Low-latitude western boundary current anomalies in the North Pacific associated with the ENSO cycle

Based on SODA ocean assimilation and NCEP atmospheric reanalysis data, relationships of the main modes of tropic Pacific subsurface ocean temperature anomalies with variations of the upper ocean circulation of the North Pacific low-latitude western boundary currents (NPLLWBC) and the Asian-North Pacific atmospheric vertical and horizontal flow fields are analyzed. Major results are summarized as follows:(1) The tropical Pacific subsurface ocean variabilities are dominated by two primary modes, which make up the ENSO cycle. The first mode, represents the ENSO mature phase, and mainly prevails in winter, while the second mode represents the ENSO transition phase, prevailing in summer.(2) ENSO cycle has major impact on the upper ocean circulation in the NPLLWBC areas. During the developing phase of El Niño events or decaying phase of La Niña events, anomalous cyclonic circulation evolves in this area, accompanied by strengthening of the North Equatorial Current (NEC), northward shift of the NEC bifurcation location, enhancement of the Mindanao Current (MC), weakening of the Kuroshio Current (KC) in its source region, and the strongest phase of the North Equatorial Countercurrent (NECC). During the mature phase of El Niño, the anomalous cyclonic circulation in this area reaches its strongest phase, with NEC also strongest, NEC bifurcation location northmost, MC strongest, KC weakest, and NECC weakened. Vice versa, during the decaying of El Niño or developing of La Niña, anomalous anti-cyclonic circulation appears in the area, the NEC weakens, NEC bifurcation location shifts southward, MC weakens while KC strengthens, and the NECC becomes weakest. During the mature phase of La Niña, the anti-cyclonic circulation anomalies become strongest, with NEC weakest, NEC bifurcation location southmost, MC weakest, KC strongest, and NECC strengthened. (3) The influence of ENSO cycle on the upper ocean circulation of NPLLWBC are mainly achieved through the change of atmospheric circulation in response to thermal condition anomalies in the tropical Pacific during ENSO events. The ENSO events first generate anomalous thermal conditions in the tropical Pacific Ocean, which lead to anomalous convection activities in the above atmosphere. The latter in turn alters atmospheric circulation field in related area directly or indirectly by energy transport through the "atmospheric bridge" resulting in anomalous sea surface wind stress fields, and thus forces corresponding variations in the upper ocean circulation. In the end of the paper, the reasons for generation and sustainment of the anomalous anti-cyclonic/cyclonic wind fields near Philippines during the ENSO events are also analyzed, and the contributions of air-sea interaction in the NPLLWBC areas to the ENSO cycle are discussed.