Formation of North Pacific Intermediate Water in Meteorological Research Institute ocean general circulation model: 2. Transient tracer experiments

Abstract

Spreading of North Pacific Intermediate Water (NPIW) is investigated using transient tracers, such as chlorofluorocarbons (CFCs), tritium‐type tracers, and dye tracers. Tracer advection and diffusion are calculated based on flow and density fields obtained using an Meteorological Research Institute (MRI) ocean general circulation model for the North Pacific Ocean. Historical CFCs simulation generally agrees well with observed distributions. The subsurface CFCs maximum, the most striking feature of observed CFCs distribution in the North Pacific, is well reproduced by the model. Tritium‐type tracers with a sink due to radioactive decay and a source restoring to a uniform value, are used to determine the path of NPIW spreading. Two tracer source regions are assumed: the subarctic sea surface and the Okhotsk Sea. Tracers flowing from source regions spread southward with paths different in the 26.4–26.7 σθ surfaces, which outcrop only in the subarctic gyre, and in the 26.8–27.4 σθ surfaces, which do not outcrop in the North Pacific. In the 26.4–26.7 σθ surfaces, tracers are mostly advected eastward by the clockwise intermediate layer circulation, and eventually reach the subtropical gyre mostly in the eastern North Pacific. In the 26.8–27.4 σθ surfaces, however, tracers are carried southeastward by isopycnal eddy diffusion into the subtropical gyre beneath the Kuroshio Extension flow. Transient dye tracers whose source varies over time are used to estimate timescale of the NPIW spreading, defined as a time span over which an assumed temporal peak of the source reaches the basin interior. This timescale is free from contamination by mixing with surrounding waters. It is found that the timescale increases with density, <10 years at 26.4 σθ (advection timescale) to 15–25 years at 27.2 σθ (diffusion timescale).