The impact of freshening over the Antarctic Ocean on Pacific Decadal Oscillation

Abstract

The profound influence of the Antarctic Ocean freshening on the Pacific Decadal Oscillation (PDO) is investigated in this study by utilizing a series of fully coupled ocean-atmosphere 400-year-modeling experiments. The simulated results derived from the Fast Ocean-Atmosphere Model (FOAM) can reasonably identify the spatial pattern and time period (10–20 years and 20–50 years) of the observed PDO with slightly weak amplitudes. In the sensitivity experiment (Southern Ocean Water Hosing), 1.0 Sv (Sverdrup, 1Sv = 1.0 × 106 m³/s) freshwater flux is uniformly imposed over the Antarctic Ocean for 400 years. As a response to this Antarctic Ocean freshening, the Tropical Pacific Ocean displays a normal “La Niña pattern”, while the low-frequency variability within the North Pacific Ocean is much weakened. Preserving the PDO’s spatial pattern, the multidecadal (20–50 years) magnitude becomes weak and shifts toward higher frequency. In contrast, the decadal magnitude of the PDO (10–20 years) is slightly reinforced and also shifts towards higher frequency. Dynamical analysis indicates that the shortening of the PDO multidecadal variability is mainly caused by the acceleration of the first-baroclinic-mode Rossby waves. The spreading of the fresh anomalies and associated increasing stratification in the North Pacific Ocean result in the shortening of the long Rossby wave propagation to cross the subtropical North Pacific basin. A heat budget analysis further shows that the upper-ocean thermodynamic variability in relationship to the stratification oscillation in the North Pacific Ocean is mainly associated with the anomalous behaviors of the meridional advection, heat flux and ocean mixing.