Changes In Atmospheric Angular Momentum Research Articles

Skillful prediction of length of day one year ahead in multiple decadal prediction systems

Despite a small amplitude, Length of Day (LOD) change, which varies from one year to another due to changes in Atmospheric Angular Momentum (AAM), determines the accuracy of Global Positioning System (GPS) time calculation. In this study, we examine the prediction skill of LOD and AAM in nine decadal prediction systems archived for the Decadal Climate Prediction Project. A persistence and rebound in LOD prediction skill at one year or longer lead time is found in most models. A poleward propagation of AAM anomaly via wave-mean flow interaction is also qualitatively well reproduced. This long-lead prediction of LOD and AAM is attributed to reliable predictions of the El Niño–Southern Oscillation (ENSO) and the Quasi-Biennial Oscillation (QBO), the former being more systematically related than the latter. This result indicates that the improved ENSO prediction and atmospheric wave-mean flow interaction may lead to better prediction of LOD, AAM and related extratropical climate in the decadal prediction systems.

Analysis of Relationships Between ENSO Events and Atmospheric Angular Momentum Variations

AbstractThe El Niño–Southern Oscillation (ENSO) event has a long incubation process, during which the interannual variation of length‐of‐day (LOD) and the atmospheric angular momentum (AAM) series will be quickly affected due to the interaction between sea and air. Based on the comparisons between filtered interannual LOD variation, the AAM, the Oceanic Niño Index (ONI), and the Southern Oscillation Index (SOI) for the period of January 1953 and December 2013, the relationships among them and ENSO are studied. The results demonstrate that AAM changes and SOI/ONI series have similar waveform structures and trends. Interannual LOD variations, atmospheric LOD excitations, and ENSO indices such as the SOI and ONI are well correlated as a consequence of angular momentum conservation. AAMs can be accurately modeled at present, taking ESMGFZ as an example, its sampling interval can reach 3 hr, and can realize accurate prediction in the next 90 days. Consequently, AAM series also can be used as an index for ENSO events and has its own advantages compared with SOI/ONI.

Spin state and moment of inertia of Venus

Fundamental properties of the planet Venus, such as its internal mass distribution and variations in length of day, have remained unknown. We used Earth-based observations of radar speckles tied to the rotation of Venus obtained in 2006–2020 to measure its spin axis orientation, spin precession rate, moment of inertia and length-of-day variations. Venus is tilted by 2.6392 ± 0.0008 deg (1σ) with respect to its orbital plane. The spin axis precesses at a rate of 44.58 ± 3.3 arcsec per year (1σ), which gives a normalized moment of inertia of 0.337 ± 0.024 and yields a rough estimate of the size of the core. The average sidereal day on Venus in the 2006–2020 interval is 243.0226 ± 0.0013 Earth days (1σ). The spin period of the solid planet exhibits variations of 61 ppm (~20 min) with a possible diurnal or semidiurnal forcing. The length-of-day variations imply that changes in atmospheric angular momentum of at least ~4% are transferred to the solid planet.

Links between the mountain torque and the Arctic Oscillation in the Laboratoire de Météorologie Dynamique (LMDz), general circulation model

Recent papers suggest that in the National Center for Environmental Prediction (NCEP) reanalysis data there is a dynamical link between the mountain torque and the Arctic Oscillation (AO) at periodicities near and below 30 days. This link essentially occurs because the AO is associated with a redistribution of mass from the polar regions to the midlatitudes, hence giving a substantial contribution to the mass term of the atmospheric angular momentum (AAM). These results are confirmed here by using a 30‐year simulation done with the Laboratoire de Météorologie Dynamique, zoom (LMDz), general circulation model. In particular, we verify that the changes in mass AAM occurring during intraseasonal variations of the AO are in good part driven by the mountain torque in the model also. In this respect, the LMDz model has the great advantage of closing the AAM budget nearly exactly, which is not the case with the NCEP reanalysis data. As the Antarctic Oscillation (AAO) is also associated with a redistribution of mass from the polar regions to the midlatitudes, its contribution to the AAM budget is also presented. As there are many fewer mountains in the Southern Hemisphere, we show that in the model as well as in the reanalysis the changes in mass AAM during intraseasonal variations of the AAO are in good part equilibrated by changes of opposite sign in wind AAM. The main interest of these results is that the mountain torque drives the changes in AAM, so it can sometimes participate actively in changes of the AO. It has a predictive value that is significant but small, around 10–15% for periodicities near and below one month, while a good fraction of the AO variability occurs at longer timescales.

