Global surface-water-induced seasonal variations in the earth's rotation and gravitational field

SUMMARY The seasonal cycle in continental surface water storage is modelled first using a global meteorological data set of precipitation (snow and rain) and evapotranspiration compiled by Willmott et af. Its effects on the annual wobble excitation (Y), and the seaonal variations in the length of day (LOD) and low-degree zonal gravitational coefficients (JI) are then computed. The resulting Y agrees well with the widely referenced estimate by Van Hylckama. However, these results are questionable because an anatomy of the computation indicates a near-complete East-West (longitudinal) cancellation ia the snow contribution, in contradiction to the satellite remote-sensing data study by Chao et af. The problem arises from a chronic underestimation of Asian snow mass by conventional methods. In order to obtain an optimal estimate of Y under present circumstances, we combine the contribution of rainfall and evapotranspiration computed from Willmott et af. ’s data set with the snow load contribution computed by Chao et al. from satellite data, The result indicates a great deal of cancellation between these two contributions (especially in the prograde component) because of their opposite seasonal cycles and the similar quantities of water mass involved. The surface water contribution to Y is thus far smaller than previously computed. The seasonal variations in J2, J3, J4, and LOD are similarly estimated. Because of the zonal nature of their weighting functions, they suffer no longitudinal cancellation so that the chronic underestimation of Asian snow is less of a problem. Furthermore, their rainfall and snow load contributions largely reinforce each other. Quantitatively, the surface water-induced variation may prove to be a significant contributor to the overall seasonal, non-tidal variations in low-degree JI coefficients and LOD. This study excludes Greenland and Antarctica where no pertinent hydrological data are available at present.

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