Variations In Atmospheric Angular Momentum Research Articles

Contributions of oceanic and continental AAM to interannual variation in ΔLOD with the detection of 2020–2021 La Nina event

As a strong climate element on interannual scales, the El Nino-Southern Oscillation (ENSO) is a major component of global weather and climate change, and it is also closely related to the interannual atmospheric angular momentum (AAM) and length-of-day changes (ΔLOD). Here, we reprocess and compare the interannual variations of AAM, ΔLOD with ENSO indices, with AAM mass and motion terms calculated over land separately from those over the ocean. Three oscillatory components (at ~ 6, ~ 7, ~ 8 years), due to angular momentum changes in Earth's interior, are removed to obtain the interannual ΔLOD solely related to climatic variations. Our results show that the AAM motion term over the ocean contributes the most to interannual ΔLOD, and that the oceanic AAM has larger variability than that over land, especially during the periods of strong ENSO events. After subtracting contributions associated with interior processes, the interannual ΔLOD anomalies corresponding to extreme ENSO events (1982–1983 ~ 0.43, 1997–1998 ~ 0.36, 2015–2016 ~ 0.42 ms) are about half as strong as those found in previous studies (~ 0.91, ~ 0.76, ~ 0.81 ms). Furthermore, we detect an intermediate La Nina event that occurred from August 2020 to May 2021, forcing the interannual ΔLOD to a minimum value of approximately -0.21 ms.

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.

Long-term variations of atmospheric angular momentum and torque

In the absence of external torques, the earth system is a closed system and its angular momentum is conserved. The atmospheric angular momentum (AAM) exchange with the solid/liquid earth is achieved by friction and mountain torques. The variations of the AAM and torques are important indicators of global climate change. Using the NCEP/NCAR reanalysis data for 1948–2015, the long-term variations of the AAM and torques are analyzed. A weak positive AAM trend is detected, but an examination of the AAM budget shows that on annual to decadal scales, the signals of the AAM and total torque are inconsistent. During the study period, the total torque was mostly negative and had a decreasing trend, suggesting a decrease of the AAM. To check this inconsistency, we analyze the time series of the length-of-day anomalies, ΔLOD. It is found that ΔLOD is weakly correlated with the AAM, while the derivative of the earth’s core-induced ΔLOD is strongly correlated with the torque. If this is correct, then a core-induced climate change can indeed happen.

Effects of zonal wind on stratospheric ozone variations over Nigeria

ABSTRACTThe effects of zonal wind on stratospheric ozone (O3) variation over Nigeria have been studied. The areas covered in this study include: Maiduguri (11.83° N, 13.15° E), Ikeja (6.45° N, 3.40° E), Port-Harcourt (4.75° N, 7.00° E), Calabar (4.95° N, 8.33° E), Makurdi (7.73° N, 8.54° E), Ilorin (8.50° N, 4.55° E), Akure (7.17° N, 5.08° E), Yola (9.23° N, 12.46° E), Minna (9.61° N, 6.56° E), Jos (9.93° N, 8.88° E), Kano (12.00° N, 8.52° E), and Enugu (6.45° N, 7.51° E), from 1986 to 2008. Zonal wind data was computed from the iso-velocity map employing Matrix Laboratory (MATLAB) software. The mean monthly variations of atmospheric angular momentum (AAM) and length of day (LOD) at pressure levels of 20, 30, and 50 mbar in the atmosphere mostly depict a trend of maximum amplitude between April and September, and minimum amplitude between December and March. The trend observed in seasonal variation of column ozone data in the low latitude had maximum amount from May through August and minimum values from December through February. The mean monthly maximum O3 concentrations was found to be 284.70 DU occurring at Kano (12.00° N, 8.52° E) in May 1989 while an average monthly minimum O3 concentration was found to be 235.60 DU occurring at Port-Harcourt and Calabar (4.75° N, 7.00° E and 4.95° N, 8.33° E, respectively) in January 1998. It has been established in this study that the variation in LOD caused by AAM mostly transfer O3 by means of zonal wind from the upper troposphere to the lower stratosphere in the stations under study. The strong effect of the pressure levels of the atmosphere on O3 variation could be attributed to its effect on the AAM and LOD. Variation in the LOD is significant in the tropics, suggesting that the effects of the extra-tropical suction pump action are not the only driver responsible for O3 transportation from the tropics to extra-tropical zones. Analyses show a relationship with strong correlation between rainfall intensities and total ozone throughout the year under study. For instance, the obtained value of r ranges between 0.676 and 0.957 and p-value <0.05. This most likely indicates that the phenomenon could probably contribute to total ozone variations in Nigeria. Consequently, these findings lead to a deduction that weather pattern alteration observed due to these changes could lead to climate change.

