Regional geophysical excitation functions of polar motion over land areas

Abstract

Here we estimate hydrological polar motion excitation functions over land areas regionally from hydrological models and from Gravity and Climate Recovery Experiment (GRACE) gravity fields. The models include equivalent water heights fields determined from groundwater, soil moisture and snow estimates on continents. In this regard, we consider land data from the Climate Prediction Center (CPC) hydrological model and from the surface modelling system Global Land Data Assimilation System (GLDAS), both of which produce monthly estimates. Also we used satellite -gravimetry data, in the form of the GRACE RL04 equivalent water heights from Center for Space Research. The mass effects of the ocean and atmosphere and postglacial rebound are being removed, so in this way hydrological excitation of polar motion can be estimated from the gravimetric data. The monthly step of the data restricts our analysis to seasonal signals only. Large hydrological variability in equivalent water thickness occurs in the lower latitude Southeast Asia, South Asia, and the South American Amazon regions, and remain important in polar excitation even after multiplication by polar motion transfer functions, with the exception of the band very close to the equator. Differences among models and GRACE related values are still considerable, and need to be reconciled to form the best estimates of hydrological variability. Additionally, variations from the atmosphere are determined over land areas from NCEP/NCAR reanalyses; they are noted to be strongly dependent on variability over the high topography regions of Eurasia and North America. © Springer-Verlag Berlin Heidelberg 2012.