Interdecadal oscillations in Atmospheric Angular Momentum variations

Abstract

Interdecadal oscillations in Atmospheric Angular Momentum variationsGlobal Atmospheric Angular Momentum (AAM) is an intrinsic index for describing processes that affect the atmospheric circulation on time scales ranging from intraseasonal to secular. It is associated with length-of-day (LOD) variability through conservation of global angular momentum in planet Earth and thus is of considerable importance for quantifying how the Earth acts as a system. The availability of lengthy AAM time series computed from the recent 20th Century atmospheric reanalyses (1870-2008), complemented by the NCAR-NCEP reanalysis in the overlapping period of 1948-2008 allows the investigation of the role of decadal and interdecadal cycles as well as the recent overall trend in AAM. Thus, we extend to the entire 20th century (and prior, back to 1870) results concerning decadal time scales and a secular positive trend detected over recent decades by different authors. In addition, we also note that AAM has features of interdecadal time scales that modulate the lower frequency variability. These interdecadal time signals oscillate with periods of about 30-50 years, and we found an indication of an 80-90 year period. Short term signals interact with the long-term (secular) trend. Particularly over the years 1950-1985 the global positive trend in AAM appears to result from a conjunction of constructive positive slopes from all lower frequency signals (interdecadal short-term trends and the long-term positive secular trend). Since the mid 1980s, however, the interdecadal oscillation short-term trend contribution decreases, as does the total signal in global AAM. These oscillations appear as two interdecadal modes originating within the Pacific (associated principally with the Pacific Decadal Oscillation and also ENSO) from which they propagate poleward, with differing characteristics in each hemisphere.