The Mindanao Undercurrent (MUC) as a subsurface northward flowing current offshore and below the Mindanao Current (MC) was examined based on the comparison between 7-survey observations gathered in autumn 1986~1991 and the assimilation data produced with the GFDL MOM3 and the NODC data set. The MUC with two velocity cores were similarly described by the observation and the assimilation along 8ZN. The MUC consisted of two water masses with respective salinity 34.52 psu at 27.2σt and 34.6 psu at 26.9σt, which were remnants of the Antarctic Intermediate Water (AAIW) and the lower part of the Southern Pacific Subtropical water (SPSW), showing that the MUC was not a local transient but originated from the South Pacific. The MUC found in the assimilation data flows from 5ZN to 11ZN along 127~128ZE and deepens along isopycnals. Its two velocity cores seemed to be related to the eddy-like circulation at depth southeast of the Mindanao, especially the east one connected with the southern recirculation of the Mindanao Eddy (ME) below 200m. The MUC had significant seasonal variation. It was noteworthy that the two velocity cores revealed different seasonal cycles to each other, implying that the formation of the MUC might be composed of different processes.
Based on a simple conceptual model of the stratified ocean, the criterion of the geostrophic velocity reverse in and below the thermocline was derived as h ′•η′< 0 and ρ |η′|≤Δρ| h ′|, meaning that the slopes of the thermocline( h ′ ) and the sea surface(η′) must be opposite to each other, and that the h ′ must be strong enough to satisfy the latter inequality. The features and dynamics of the western boundary undercurrents below the thermocline, such as the MUC, were discussed with the criterion, which indicated that the MUC is probably attributed to the combined effect of the basin-scale wind-driven circulation and local geostrophic balance. |
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