Improved ocean tide loading corrections for gravity and displacement: Canada and northern United States

Within 100 km of the North American coastline, ocean tide loading effects a re similar to 1 order of magnitude larger than the present-day precision of absolute gravity (1 mu Gal) and continuous, differential Global Positionin g System (GPS) measurements (3-8 mm). In relation to the precision of these measurements, ocean loading effects on gravity tend to be larger than the effect of loading on GPS measurements. Even at sites hundreds of kilometers from the coast, ocean tide loading can affect superconducting gravimeters at a level of 1 order of magnitude higher than the precision with which tid al constituents amplitudes can be resolved (0.1 mu Gal). In order to reduce contamination of high-precision geodetic measurements by ocean tide loadin g, a numerical representation of the nearshore ocean tide load constituents , M-2, S-2, N-2, O-1, K-1, has been developed using available ocean tide, n umerical, modeling results for Canadian waters. The nearshore ocean tides a re represented by a grid of cells, ranging in size from 0.125 degrees to 0. 5 degrees, in which the amplitude and phase lag of tidal constituents are c onstant and which are bounded, in general, by lines of constant latitude an d longitude. The detailed nearshore tidal representation was originally des igned to supplement the global 1 degrees X 1 degrees model of Schwiderski, but computations have also been carried out using the FES95.2, global model . Ocean tide loading computations that include the more detailed coastal oc ean tide representations yield 10-20% changes in tidal correction values. A significant reduction in the variance of absolute gravity residuals is obs erved at Vancouver Island absolute gravity stations when the more detailed coastal model is used, but a considerable, unexplained residual remains at Holberg at the northwestern tip of Vancouver Island. Computations based on the FES95.2 model yield slightly smaller gravity residuals at west coast si tes but only at the measurement precision level. Ocean loading effects on d aily GPS positions in the Western Canada Deformation Array are adequately p redicted by the Schwiderski global ocean tide loading model. Data from the Canadian Superconducting Gravimeter Installation, Gatineau, Quebec, for M-2 and O-1 were compared with ocean tide load modelling results for the Schwi derski and FES95.2 global ocean tide models enhanced by the nearshore model . With the enhancement, the two models agree with each other to within the expected computational uncertainty of 0.1 mu Gal for both M-2 and O-1. Both models predict an ocean loading effect at O-1 within 0.1 mu Gal of the obs erved effect; however, the predicted loading effect for M-2 is still 0.2 mu Gal smaller than the observed effect, which suggests that there is still r oom for improvement in semidiurnal ocean tide models for the east coast of North America. The enhanced ocean tide loading models will become more impo rtant in the future when gravity and position are measured at subcentimeter precision over periods shorter than 24 hours.