Comparison of Earth rotation excitation in data‐constrained and unconstrained atmosphere models

Abstract

[1] Changes in Earth rotation are strongly related to fluctuations in the angular momentum of the atmosphere, and therefore contain integral information about the atmospheric state. Here we investigate the extent to which observed Earth rotation parameters can be used to evaluate and potentially constrain atmospheric models. This is done by comparing the atmospheric excitation function, computed geophysically from reanalysis data and climate model simulations constrained only by boundary forcings, to the excitation functions inferred from geodetic monitoring data. Model differences are assessed for subseasonal variations, the annual and semiannual cycles, interannual variations, and decadal-scale variations. Observed length-of-day anomalies on the subseasonal timescale are simulated well by the simulations that are constrained by meteorological data only, whereas the annual cycle in length-of-day is simulated well by all models. Interannual length-of-day variations are captured fairly well as long as a model has realistic, time-varying SST boundary conditions and QBO forcing. Observations of polar motion are most clearly relatable to atmospheric dynamics on subseasonal to annual timescales, though angular momentum budget closure is difficult to achieve even for data-constrained atmospheric simulations. Closure of the angular momentum budget on decadal timescales is difficult and strongly dependent on estimates of angular momentum fluctuations due to core-mantle interactions in the solid Earth.

Earth Rotation Variations – Long Period

“…Chao and Au (1991) showed that during 1980-88, the amplitude of the prograde annual polar motion excitation can be accounted for by atmospheric wind and pressure fluctuations, with equatorial winds contributing about 25% and pressure fluctuations contributing about 75% to the total atmospheric excitation. Discrepancies as large as a factor of two in amplitude were also found by Chao and Au (1991) between observed semiannual polar motion excitation and that due to atmospheric processes (also see Aoyama and Naito, 2000;Barnes et al, 1983;Chao, 1993;Dobslaw et al, 2010;King and Agnew, 1991;Kolaczek et al, 2003;Merriam, 1982;Nastula and Kolaczek, 2002;Nastula et al, , 2009Neef and Matthes, 2012;Stuck et al, 2005;Wahr, 1983;Wilson and Haubrich, 1976;Zhou et al, 2006Zhou et al, , 2008. Furthermore, no agreement was found by Chao and Au (1991) between the observed retrograde annual polar motion excitation and that due to atmospheric processes, with atmospheric excitation being about twice as large as the observed excitation.…”

“…Like seasonal variations in LOD, variations on interannual timescales are also predominantly caused by changes in the angular momentum of the zonal winds (e.g., Eubanks, 1993;Hide and Dickey, 1991;Lambeck and Hopgood, 1981;Neef and Matthes, 2012;Rosen, 1993;Yu et al, 1999). The most prominent feature of the climate system on these timescales is the ENSO phenomenon.…”

“…Like the seasonal and interannual variations in the length of day, variations on intraseasonal timescales are also predominantly caused by changes in the angular momentum of the zonal winds (e.g., Dickey et al, 1992bDickey et al, , 2010Eubanks, 1993;Hide and Dickey, 1991;Lambeck and Hopgood, 1981;Neef and Matthes, 2012;Rosen, 1993;Rosen et al, 1990;Yu et al, 1999;Zhou et al, 2008). The Madden-Julian oscillation (Madden and Julian, 1971 with a period of 30-60 days is the most prominent feature in the atmosphere on these timescales and a number of studies have shown that fluctuations in the zonal winds associated with this oscillation cause the length of day to change (Anderson and Rosen, 1983;Chao and Salstein, 2005;Dickey et al, 1991;Feissel and Gambis, 1980;Feissel and Nitschelm, 1985;Hendon, 1995;Langley et al, 1981b;Madden, 1987;Marcus et al, 2001).…”

“…Wü nsch (2002) summarized the contribution Table 13 Observed and modeled nontidal polar motion excitation at annual and semiannual frequencies of soil moisture and snow load to exciting the annual and semiannual wobbles. Climate models have also been used to study atmospheric, oceanic, and hydrologic excitation of seasonal polar motion (Celaya et al, 1999;Neef and Matthes, 2012;Zhong et al, 2003). Climate models have also been used to study atmospheric, oceanic, and hydrologic excitation of seasonal polar motion (Celaya et al, 1999;Neef and Matthes, 2012;Zhong et al, 2003).…”

Low‐frequency ocean bottom pressure variations in the North Pacific in response to time‐variable surface winds

“…To explore this in detail, however, tailored data assimilation studies following, e.g., the experimental set-up of Neef and Matthes [2012] need to be performed. Best results are expected during boreal winter, where barotropic conditions dominate and surface wind characteristics are therefore representative for the large-scale atmospheric flow in the lower troposphere.…”