves. In the western tropical Pacific Ocean, local upward Ekman pumping and the upwelling Rossby waves which spread from central and eastern Pacific decrease the sea surface height and generate a cyclonic flow anomaly enhancing the NEC transport. But for the other hand, during CP-El Ni?o, both the strengths of the wind and associated Ekman pumping velocity are very weak. The centers of the negative sea surface height and cyclonic flow anomalies induced by the wind amomalies are located slightly north of those positions during EP-El Ni?o and this will go against the enhancement of the NEC transport.(2) During four El Ni?o events in the recent ten years, the NEC transport significantly increases in the peak months of 02/03, 06/07 and 09/10 El Ni?os, but the case is not in the peak month of 04/05 El Ni?o.Further analyses find that the strong negative sea surface height anomaly and a cyclonic gyre anomaly take place south of 10°N in the northwest tropical Pacific Ocean during the mature phase of 02/03, 06/07 and 09/10 El Ni?os. These are mainly induced by the westerly wind anomaly and positive Ekman pumping in the tropical circulation area through the westward propagating Rossby waves. But during the mature phase of 04/05 El Ni?o, the distribution of sea surface temperature anomalies is obviously different. The westerly wind anomaly and positive Ekman pumping anomaly move northward obviously and induce the negative sea surface height anomaly and cyclonic gyre anomaly occuring north of 10°N in the northwest Pacific Ocean. So the NEC transport does not increase obviously during the mature phase of 04/05 El Ni?o.