Tropical Subsurface Salinity and Tritium Distributions in the Pacific: Their Differences and Formation Mechanisms

Abstract

While high salinity water extends to the equator in the upper thermocline of the Pacific in the Southern Hemisphere (SH), it hits the western boundary (WB) farther north of the equator in the Northern Hemisphere (NH), suggesting that no interior pathway exists to the equatorial region. By contrast, high tritium water appears on the equator in the central Pacific, apparently through a NH interior pathway within the thermocline. The mechanisms of forming these salinity and tritium distributions and the causes of their difference are investigated using a realistic ocean general circulation model (OGCM). The OGCM reproduces the properties of tropical salinity distribution quite well and displays interior pathways in the NH. Analysis indicates that the observed salinity distribution is compatible with the existence of a NH interior pathway. Key to the hemispheric difference in thermocline salinity is the sea surface salinity (SSS) distribution in relation to the so-called WB (interior) exchange window, from which subducted water goes to the equatorial region through the WB region (interior ocean). In the NH, high SSSs are found only in the WB exchange window, and high salinity water thus appears to turn onto the WB before reaching the equator. In the SH, on the other hand, high SSSs are found in both the WB and interior exchange windows, and, as a result, high salinity water extends to the equatorial region through both the WB region and interior ocean. The sea surface tritium field has high values near the eastern boundary within the interior exchange window in the midlatitude North Pacific. Thus, high tritium water takes the NH interior pathway to the equatorial region after the subduction. This is demonstrated by a passive tracer experiment with a sea surface distribution resembling that of tritium. This result suggests that the apparent differences between the isopycnal salinity and tritium distributions are largely due to differences in surface distribution, raising caution about interpreting ocean circulation with tracer fields alone.