The origin and fate of mode water in the southern Pacific Ocean

Understanding the origin and fate of mode and intermediate waters (MW) in the subtropical Pacific Ocean is critical for climate, as MW store and export a large volume of CO₂, heat, nutrients and salinity to lower latitudes at depths isolated from the atmosphere. A realistic 4D simulation has been used to track and quantify the MW routes and their property characteristics at the last region of subduction. It also allows us to quantify the water transformation after subduction. The simulation has been compared to available observations using a collocation method that interpolated model data onto observations in time and space. The comprehensive comparisons gave us confidence in the model’s capacity to reproduce MW characteristics. A quantitative Lagrangian analysis was performed on the model output to depict the origin, the fate and the route of MW circulating in the southern Pacific Ocean, selected in the density range of 26.8–27.4 kg m⁻³. We found 18 Sv of MW were transported northward in patches through the 42° S section, mostly between 200 and 800 m depth. Of this transport, 8 Sv enters the Pacific Ocean in the upper layer south of Tasmania and subducts in the Pacific. The remainder is not ventilated in the Pacific sector: 4 Sv is advected from the Indian Ocean south of Tasmania at intermediate depth and finally 6 Sv is part of an intermediate depth recirculation within the Pacific Ocean. Particles take up to 30 years to travel northward through our domain before crossing the 42° S section. Southward transport branches also exist: 3 Sv flows southward following the eastern New Zealand coast and then exits through Drake Passage. An additional 4 Sv passes southward in the Tasman Sea, following the eastern Tasmanian coast and enters the Indian Ocean south of Tasmania, as part of the Tasman Leakage. Four different formation sites have been identified, where the MW are last ventilated. These formation sites have different water masses with specific salinity ranges. A study on the evolution of the physical characteristics of each of these water masses has been performed. All MW characteristics become more homogeneous at 42° S than they were when they left the mixed layer. This study confirms the homogenisation of mode waters at intermediate depth in the Pacific Ocean as previously revealed in the Indian Ocean using the same methodology. Transformations are shown to be mostly isopycnal in the Tasman Sea and diapycnal farther east.