Process Decomposition of Ocean-Bottom Pressure Variability: What OFES2 Reproduces and Misses

 Ocean-bottom pressure (OBP) gauges are a key tool in seafloor geodesy, providing time-series observations of vertical seafloor motion by continuously recording pressure variations associated with changes in water depth. However, OBP time series contain multiple superimposed signals in addition to crustal deformation, including tides, instrumental drift, and non-tidal oceanic fluctuations driven by atmospheric and oceanic processes. In particular, non-tidal oceanographic fluctuations have time scales comparable to long-term crustal deformation, making it difficult to separate them and accurately estimate long-term seafloor deformation.

 Ocean-model-based corrections have been proposed to isolate and remove non-tidal oceanographic components, offering a physically based approach. However, model–observation mismatches can leave large residuals after correction, making it harder to identify deformation-related signals. As a fundamental step toward understanding these model–observation discrepancies, this study quantifies the physical processes controlling OBP variability using an ocean general circulation model named OFES2 (Sasaki et al., 2020), which does not assimilate oceanic observations.

 As a first step, we assess the extent to which OFES2 can account for the observed OBP variability by directly comparing modeled and observed time series. The modeled OBP is compared with observed OBP records from pressure gauges deployed off northeastern Japan. Our results show that OFES2 reproduces the seasonal cycle of the observed OBP time series to some extent, whereas agreement at periods of several days to about a month remains limited in both amplitude and phase.

 To investigate the physical processes controlling the modeled OBP variability, we decompose the modeled OBP into four components: atmospheric pressure loading, sea surface height variability, horizontal density advection, and vertical advection, and quantify the relative contribution of each component. The decomposition indicates that atmospheric pressure loading and sea surface height variability dominate the modeled OBP fluctuations, while horizontal density advection contributes to part of the seasonal variability. Moreover, correlation analyses using the time-derivative of the observed OBP and the decomposed pressure components reveal episodic enhancements in correlation with the horizontal density advection term, suggesting that its contribution can temporarily increase during specific periods.

 We will extend the analysis by comparing OFES2 with longer-term OBP observations and discussing in more detail the contributions of each component to seafloor pressure variability.