Analysis of ¹³⁷Cs concentration in the Pacific using a Lagrangian approach

A hindcast of the 137 Cs concentration in seawater is conducted using a 0.5° × 1° global ocean general circulation model. A significant amount of 137 Cs concentration appears in the subtropical gyre in the South Pacific even though the source of 137 Cs is limited to the Northern Hemisphere. The pathway from the North Pacific to the South Pacific is examined using the Lagrangian approach. The 137 Cs in the South Pacific water is brought by the water that enters the equator through the interior pathway and the Mindanao Current from the subtropical gyre in the North Pacific. The water then flows along the Equatorial Undercurrent, and spreads to the South Pacific through the Ekman transports at the surface. A cross section of 137 Cs along 165°E is examined by the Lagrangian approach and categorized based on the source region. The maximum 137 Cs concentration in 1975 comes from the formation region of the Eastern subtropical Mode Water (EMW) along the 24.7 σ 0 surface, while the origin of the 137 Cs maximum in 2002 spreads over the Central Mode Water (CMW) and the Tropical Water (TW) along the 25.8 σ 0 surface. The change of the maximum 137 Cs concentration from the water of EMW origin to the waters of CMW origin and TW origin is explained by the difference in the time evolution of 137 Cs in the ventilated region. The 137 Cs concentration decreases much more slowly in the formation regions of CMW and TW than in that of EMW.