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

The remarkable relation well observed between variation in the length of day (ΔLOD) and variation in the axial atmospheric angular momentum (ΔAAM) (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 ΔLOD‐ΔAAM 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 ΔAAM from the mantle to the core. If, after conversion to the equivalent ΔLOD assuming core‐mantle decoupling (as in current standard practice), the calculated axial ΔAAM (according to atmospheric general circulation models [GCMs]) is systematically greater than the observed ΔLOD, 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 ΔAAM (plus the small oceanic counterpart) is smaller than the observed ΔLOD by 10%–20% consistently across the intraseasonal and seasonal time scales. Our main logical conclusion is that the atmospheric GCMs in general underpredict the ΔAAM, 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 ΔAAM‐predicted and the observed ΔLOD masks the relevant information on core‐mantle coupling that we seek.