Time‐varying parametric subharmonic instability from repeat CTD surveys in the northwestern Pacific Ocean

Abstract

Time‐varying diapycnal diffusivity signals are estimated using the fine‐scale parameterization method from high‐resolution CTD data along three repeat sections in the northwestern Pacific Ocean. Indicative of the parametric subharmonic instability (PSI), locally‐elevated diapycnal mixing is detected alongallthree sections within the 25°–29°N band where bottom topography is relatively featureless. Due to their proximity to the semidiurnal internal tide generation sites, the two sections along ∼137°E have a time‐mean diffusivity value 2.5 × 10−5 m2 s−1 in the 300–2,000 m upper ocean. In contrast, it is 1.2 × 10−5 m2 s−1along the 165°E section. The time‐varying diffusivity along both sections is dominated by signals whose vertical structure resembles the local first dynamic normal mode profile. At the 137°E site of 25°–29°N, the local spring‐neap modulated semidiurnal tidal current is found to lead the CTD‐derived diffusivity time series by 6 days and can alter the diffusivity level by 0.24 × 10−5 m2 s−1. Similarly, the concurrent surface wind work is found to modify the diffusivity level by 0.35 × 10−5 m2 s−1. The combined spring‐neap tide and wind work forcing explains 47% of the observed, time‐varying diffusivity signals.