High frequency variations of Earth Rotation Parameters from GPS and GLONASS observations.

The Earth's rotation undergoes changes with the influence of geophysical factors, such as Earth's surface fluid mass redistribution of the atmosphere, ocean and hydrology. However, variations of Earth Rotation Parameters (ERP) are still not well understood, particularly the short-period variations (e.g., diurnal and semi-diurnal variations) and their causes. In this paper, the hourly time series of Earth Rotation Parameters are estimated using Global Positioning System (GPS), Global Navigation Satellite System (GLONASS), and combining GPS and GLONASS data collected from nearly 80 sites from 1 November 2012 to 10 April 2014. These new observations with combining different satellite systems can help to decorrelate orbit biases and ERP, which improve estimation of ERP. The high frequency variations of ERP are analyzed using a de-trending method. The maximum of total diurnal and semidiurnal variations are within one milli-arcseconds (mas) in Polar Motion (PM) and 0.5 milli-seconds (ms) in UT1-UTC. The semidiurnal and diurnal variations are mainly related to the ocean tides. Furthermore, the impacts of satellite orbit and time interval used to determinate ERP on the amplitudes of tidal terms are analyzed. We obtain some small terms that are not described in the ocean tide model of the IERS Conventions 2010, which may be caused by the strategies and models we used or the signal noises as well as artifacts. In addition, there are also small differences on the amplitudes between our results and IERS convention. This might be a result of other geophysical excitations, such as the high-frequency variations in atmospheric angular momentum (AAM) and hydrological angular momentum (HAM), which needs more detailed analysis with more geophysical data in the future.

Spatial and temporal variations of atmospheric angular momentum and its relation to the earth length of day

The characteristics of atmospheric-angular-momentum (AAM) and length-of-day (LOD) on different timescales are investigated in this paper, on the basis of the NECP/NCAR reanalysis data and an LOD dataset for 1962–2010. The variation and overall trend of the AAM anomaly (AAMA) at different latitudes are presented, and the relationship between AAMA and LOD is discussed. The AAMAs in different latitude regions exhibit different patterns of variation, and the AAMA in the tropics makes a dominant contribution to the global AAMA. In the tropics, the AAMA propagates poleward to the extratropical regions. It is confirmed that a downward propagation of the AAMA occurs in the lower stratosphere. Correlation analysis shows that the relationship between AAMA and LOD varies significantly on different timescales. Specifically, the tropical AAMA is positively correlated with LOD on short timescales, but they are not obviously correlated on long timescales. This indicates that the interaction between AAM and the earth’s angular momentum follows the conservative restriction on short timescales, but the influence of the earth angular momentum on that of the atmosphere depends on the interaction process on long timescales.

Atmospheric angular momentum variations of Earth, Mars and Venus at seasonal time scales

Atmospheric angular momentum variations of a planet are associated with the global atmospheric mass redistribution and the wind variability. The exchange of angular momentum between the fluid layers and the solid planet is the main cause for the variations of the planetary rotation at seasonal time scales. In the present study, we investigate the angular momentum variations of the Earth, Mars and Venus, using geodetic observations, output of state-of-the-art global circulation models as well as assimilated data. We discuss the similarities and differences in angular momentum variations, planetary rotation and angular momentum exchange for the three terrestrial planets. We show that the atmospheric angular momentum variations for Mars and Earth are mainly annual and semi-annual whereas they are expected to be “diurnal” on Venus. The wind terms have the largest contributions to the LOD changes of the Earth and Venus whereas the matter term is dominant on Mars due to the CO 2 sublimation/condensation. The corresponding LOD variations (Δ LOD) have similar amplitudes on Mars and Earth but are much larger on Venus, though more difficult to observe.

Recently discovered small polar motion loops and their atmospheric excitation mechanism

Polar motion is the relative motion of the solid Earth with respect to its rotational axis. Recently, small loops have been discovered in unfiltered polar motion observations. These loops with periods less than 21 d and sizes ranging from a silver dollar coin to a compact disc (when projected onto the surface of the Earth) are currently observable because every ∼6.4 years the Chandler, annual, and semiannual variations that dominate this motion destructively interfere and act like a high‐pass filter. These loops, as well as diurnal polar motion variations, are shown to be significantly correlated to variations in atmospheric angular momentum that result primarily from rapid fluctuations in atmospheric pressure fields with the contributions from the winds playing a smaller variable role. Finally, the results show that variations in the speed of weather patterns and whether or not the surface winds are interacting with land or oceans have an effect on polar motion that is observable with current geodetic observational techniques.

