FIG 2 - uploaded by Francois Lott
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The AAM in the ECMWF model, 10-day forecast starting 1 Feb 2004, and its corresponding analysis of total AAM (M: black solid), mass AAM (M O : gray solid), wind AAM (M r : gray dashed), and mass AAM from the analysis (solid with dots). For clarity, each curve has been shifted vertically.
Source publication
The diurnal and subdiurnal variations of the mass and wind terms of the axial atmospheric angular momentum (AAM) are explored using a 1-yr integration of the Laboratoire de Météorologie Dynamique (LMDz) GCM, twelve 10-day ECMWF forecasts, and some ECMWF analysis products. In these datasets, the wind and mass AAMs present diurnal and semidiurnal osc...
Contexts in source publication
Context 1
... oscillations between the mass and the wind AAM are not specific to the LMDz GCM. This is illus- trated in Fig. 2, which presents M, M O , and M R issued from the ECMWF 10-day forecast, starting 1 February 2004, and for which the AAM data have been provided every hour. It shows that the daily oscillations between M R and M O are also substantial in the ECMWF model. They compare rather well in amplitude and phase with the corresponding oscillations ...
Context 2
... It shows that the daily oscillations between M R and M O are also substantial in the ECMWF model. They compare rather well in amplitude and phase with the corresponding oscillations in the LMDz GCM (Fig. 1). It is also noticeable that the oscillations of the mass AAM that we analyze are also present in the ECMWF analysis (solid line with dots in Fig. 2). This indicates that they are not related to an initial adjustment of this model at the beginning of the fore- casts and that they are somehow present in the datasets that are used to produce the ...
Context 3
... of magnitude below 1 Hd, that is, very small compared to the amplitude of the natural variations of M. Neverthe- less, this very small daily signal in M results, in fact, from the cancellation between much stronger daily cycles of the mass AAM M O and of the wind AAM M R (their amplitudes are of a few hadleys, see Fig. 1 for the LMDz GCM and Fig. 2 for the ECMWF forecasts and ...
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Citations
... Inter-model discrepancies are larger for the atmospheric component, likely because mass and motion terms are subject to a distinct cancellation and the total atmospheric effect (Ä 0:5 10 23 kg m 2 s 1 in terms of AAM) is thus not well determined. In fact, at the frequencies of atmospheric tides, Earth-atmosphere interaction torques appear to be more robust excitation measures than angular momentum (Lott et al. 2008;Schindelegger 2014). We have therefore validated all our AAM estimates with the corresponding torques on the atmosphere by means of the AAM budget equation; see Schindelegger (2014) for the analytical expressions. ...
The contribution of the diurnal atmospheric S1 tide to Earth’s wobble is assessed by tidally analyzing hourly polar motion (PM) estimates from approximately 25 years of geodetic Very Long Baseline Interferometry (VLBI) observations. Special emphasis is placed on the dependency of S1 estimates on various settings in the a priori delay model and on the method of time series analysis in post-processing. The considered VLBI solutions differ with regard to the inclusion/exclusion of weak network geometries and the choice of a priori geophysical corrections such as thermal antenna deformation. Prograde PM coefficients A++iB+ of S1 are on the level of 9 + i10 μas (microarcseconds) for all solutions and none of the changes in the processing strategies perturbs this estimate beyond the twofold S1 standard deviation (∼ 2.6 μas). An independent validation of the deduced harmonics against excitation estimates from atmosphere-ocean models shows that space geodetic and geophysical accounts of the S1 effect in PM are still inconsistent by about 10 μas.
... Generally, the mountain torque generates the axial AAM variations, which are eventually damped away by the friction torque [de Viron et al., 2001;Lott et al., 2008;Marcus et al., 2011]. A noticeable exception is the seasonal AAM anomaly, which is generated by an anomalous friction torque over the Indian Ocean [de Viron et al., 2002]. ...
At the interannual to decadal timescale, the changes in the Earth rotation rate are linked with the El-Niño Southern Oscillation phenomena through changes in the Atmospheric Angular Momentum. As climatic studies demonstrate that there were two types of El-Niño events, namely Eastern Pacific (EP) and Central Pacific (CP) events, we investigate how each of them affect the Atmospheric Angular Momentum. We show in particular that EP events are associated with stronger variations of the Atmospheric Angular Momentum and length-of-day. We explain this difference by the stronger pressure gradient over the major mountain ranges, due to a stronger and more efficiently localized pressure dipole over the Pacific Ocean in the case of EP events.
