[A-CG48] Water and sediment dynamics from land to coastal zones

[ACG48-09] Effects of freshwater discharge into the Northeast Pacific from the Alaskan mountains on the subtropical and equatorial ventilated thermocline

*Peng Xin¹, Humio Mitsudera¹, Takuro Matsuta¹ (1.Hokkaido University)

Keywords:Freshwater, Pacific Ocean, Subpolar gyre, Equatorial ventilated thermocline

Freshwater flux plays an important role in the ocean circulation. Previous studies found that freshwater input into the subtropical gyre leads to a deepening of the equatorial thermocline and a shrinking of the cold tongue in the eastern equatorial Pacific. This process results in a reduction of heat gain in the eastern equatorial Pacific and potentially contributes to an El Niño-like warming pattern. However, exploration of the effects of freshwater input at higher latitudes such as the subpolar gyre on the ocean circulation remains poorly understood. This study investigates the effects of continental freshwater discharge mimicking those from the Amur River, Kamchatka peninsula, and Alaska coastal area into the northern subpolar gyre on the ocean circulation, especially on the subtropical and equatorial thermocline, using numerical experiments with the MITgcm. Under freshwater forcing, low-salinity water is transported along the isopycnal surfaces from the subpolar gyre to the subtropical gyre, leading to cooling effects in the upper western equatorial Pacific because of the compensation between temperature and salinity on an isopycnal surface. Meanwhile, freshwater input into the subpolar gyre causes the deepening of isopycnal surfaces; the effect of this deepening dominates in the South Pacific, contributing to subsurface warming at the equator. In extreme cases with the Cordilleran Ice Sheet collapse during the Last Glacial period, these effects become more pronounced. Consequently, increased freshwater input weakens the east-west SST gradient and deepens the thermocline along the equator, creating an El Niño-like state. These findings highlight the significant role of high-latitude freshwater fluxes in shaping equatorial thermocline dynamics and their broader impacts on regional and global climate systems.