CO2-Induced Changes in Atmospheric Angular Momentum in CMIP2 Experiments

The response of atmospheric angular momentum to a gradual doubling of CO2 is studied using 16 model experiments participating in the second phase of the Coupled Model Intercomparison Project (CMIP2). The relative angular momentum associated with atmospheric zonal winds increases in all but one of the models, although the magnitude of the change varies widely. About 90% of the 16-model mean increase comes from increasing westerly winds in the stratosphere and the uppermost low-latitude troposphere above 200 hPa. This increase in westerly winds reflects a steepening of the meridional temperature gradient near the tropopause and in the upper troposphere. The simulated temperature gradient at this height increases partly as an indirect consequence of the poleward decrease in the tropopause height, and partly because convection induces a maximum in warming in the tropical upper troposphere. The change in the omega angular momentum associated with the surface pressure distribution is in most models smaller than the change in the relative angular momentum, although its exact value is sensitive to the method of calculation.

Diurnal angular momentum budget of the atmosphere and its consequences for Earth's nutation

The three‐dimensional diurnal atmospheric angular momentum (AAM) budget is investigated, and its geographical features and meteorological dynamics are explored. It is shown that the diurnal atmospheric torques are associated with mass effect (ellipsoidal torque for the equatorial component and mountain torque for the axial component). It is shown that there is an important difference between the torque and angular momentum approaches on diurnal timescales, with the AAM time derivative generally exceeding the calculated torques. For the retrograde diurnal band, however, which is of interest for Earth nutation, the torque approach predicts much larger AAM changes than those observed. The AAM budget equation is also investigated theoretically, in order to give condition on the torques and AAM in order to verify the equation. Alternatively to the condition proposed by Bell, [1994], a condition on the local torque is shown sufficient to verify the AAM budget equation in the retrograde band, that is, the local torque have to balance the ellipsoidal effect. This condition is shown to be verified qualitatively. However, when separating the AAM budget equation into prograde and retrograde part, this last part become very sensitive to the error on the computation of the local torque. This explains why the torque approach always give one order of magnitude larger than the AAM approach at the retrograde diurnal frequencies.

Coupling of long oceanic waves in the Pacific Ocean and the rotating elastic Earth during the 1986–1987 El Niño

We investigate interannual coupling between the rotating elastic Earth and the ocean during the 1986–1987 El Niño. We find the E‐W component of the ground tilt at the Esashi Earth Tides Station up toward the east about 50 milliarc seconds (mas), a rise of sea level about 5 cm at Chichijima and Oshima in the northwestern Pacific Ocean, and an increase of ∼1 cm in the very long baseline interferometry (VLBI) baseline distance between Kashima (Japan) and Kauai (Hawaiian Islands) as well as the residual length of day (LOD) changes of ∼50 μs during the 1986–1987 El Niño. The residual LOD is derived from elimination of changes in atmospheric angular momentum. Our discussions are based on the coupling between torsional motions of the elastic Earth and reflection of the eastward traveling equatorial Kelvin waves at the eastern boundary of the Pacific Ocean. These equatorial waves exchange the momentum and the axial and zonal component of vorticity with the elastic Earth. During the 1986–1987 El Niño, reflection of the Kelvin wave is expected to excite torsional motions and to couple with differential zonal rotation in the elastic Earth and to generate anomalies of ∼1.6 μs in LOD. The differential zonal rotation due to the torsional motion can produce the same order of the magnitude of changes in the VLBI baseline distance between Kashima and Kauai. Variations of the E‐W component of the tilt cannot be explained by torsional motion. We infer that the midlatitude Rossby waves are generated in the central Pacific Ocean. This conclusion is consistent with sea level changes at Chichijima and Oshima and changes of the tilt at Esashi.

The Dynamics of Intraseasonal Atmospheric Angular Momentum Oscillations

Abstract The global and zonal atmospheric angular momentum (AAM) budget is computed from seven years of National Centers for Environmental Prediction data and a composite budget of intraseasonal (30–70 day) variations during northern winter is constructed. Regressions on the global AAM tendency are used to produce maps of outgoing longwave radiation, 200-hPa wind, surface stress, and sea level pressure during the composite AAM cycle. The primary synoptic features and surface torques that contribute to the AAM changes are described. In the global budget, the friction and mountain torques contribute about equally to the AAM tendency. The friction torque peaks in phase with subtropical surface easterly wind anomalies in both hemispheres. The mountain torque peaks when anomalies in the midlatitude Northern Hemisphere and subtropical Southern Hemisphere are weak but of the same sign. The picture is different for the zonal mean budget, in which the meridional convergence of the northward relative angular moment...

