Intraseasonal Variations in Atmospheric and Oceanic Excitation of Length‐of‐Day

Abstract

AbstractLike the seasonal and interannual variations in length‐of‐day (LOD), variations on intraseasonal timescales (periods shorter than 183 days) are also predominantly caused by changes in the angular momentum of the zonal winds. But the smaller intraseasonal LOD excitations from atmospheric surface pressure, oceanic currents, and the ocean bottom pressure are less clear. Here sliding window average filtering is applied to isolate intraseasonal signals from both geodetically observed LOD excitation (shorten to geodetic excitation for convenience) and atmospheric and oceanic LOD excitations. Based on careful comparison between these two‐excitation series, we find that during 1993–2018, atmospheric winds reduce the RMS of observed intraseasonal LOD series from 276.1 to 55.6 μs with a correlation coefficient of 0.9795. Including the effect of surface pressure changes with that of the winds reduced the RMS from 55.6 to 43.3 μs, and increased the correlation coefficient with the observations from 0.9795 to 0.9884. Additionally, including the effects of changes in oceanic currents and bottom pressure further reduced the RMS from 43.3 to 33.9 μs, and further increased the correlation coefficient from 0.9884 to 0.9931. Thus, although the impact of oceans is relatively minor, closer agreement with the observations in the intraseasonal frequency band is obtained when the effects of oceanic processes are added to that of atmospheric. The RMS values and correlation coefficients of ESMGFZ and MPIOM residual series are better than NCEP and ECCO in every component, indicating that NCEP model set is generally poorer than ESMGFZ in the intraseasonal band.