On Some Aspects In Western Tropical Paciifc Circulation Variability And Dynamics

The western tropical Pacific Ocean is one of the regions with complexcirculation system in the world ocean, and plays an important role in the variability ofthe global and regional ocean and climate. Based on previous studies, some aspects inthe western tropical Pacific circulation variability and dynamics are investigated withuse of subsurface current observations east of the Luzon Island, observations withsatellites, and ocean data assimilation. The topics include interannual and decadalvariability of the North Equatorial Current (NEC), seasonal variability of theHalmahera Eddy (HE), and the tidal characteristics east of the Luzon Island. The mainresults are as follows.(1) Interannual variability of the NEC. West of170oE in the tropical PacificOcean, both the amplitude and root-mean-square (RMS) of interannual anomalies ofthe NEC transport increase westward. This interannual variability is also closelyassociated with the El Ni o-Southern Oscillation (ENSO), and this correlation can bewell expressed by a1.5layer reduced gravity model.In the antecedent phase of El Ni o events, negative SSH anomalies in the westerntropical Pacific Ocean take place first in the Mindanao Eddy (ME) region, and thengradually spread zonally and meridionally during El Ni o germination, development,onset phases, and peak around the mature phase and weaken in the decay phase. Themaximum changes occur mainly in the area of015°N,130°E160°E. Falling of theSSH gives rise to a cyclonic gyre anomaly, resulting in increase of the NEC transportand NEC bifurcation moving northward. During La Ni a years, the oceanic variationsin the first three phases are the same as those in the last three phases of El Ni o events.From the onset phase of La Ni a, however, the positive SSH anomalies replacenegative ones, peak around the mature phase and then weaken in the decay phase. Rising of the SSH gives rise to an anticyclonic gyre anomaly, resulting in decrease ofthe NEC transport and NEC bifurcation moving southward. The oceanic variations aremainly in response to anomalous wind forcing in the west-central tropical NorthPacific Ocean and relate to ENSO. During El Ni o years, westerly wind anomaliesdevelop and move eastward mainly between the equator and10°N in the westerntropical North Pacific Ocean, which is accompanied by cyclonic wind stress curlanomalies between10°N and20°N. Westerly wind anomalies and cyclonic windstress curl anomalies induce upward Ekman pumping anomalies in the ocean, whichfurther induces the first baroclinic mode upwelling Rossby waves; the upwellingRossby waves propagate westward, shoal the pycnocline and lower the SSH. Severalmonths before the mature phase, the westerly wind anomalies get to their easternmostposition, around the dateline. As a result of the accumulative effect of the westwardpropagating upwelling Rossby waves, the SSH south of20°N and west of170°Ereaches its lowest around the mature phase. Falling of the SSH induces cyclonic gyreanomalies, which results in increase of the NEC transport. The situation is reversedduring La Ni a events.(2) Decadal variations of the NEC. From1962to2006, in decadal time scale,the NEC transport in the far western tropical North Pacific Ocean shows four maxima,occurring in1966/1967,1980/1981,1994/1995, and2004/2005, respectively, andthree minima, in1970/1971,1990/1991, and1999/2000, respectively. Except thetransport minima around1970/1971, the decadal variability of the NEC transportshows good association with the decadal variability in the tropical Pacific Ocean(TPDV). During the warm phase, the western tropical Pacific Ocean shows negativeSSH anomalies and cyclonic gyre anomalies, and the NEC transport increases, whileduring the cold phase, the situation is reversed.(3) Tidal characteristics east of the Luzon Island. Around the mooring siteeast of the Luzon Island (122°38.0E,18°1.5N), the high frequency currents withperiods shorter than3days are mainly near-inertial currents, diurnal (O1and K1) andsemidiurnal (M2) tidal currents. The vertical structures of the diurnal and semidiurnaltides are different from each other. Within the observed depth range (330-670m), the semi-major axis of M2tide decreases with depth, while those of O1and K1tides showminima around520m and480m, respectively.(4) Seasonal variability of the Halmahera Eddy (HE). The model resultsindicate that the HE first appears around May, peaks in July, and dies out in February,March and April in the following year, which mainly results from the New GuineaCoastal Current (NGCC) seasonality driven by monsoon. The NGCC flowsnorthwestward from April to November, while southeastward from December to nextFebruary. The HE begins to form and intensify with the strengthening of thenorthwestward NGCC, but decay and disappear with the weakening and reversing ofthe northwestward NGCC. In summer, that the Mindanao Current strengthens alsoenhance the HE.