The Pole Tide and its Effect on GRACE Time‐Variable Gravity Measurements: Implications for Estimates of Surface Mass Variations

Gravity Recovery and Climate Experiment (GRACE) provides monthly solutions for the Earth's gravity field in the form of spherical harmonic coefficients. These can be used to infer changes in mass at the Earth's surface. The pole tide (the response of the Earth and oceans to polar motion) causes gravity signals dominated by harmonics of degree 2, order 1. If the pole tide is not removed from GRACE data, it affects the coefficients of those harmonics (C-21, S-21) and introduces errors when using those coefficients to determine surface mass variations. The pole tide is partially removed by GRACE processing centers before solving for the gravity field. But long-period pole tide signals are not usually included in the GRACE pole tide correction, and so those signals are still present in the GRACE coefficients. We discuss this issue from the standpoint of somebody who uses the GRACE gravity fields to infer changes in surface mass. We arrive at a recommendation for an optimal GRACE pole tide correction. We describe how to modify the C-21, S-21 coefficients provided by the processing centers, so that they conform with our recommendation. We discuss the size of the pole tide contributions to C-21, S-21, compared to those of the direct load-induced contributions. As an example, we show how an incompletely removed pole tide can impact GRACE results for the trend in ocean mass. We consider the impact of mantle anelasticity on long-period pole tide corrections and conclude that it is unlikely to affect those corrections by more than 20%.