Two decades of mixing tritium into the North Pacific Ocean

Abstract

Tritium data collected in the North Pacific Ocean mostly from Long Lines cruises (1983–1985) are compared with GEOSECS data (1973–1974) to characterize the dominant circulation processes in the second decade after the major bomb injection. The total decaycorrected inventory for the North Pacific Ocean remained nearly constant during the decade between GEOSECS and Long Lines, although the spatial distribution changed. Tritium was lost from the subpolar region and was gained by the tropics. Exchange between the subpolar and subtropical regions appears to be occurring at both the extreme eastern and western boundaries. On isopycnals that are not thought to outcrop ≥26.80 σθ), the tritium inventory is about 30% higher in the Long Lines data.The GEOSECS tritium data show that the ventilation time scale of the thermocline is rapid; less than 10 years. The GEOSECS tritium distribution is characterized by north-south gradients, which are strongest in the east. The Long Lines tritium data show that the eastern portion of the subtropical gyre is well mixed and has similar values to the subpolar region. We attribute this difference in the data sets to the sharp decrease in the tritium source function after the mid-1960s. Without the addition of tritium at northern outcrops, gradients cannot be maintained in the presence of mixing. Higher tritium values in the eastern, as compared with the western, portion of the basin are consistent with Subtropical Mode Water and shallow salinity minimum water formation. Furthermore, differences between the eastern and western portions of the basin suggest that mixing and/or recirculation may be more important in the west.On non-outcropping isopycnals there is poleward and westward contraction of the subtropical gyre in both data sets. Cross i~opycnal mixing in the subpolar region ventilates isopycnals (as evidenced by the presence of bomb tritium) which do not outcrop. The Long Lines data suggest that after formation in the northwest, intermediate waters may be further ventilated within the Alaskan Gyre. The residence time in the subpolar region is estimated to be less than 14 years for North Pacific Intermediate Water and greater than 20 years for deeper waters. In the Long Lines data tritium penetration reaches at least to the oxygen minimum (27.40 σθ) in the subpolar and most of the SUbtropical region. The average increase in vertical tritium penetration in the Long Lines data is approximately 100 m. This is consistent with the small increase (from 12 to 14%) in the volume of the North Pacific Ocean exposed to bomb tritium. Fourteen per cent is a lower bound estimate of the ventilated volume of the North Pacific Ocean