Subsurface Subtropical Fronts of the North Pacific as Inherent Boundaries in the Ventilated Thermocline

Abstract

The two subsurface temperature–density fronts and the accompanying eastward currents in the central and southern part of the North Pacific subtropical gyre are identified using the repeat hydrographic sections in the western part of the gyre and the sections over the whole gyre from the World Ocean Circulation Experiment (WOCE) Hydrographic Program (WHP). The northern eastward current corresponds to what has been known as the subtropical countercurrent (STCC). The countercurrent/front is located typically near 24°N and extends from the western boundary approximately to the international date line. The previous suggestion that the STCC is located at the southern edge of North Pacific Subtropical Mode Water (STMW) is confirmed; the front appears as the southern boundary of the lower potential vorticity (PV)/apparent oxygen utilization (AOU) waters on the isopycnals within the STMW layer. The southern eastward current corresponds to what was recognized earlier but has not been documented in detail. The southern countercurrent/front is located near 18°N west of the date line, but shifts to the north east of the date line. Its eastern limit is around 165°W at 25°N. The southern front corresponds to the southern boundary of the lower PV/AOU waters on the isopycnal surfaces of the wide σθ range including those of North Pacific Central Mode Water (CMW). While CMW seems to contribute largely to this low PV/AOU layer, the contribution from other waters is still significant. Judging from the associated PV/AOU features in the thermocline, both fronts can be regarded as inherent boundaries in the ventilated thermocline, dividing the regions ventilated to distinctive degrees. Correspondence between the observational features of the countercurrents/fronts and some of the previously proposed theories concerning the STCC are discussed.