... Moreover, AAM and torque estimates can be crosschecked, and information on the quality of atmospheric models can be deduced since none of the existing analysis systems explicitly accounts for conservation of angular momentum. Extensive efforts have been devoted to the closure of the AAM budget in the axial direction, but, dedicated simulations aside (Lott et al., 2008), departures from an exact balance exist on various time scales (Swinbank, 1985;Salstein and Rosen, 1994;de Viron and Dehant, 2003). The last study also represents the most recent assessment of the (intra-)seasonal AAM balance in the equatorial direction, and offers considerable room for improvement, too. ...
The use of Earth-atmosphere interaction torques is a potential but generally less addressed alternative to the classical angular momentum approach for modeling variations in Earth rotation. We present an update on this subject for the purpose of explaining seasonal and intraseasonal polar motion variability based on the output of the most recent meteorological reanalysis systems of the ECMWF (European Centre for Medium-Range Weather Forecasts) and NASA's Global Modeling and Assimilation Office. The agreement of both models in terms of the three prime torque constituents is shown to be far superior to that of the conventionally deployed wind term of atmospheric angular momentum (AAM). A sufficiently good closure of the equatorial AAM budget equation within the ECMWF reanalysis provides additional endorsement for the use of atmospheric torques as excitation measures. When used as such, polar motion residuals after reduction of the AAM pressure term, as well as oceanic and hydrological excitation, are considerably better modeled by the torque-based quantities than by the standard wind term of AAM, in particular at intraseasonal periodicities. This finding is obtained by means of a newly proposed, hybrid excitation formalism, which derives the AAM counterparts of torque terms from inversion of the AAM budget equation in the frequency domain.
Space geodetic determinations of a 6-μs length-of-day (LOD) anomaly at the diurnal S1 frequency are reconciled with excitation estimates from geophysical fluid models. Preference is given to a hybrid excitation scheme that combines atmospheric torques with oceanic angular momentum (OAM) terms from hydrodynamic forward modeling. A joint inversion of all datasets yields an LOD in-phase and quadrature estimate of μs, matching space geodetic S1 terms well within their formal uncertainties. Non-harmonic LOD excitations, while less than 30% of the time-averaged rotation rate contribution, are conclusively linked to El Niño–Southern Oscillation (ENSO) as the main perturbation of diurnal cycle characteristics in the troposphere. ENSO modulations of particular relevance are those in OAM, associated with the barotropic ocean response to regional modifications in the diurnal atmospheric pressure wave. The study thus highlights previously unexplored aspects of non-tidal mass-field variability in the Earth system.
The dynamical relations between equatorial atmospheric angular momentum (EAAM), equatorial mountain torques, and cold surges are analyzed in a general circulation model (GCM). First, the authors show that the global EAAM budget is well closed in the GCM, much better than in the NCEP–NCAR reanalysis. They then confirm that the equatorial torques due to the Tibetan Plateau, the Rockies, and the Andes are well related to the cold surges developing over East Asia, North America, and South America, respectively. For all these mountains, a peak in the equatorial mountain torque components precedes by few days the development of a cold surge, confirming that the cold surge’s “preconditioning” is dynamically driven by large-scale mountains.
The authors also analyze the contribution of the subgrid-scale orography (SSO) parameterizations and find that they contribute substantially to the torques. In experiments where these parameterizations are almost entirely reduced over a given massif, the authors find that the explicit pressure torques produced by that massif largely compensate the reduction in the parameterized torques. On the one hand, this proves that the explicitly resolved equatorial mountain torques are effective dynamical drivers of the flow dynamics, since they are enhanced when a parameterized torque is reduced. On the other hand, this shows that the cold surges can be captured in GCMs, provided that the synoptic conditions prior to their onset are realistic. The compensation between torques is nevertheless not complete and some weakening of the cold surges is found when the parameterized mountain torques are reduced.
The effect of mountain gravity waves breaking at critical levels, upon the large scale flow, is studied as a production of potential vorticity in the framework of the Eady model. Concerning the synoptic scales, we use a linearized semi-geostrophic model forced by a parameterized force. We show notably that the effect is cyclolytic for a cold front, and cyclogenetic for a warm front. Then the complete nonlinear 2D dynamics is computed with anelastic primitive equations. The subsynoptic scale response to the mountain forcing (parameterized or direct) contains notably nongeostrophic Eady modes, coupling a surface Eady wave with an inertia-gravity wave of the same scale, and inertia-gravity wave trains generated spontanously by the balanced flow associated to the dipole of potential vorticity. The study of the nonlinear saturation of these responses yields amplitudes rather weak but significant.