Diurnal and subdiurnal effects of the atmosphere on the Earth rotation and geocenter motion

The diurnal cycle in the atmospheric angular momentum (AAM) and in the wind and surface pressure fields is studied with a realistic atmospheric general circulation model (GCM) in which the AAM budget is very well closed. For this, we used a 1 year simulation. From a geodetic point of view, we find that this model predicts AAM variations at diurnal timescale which produce a polar motion near 0.2 milliarc second. Additionally, at the same period, the model predicts a geocenter motion of the order of a millimeter. These results are compared with those obtained with the National Centers for Environmental Prediction/National Center for Atmospheric Research and the European Centre for Medium‐Range Weather Forecasts operational analysis data sets. As the AAM budget is not exactly closed in those two data sets, large quantitative differences with the GCM are found. These results witness that there are problems in using AAM values from the major weather prediction center to estimate the AAM and torques variation at diurnal and subdiurnal timescales. We have also computed, for the three models, the spherical harmonics decomposition of the diurnal and semidiurnal surface pressure signals. The results show large differences from one model to another, which advices carefulness when correcting gravity missions (as Gravity Recovery and Climate Experiment (GRACE), for instance) from the high‐frequency effect of the atmosphere on the orbit, using operational analysis.

Global mass balance and the length‐of‐day variation

Variations of atmospheric angular momentum are dominated driving forces behind the length‐of‐day change, while mass redistribution in the oceans and hydrosphere is expected to cause the length‐of‐day to change as well. We estimate atmospheric, oceanic, and hydrological contributions to the length‐of‐day variations during 1993–2004 using the National Centers for Environmental Prediction reanalysis atmospheric model, the Estimating the Circulation and Climate of the Ocean consortium's data assimilating ocean general circulation model, and the U.S. Climate Prediction Center's land data assimilation system model. A coherent mass balance among the atmosphere, ocean, and continental water is implemented. At seasonal timescales the mass balance among the atmosphere, land, and oceans is very important and nearly cancels all the contributions to the length‐of‐day variation predicted by the oceanic and hydrological models, although at intraseasonal timescales, good correlations exist between observed length‐of‐day variations unaccounted for by the atmosphere and contributions from the ocean and continental water. This study indicates that in a fully mass balanced Earth system the combined seasonal oceanic and hydrological contributions to the length‐of‐day variation are too small to explain the residual length‐of‐day variations unaccounted for by the atmosphere. The discrepancies between observed length‐of‐day variations and atmospheric contributions appear more likely caused by the errors of the atmospheric models, in particular of the wind fields.

Hydrological excitations of polar motion, 1993-2002

SUMMARY Changes in continental water storage obtained from an advanced land data assimilation system (LDAS) are employed to compute hydrological excitations of polar motion for the 10-yr period of 1993‐2002. The results are compared with observed excitations of polar motion and contributions from the atmosphere and ocean, and with results from previous studies. There is remarkably good agreement between LDAS-derived results and remaining observed excitations, i.e. after atmospheric and oceanic effects are removed from the observations. LDAS-predicted hydrological excitations play a major role in closing the excitation budget for prograde and retrograde annual wobble, especially for the prograde component, and show considerably better agreement with observations than previous results. LDAS-predicted results show significantly larger interannual variability and also agree well with observed excitations not accounted for by the atmosphere and ocean. As demonstrated in this study, LDAS shows major improvement in modelling large-scale change in land water storage at both seasonal and interannual timescales. 1I N T R O DUCTION Under the conservation of angular momentum, any change in the Earth’s inertia tensor due to redistribution of mass will cause changes in the Earth’s rotation. Variations of atmospheric angular momentum (AAM) or oceanic angular momentum (OAM) will also change the Earth’s rotation via exchange of angular momentum between the solid Earth and its geophysical fluid envelope. At interannual or shorter timescales, redistribution of air and water mass and movement within the Earth system, including the atmosphere, ocean and hydrosphere (plus snow/ice sheet), are primary driving forces behind changes in the Earth’s rotation, while redistribution of mass within the core and mantle plays a more important role at decadal or longer timescales.

