Dynamical Ocean Response Controlling the Eastward Movement of a Heat Content Anomaly Caused by the 18.6-Year Modulation of Localized Tidally Induced Mixing

摘要

Numerical experiments based on a long-term ocean state estimate have been used in previous work to demonstrate that the 18.6-year modulation of localized tidally induced vertical mixing can influence wintertime sea surface temperature (SST) over a broad area of the northern North Pacific, and may play a role in determining the spatial structure of the Pacific Decadal Oscillation (PDO). This study investigates the mechanism driving the model annual mean surface layer heat content anomaly (SHCa), which is correlated with the SST anomaly, at a location near the center of the PDO in the eastern Pacific. The modulation of mixing first induces an 18.6-year oscillatory SHCa in the western boundary region, which moves eastward along the boundary between the subtropical and subarctic gyres, taking similar to 10 years to reach the eastern region. Adjoint sensitivity analysis is applied to confirm the importance of the dynamical response of the ocean to the eastward motion of the SHCa. It is shown that the generation of a temperature anomaly by wave-related current anomalies works to delay the eastward motion of the SHCa. Results also suggest that the generation of a spiciness anomaly in the open ocean may play a role. Although further studies are required to clarify whether the mixing modulation has an impact on the actual climate variability, the mechanism can be applied to any location along an eastward current, independent of the periodicity of the phenomena, and thus might have wide applicability in efforts to understand the role of the ocean in the climate system.