Characteristics And Dvnamics Of The North Equatorial Current In The Western Tropical Ocean

Based on the hydrographic measurements along the137°E section of JapanMeteorological Agency (JMA) from1967to2009and the calculated geostrophiccurrent, combining with satellite altimetric sea surface height (SSH) data, interannualvariability of the North Equatorial Current (NEC) is investigated, with the controllingmechanisms also explored. Compared with previous works, the study emphasizes thechanges of the velocity structure of the NEC and the differences between theinterannual variations in winter and summer.For the mean state, the meridional position of the NEC is between8°N and18°N,with westward velocity core situated at around10°N-12°N. The mean transport of theNEC across the section is about55Sv (50Sv) in summer (winter), with the summertransport larger by5Sv. An EOF analysis reveals similar patterns in leading modes insummer and winter, showing out-of-phase interannual variations for the southern andnorthern parts of the NEC with the winter pattern situating more northly by~1°thanthe summer one. Correlations analysis between velocity field at the section and thetransport series shows that, in winter, the NEC transport changes are mainly caused byincrease/decrease of the NEC width; in summer, except the width changes, the effectof the strengthening/weakening of the velocity core is also prominent. In order toquantify the structural changes of the NEC, we define the main body of NEC bywestward vertically-averaged velocities greater than7cm/s, and thereby define itswidth and southern/northern boundary. The interannual changes of the boundaries arebetter correlated with the NEC transport in winter than in summer, suggesting againthe more important role of the current width in deciding the NEC transport in winter.Further analysis reveals that the interannual changes of the NEC are wellcorrelated with SSH variations in the western Pacific. In summer, the highly relatedareas are mainly in the subtropical region of the western Pacific. The interannualchanges of the NEC are mainly controlled by influences from subtropical gyre. In thewinter, on the other hand, the highly related areas mainly appear in the westerntropical Pacific, centered around the Mindanao Dome (MD), indicating the dominantrole of tropical gyre interannual signals in winter.The impact of the ENSO events on the interannual variability of the NEC is also explored. The results show that, in the summer, the impact of ENSO is reflected byboth the poleward expansion/contraction of the NEC and strengthening/weakening ofits velocity core. These transport changes of the NEC are mainly compensated by theeastward flow variations in the subtropical gyre. In winter, the role of ENSO isprimarily deciding the width of the NEC through its poleward/equatorwardexpansion/contraction, which is compensated by the changes of the eastward flowingNorth Equatorial Countercurrent (NECC) in the south of the NEC.