Relation between length-of-day variation and angular momentum of geophysical fluids

Abstract

The remarkable relation well observed between variation in the length of day (LOD) and variation in the axial atmospheric angular momentum (AAM) (plus the oceanic counterpart, to a much lesser extent) is a consequence of the conservation of angular momentum on planet Earth. Quantification of the exact LOD-AAM relation, which we seek in the present study, depends significantly on the extent to which the core participates, or is dynamically coupled with the mantle, in transmission of the axial AAM from the mantle to the core. If, after conversion to the equivalent LOD assuming core-mantle decoupling (as in current standard practice), the calculated axial AAM (according to atmospheric general circulation models [GCMs]) is systematically greater than the observed LOD, then we can conclude the presence, and, furthermore, estimate the strength, of the said dynamic core-mantle coupling. However, in this study we find the opposite instead, that the calculated AAM (plus the small oceanic counterpart) is smaller than the observed LOD by 10%-20% consistently across the intraseasonal and seasonal time scales. Our main logical conclusion is that the atmospheric GCMs in general underpredict the AAM, by at least 10%-20% at the mentioned time scales, a fact of importance with respect to the assessment of GCMs. Therefore, the systematic discrepancy found between the AAM-predicted and the observed LOD masks the relevant information on core-mantle coupling that we seek. Copyright 2010 by the American Geophysical Union.