Semi‐decadal and decadal signals in atmospheric excitation of length‐of‐day

Abstract

AbstractAt timescales shorter than about 2~yr, non‐tidal length‐of‐day (LOD) variations are mainly excited by angular momentum exchanges between the atmospheric, oceanic and continental hydrological fluid envelopes and the underlying solid Earth. On decadal timescales, the dominant excitation sources of LOD variations are from core and mantle coupling. But the excitations of semi‐decadal (specifically 2~8 yr here) signals in length‐of‐day is less clear, and have been characterized by signals with a wide range of periods and varying amplitudes, including a peak at about 5~6 yr. Here sliding window average filtering is applied to isolate semi‐decadal signals from both geodetic technique observed LOD excitation (shorten to geodetic excitation for convenience) and atmospheric LOD excitations. Based on careful comparison between these two excitation series, we find that (1) the 5~6 yr oscillation in geodetic excitations is not periodically consistent; (2) there is a 5~yr oscillation in atmospheric excitation series, and atmosphere can explain all the semi‐decadal oscillation shorter than about 5~yr in geodetic LOD excitations; (3) contributions from atmosphere to 5~6 yr oscillation in LOD variations is small and cannot be clearly determined; (4) it appears that there are some modest long‐period (longer than 10 yr periods) signals in the atmospheric excitations, but these could be artifacts of the models. Then we compare those peaks/troughs epochs in the residual series between geodetic and atmospheric excitations (ROBS_ATM series for short) with the observed geomagnetic jerks, and find that all of the geomagnetic jerks can match one peaks/troughs in the ROBS_ATM series, and they occurred at the same time or within one year before these peaks/troughs. It implies that there is a common origin for the processes giving rise to geomagnetic jerks and the remaining 5~6 yr oscillation in the ROBS_ATM series.