Measuring gravity field variability, the geoid, ocean bottom pressure fluctuations, and their dynamical implications

Abstract

To obtain insight into bottom pressure variability and requirements for assimilating gravity data into an ocean general circulation model, we use recent results from a constrained global model. Bottom pressure variability, as contrasted to surface elevation variability, is examined at the annual, semiannual, Chandler‐wobble, and 4‐month periods. Surface elevation and bottom pressure variability occurs on all spatial and temporal scales resolvable by our analysis, and patterns can be quite different. A barotropic index, developed to quantify variability character, suggests the importance of steric effects at longer periods. Variability decomposition into standing, eastward, and westward traveling waves indicates a strong zonal component for both surface elevation and bottom pressure, with bottom pressure showing a greater overall tendency toward eastward propagation, and surface elevation showing a greater overall tendency toward westward propagation. Bottom pressure variability signals are estimated to be recoverable from space to spherical harmonic degree 27 for the annual cycle and 14 for the semiannual and Chandler‐wobble cycles. Mass exchange, arising from model approximations, is detectable to about degree 7. Polar motion excitation by ocean seafloor loading is found to be quantitatively important. Spurious oscillations encountered in using spherical harmonics to represent oceanic fields are suppressed using Lanczos factors.