Analysis and interpretation of Tritium and Helium tracer data collected in the North Pacific in 2015.

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By following the evolution of transient tracers in the upper two kilometers of the water column, it is possible to document changes in thermocline ventilation, in particular the interior circulation pathways between the subtropics and the equatorial region. These circulation cells have been argued to respond to and play an important role in modulating/maintaining major climatic phenomena, including ENSO, and are also relevant to the Pacific Decadal Oscillation. Moreover, there is mounting evidence of decade-timescale changes in the strength of these circulation cells which may have significant climate impacts on even larger scales. Transient tracers highlight the pathways of newly ventilated water into the ocean. Their time-evolving distributions provide quantitative constraints on water mass formation rates and circulation, and the rates of biogeochemical processes. Tritium is a valuable transient tracer because it has a distinctive time history and boundary condition that yields additional information to other tracers. Moreover, its daughter Helium-3 provides additional, unique information because of its complementary (nutrient-like) boundary conditions to all other transient tracers. Extending the time series to longer times provides a description of both longer time-scale ventilation processes and the opportunity to diagnose time variations in the short time-scale processes. Additionally, the evolution toward smaller spatial gradients in the tritium distribution and reduced time derivatives further minimizes non-linear behavior in the tritium/Helium-3 age. Through the reoccupation of key hydrographic sections, observation of the evolving distributions of these tracers will provide powerful constraints on theories and models of ocean circulation and ventilation. The processed data set will be archived and made publicly available through established data centers.. This data will be accessible to a wide range of research enterprises involved in modelling ocean circulation, ventilation, and climate change. Tracers, and in particular radioactive and transient tracers (e.g., chlorofluorocarbons, radiocarbon, tritium/Helium-3), have been integral to both the WOCE (World Ocean Circulation Experiment) and CLIVAR (CLImate VARiability and predictability program) hydrographic programs and are valuable tools for studying ocean ventilation and its variations on climatic scales. Recognizing that global change is already affecting the oceans, the CLIVAR program has sought to re-occupy key WOCE sections at roughly decade time-scale intervals since2003. The primary objectives of this project are to document the temporal evolution of the distributions of the transient tracers tritium and Helium-3 over the 24 years since the WOCE occupation, and to use this information to diagnose the changing rates of thermocline ventilation and meridional overturning in the shallow Pacific Ocean. Moreover, the longer term evolution of these tracers on density horizons that do not directly outcrop in the North Pacific may provide useful constraints on the effect of mixing processes on ventilation of those density horizons. The investigators will analyze and interpret helium isotope and tritium measurements on samples acquired on the 2015 occupation of the P16N section in the central North Pacific that crosses the northern subpolar and subtropical gyres as well as the northern and southern tropics.