超音波式ドップラー流速計による西部赤道太平洋での表層海流の長期観測

Abstract

Measurements of oceanic currents were performed at two sites (0°, 147°E and 0°, 154°E) in the western equatorial Pacific by upward-looking moored Acoustic Doppler Current Profilers (ADCPs) during about one year in 1992-93 to investigate current variabilities in the surface layer of a warm-pool region. The zonal currents are always eastward except in the uppermost layer above 100 m, in which the current direction changed frequently. Mean eastward currents corresponding to the Equatorial Undercurrent (EUC) have two maxima at 70-90 m and 210 m. The depth of the EUC's core tends to shift in the vertical direction with time scales from several days to a few months. These layered structures of the zonal currents are represented by those of empirical orthogonal function (EOF) modes. At the western site, the first EOF mode contributing about 60 % of the total variance has no nodes in the depth range with a maximum of 90 m, meaning that the entire layer changes with the same phase, while the second EOF mode, contributing about 20 %, has a node at about 110 m, meaning that the upper and lower currents change with an out-of-phase relation. In the uppermost layer above 100 m, strong eastward currents reaching about 80 cm s -1 are found in January 1992, and are related to the third mode which has two nodes, one at 70 m and the other at 130 m. The meridional current has a time average of about zero in almost all layers, its change being about half of the zonal change in magnitude. Spectral analyses indicate that dominant changes are found at periods of about 10 days, 15-20 days, and 30-60 days.