Detection of an ENSO signal in seasonal length‐of‐day variations

Conservation of angular momentum dictates that as the wind‐driven axial atmospheric angular momentum changes, so will the length‐of‐day (LOD). In particular, as the strength of the seasonal zonal winds change, so should the strength of the seasonal LOD signals. Here, observed changes in the strengths of the annual and semiannual LOD signals during 1963–1991 are analyzed and shown to be both significantly correlated (at the 99% significance level) with the Southern Oscillation Index (SOI), and to exhibit trends of comparable magnitude but opposite signs. This reported correlation between the SOI and changes in the amplitude of the seasonal LOD signals demonstrates a linkage between seasonal LOD (and hence seasonal zonal wind) variability and the El Niño/Southern Oscillation (ENSO) phenomenon. Furthermore, this study suggests that observed variations in the amplitudes of the seasonal LOD signals can be used to study changes in the strengths of the seasonal atmospheric zonal winds on interannual to decadal and longer time scales.

Polar motion at seasonal frequencies

Studying the planetary-dynamic behaviour of the Earth connected with geophysical processes as sources of excitation is of particular importance. In order to make a contribution concerning seasonal influences of near-surface geophysical processes on polar motion and Earth rotation, numerical investigations based on a time series of Earth Rotation Parameters and Atmospheric Angular Momentum parameters from 1976 to 1987 were performed. Atmospheric Angular Momentum changes are the dominant cause of polar motion and length-of-day variation at seasonal frequencies. Since there is another excitation source, the problem is to be considered such that the total excitation portion is composed of atmospheric and non-atmospheric portions. Particularly in polar motion, the paper deals with the separation of these portions for the seasonal constituents and presents interesting information about their character.

Adaptive filtering and maximum entropy spectra with application to changes in atmospheric angular momentum

The spectral resolution and statistical significance of a harmonic analysis obtained by low‐order maximum entropy methods (MEM) can be improved by subjecting the data to an adaptive filter. This adaptive filter consists of projecting the data onto the leading temporal empirical orthogonal functions obtained from singular spectrum analysis (SSA). The combined SSA‐MEM method is applied both to a synthetic time series and a time series of atmospheric angular momentum (AAM) data. The procedure is very effective when the background noise is white and less so when the background noise is red. The latter case obtains in the AAM data. Nevertheless, we detect reliable evidence for intraseasonal and interannual oscillations in AAM. The interannual periods include a quasi‐biennial one and a low‐frequency one, of 5 years, both related to the El Niño/Southern Oscillation. In the intraseasonal band, separate oscillations of about 48.5 and 51 days are ascertained.

Atmospheric angular momentum forecasts as novel tests of global numerical weather prediction models

The global circulation of the terrestrial atmosphere exhibits fluctuations of considerable amplitude in all three components of its total angular momentum on interannual, seasonal and shorter timescales. The fluctuations must be intimately linked with nonlinear barotropic and baroclinic energetic conversion processes throughout the whole atmosphere and it is advocated that studies of routinely produced determinations of atmospheric angular momentum (AAM) changes be incorporated into systematic diagnostic investigations of large-scale atmospheric flows, AAM fluctuations are generated by dynamical interactions between the atmosphere and the underlying planet. These excite tiny but measurable compensating fluctuations in the rotation vector of the massive solid Earth, thereby ensuring conservation of the angular momentum of the whole system. Forecasts and analyses of changes in AAM from the output of a global numerical weather prediction (GNWP) model constitute a stringent test of the model. Successful forecasts of the axial com ponent of AAM, and hence of irregular non-tidal components of short-term changes in the Earth’s rotation, would find practical applications in various areas of astronomy and geodesy, such as spacecraft navigation. Reported in this paper are the main findings of intercomparisons of analyses and forecasts of changes in all three components of AAM obtained from the operational GNWP models at the United Kingdom Meteorological Office (UKMO) and the European Centre for Medium Range Weather Forecasts (ECMWF), over the period covering the two years 1987 and 1988. Included in the results obtained is the finding that useful forecasts of changes in the axial component of AAM can be made out to 5 days and even slightly longer.