Studies of Ocean Tides with Satellite Altimetry

Author:

Edward Zaron (Oregon State University)

Co-Investigator(s):

Richard Ray (NASA Goddard Space Flight Center)

Project Summary:

We propose a series of investigations of ocean tides using data from both historical and ongoing satellite altimeter missions. The proposed research will address fundamental dynamical questions related to tidal mixing and the nonlinear interaction of tides with the ocean general circulation. We will build on insights from our previous work, and from analysis of altimetry, to refine and develop new tide models with improved predictive skill.

The first objective of this project is to describe the small changes in ocean tides which have occurred through interactions with non-astronomical processes. The astronomical ocean tides, generated through the well-known perturbations of near-earth gravity by the Sun and Moon, can be affected by non-astronomical processes related to ocean stratification, boundary currents, etc. The quantitative impact of these processes on the tides is generally at the level of centimeters, small compared to the typical tidal range of a meter. Thus, the reliable identification of these changes requires a stable observing system with well-characterized uncertainties. In contrast to coastal tide gauges, which are sensitive to coastal morphology and other localized factors, the near-global sampling of satellite altimeters is ideal for identifying tidal changes that result from large-scale ocean dynamics.

Our approach to studying tidal change will involve analysis of the complete record of satellite radar altimetry, emphasizing the long reference missions. Using harmonic analysis and related techniques we will describe the changes in tides, and evaluate all the systematic sources of error caused by spurious or real tidal signals in the geophysical corrections applied to altimetry. The significance of this objective is two-fold: first, it will provide new insights into the timescales and processes of non-tidal ocean dynamics; and, second, it will provide a basis for making tide predictions which explicitly account for non-tidal variability, thus improving the usefulness of the predictions.

The second objective of this proposal is to refine our understanding of long-period and sub-inertial ocean tides. It has been over 15 years since there was a comprehensive analysis of long-period tidal dynamics, and the quantity of data available for such an analysis has more than doubled since then. Also, the significance of tidal mixing caused by sub-inertial tides (primarily, diurnal tides poleward of their critical latitudes) has been recognized only recently, but very poorly quantified. This component of the project will lead to a better understanding of sub-inertial dynamics and the contribution of these waves to tidal mixing and mean flow generation.

The final objective of the proposed work will involve a new effort to increase the resolution of the TPXO data-assimilative tide model. The resolution of the present version of the model (TPXO10) is significantly less than that of the FES2022 model, and it may not represent small-spatial-scale features which are poorly constrained by observational data and depend on hydrodynamical constraints. We would like to improve the numerics of the shallow water model that TPXO is based on in order to better represent small-scale topography and complex coastal dynamics. Given the impending retirement of the scientific programmer who has developed the current system, implementation of the proposed low-level changes will provide a unique opportunity to train a post-doctoral scholar and sustain the scientific expertise needed to utilize altimetry and numerical models for precise tidal prediction.