Dynamic and thermohaline properties of the Mindanao Undercurrent I. Dynamic structure

Hydrographic data from eleven 1986–1991 cruises at zonal sections near 8°N from the Philippine coast to 130°E were used to examine the dynamic structure of the western boundary currents there in the present study focusing on the Mindanao Undercurrent (MUC). The MUC with maximum velocity >10 cm/s was found to be a feature of both the individual cruises and multiyear mean velocity field as a countercurrent below the Mindanao Current (MC). Usually, the MUC occupies depths below 200 dbar and consists of more than one core. Its vertical and horizontal scales are 500–1000 m and 100–250 km, respectively, and vary greatly and irregularly. The spatial distribution of the MUC agreed with that of the westward deepening isopycnals in and below the thermocline. In the individual cruises, the volume transports of the MUC relative to 1500 dbar varied from 6.2 to 28.4×106 m3/s (average of 14.4 ×106 m3/s) while those relative to 3000 dbar varied from 15.4 to 43.9×106 m3/s (average of 25.4×106 m3/s). The multiyear mean MUC transport was 5.9×106 m3/s relative to 1500 dbar and 8.7×106 m3/s relative to 3000 dbar. The difference between the multiyear mean MUC transport and that of individual cruises resulted from the MUC spatial variation.