Isolation of atmospheric effects on rapid polar motion through Wiener filtering

SUMMARY Rapid polar motions are irregular low amplitude fluctuations of the rotation pole of the Earth that occur over timescales of days to months. They have been attributed primarily to variations in atmospheric angular momentum (AAM), with a contribution from oceanic angular momentum as well. Monte Carlo experiments conducted by Stieglitz & Dickman suggest that, given the observed correlations between AAM and rapid polar motion, only two-thirds of the atmospheric excitation is actually being transferred to the solid earth's rotation. Consequently, the traditional procedure for correcting polar motion for atmospheric effects, by simply subtracting the original AAM time-series, is inefficient. Following a suggestion by Stieglitz & Dickman, we have attempted to optimally correct polar motion for atmospheric effects using the technique of Wiener filtering. With the atmospheric excitation as input and geodetic excitation (produced by deconvolving polar motion) as desired output, we use the Wiener filter to produce an atmospheric time-series that relates as meaningfully as possible to the geodetic excitation. The filter can also account for cross-component excitation of rapid polar motion by AAM, such as might result from atmospheric forcing of the oceans or groundwater. We find that the Wiener filter acts to screen non-geodetic power (mostly reddish noise) from AAM across the entire frequency spectrum. The resulting Wiener filtered AAM (WAAM) is reduced in amplitude from the original AAM by at least half and shows higher correlation and coherence with geodetic excitation. Additionally, correcting polar motion data with WAAM reveals the presence of tidal signals not seen in previous studies and allows long-period tide models to be tested.

Mountain Torques and Northern Hemisphere Low-Frequency Variability.Part II: Regional Aspects

The NCEP‐NCAR reanalysis dataset for 1958‐97 is used to analyze intraseasonal variations in mountain torques and the large-scale atmospheric circulation patterns associated with them. Spectral analysis of the atmospheric angular momentum (AAM) budget shows that the dominant variations of mountain torque have periodicities near 30 days and shorter, while the dominant AAM variations occur in the 40‐60-day band. This difference is due to the 40‐60-day AAM variations being primarily related to equatorial processes, while mountain torque variations are associated mostly with extratropical processes. The Northern Hemisphere (NH) mountain torque has enhanced power and significant spectral peaks in the 20‐30-day band. The signal in this band accounts for 33% of the NH mountain torque variance, once the seasonal cycle has been removed. Lag composites of the NH 700-hPa geopotential heights based on the 20‐30-day mountain torque signal show the latter to be associated with coherent large-scale patterns that resemble lowfrequency oscillations identified in this band by previous authors. The composite patterns that are in phase quadrature with the 20‐30-day NH mountain torque have a pronounced zonally symmetric component. These patterns are associated with substantial AAM variations, arguably driven by the NH mountain torque in this band. Principal component (PC) analysis of the NH 700-hPa heights shows that, in the 20‐30-day band, the mountain torque is also in phase quadrature with the two leading PCs; the first corresponds to changes in the midlatitude jet intensity near the subtropics, while the second corresponds to the Arctic Oscillation. The relationships with AAM of the latter essentially occurs through the mass term. Mountain torques are, furthermore, nearly in phase with dominant patterns of low-frequency variability that exhibit substantial pressure gradients across the Rockies and the Tibetan Plateau.

Mountain Torques and Northern Hemisphere Low-Frequency Variability. Part I: Hemispheric Aspects

The NCEP–NCAR reanalysis dataset for 1958–97 is used to analyze intraseasonal variations in mountain torques and the large-scale atmospheric circulation patterns associated with them. Spectral analysis of the atmospheric angular momentum (AAM) budget shows that the dominant variations of mountain torque have periodicities near 30 days and shorter, while the dominant AAM variations occur in the 40–60-day band. This difference is due to the 40–60-day AAM variations being primarily related to equatorial processes, while mountain torque variations are associated mostly with extratropical processes. The Northern Hemisphere (NH) mountain torque has enhanced power and significant spectral peaks in the 20–30-day band. The signal in this band accounts for 33% of the NH mountain torque variance, once the seasonal cycle has been removed. Lag composites of the NH 700-hPa geopotential heights based on the 20–30-day mountain torque signal show the latter to be associated with coherent large-scale patterns that resemble low-frequency oscillations identified in this band by previous authors. The composite patterns that are in phase quadrature with the 20–30-day NH mountain torque have a pronounced zonally symmetric component. These patterns are associated with substantial AAM variations, arguably driven by the NH mountain torque in this band. Principal component (PC) analysis of the NH 700-hPa heights shows that, in the 20–30-day band, the mountain torque is also in phase quadrature with the two leading PCs; the first corresponds to changes in the midlatitude jet intensity near the subtropics, while the second corresponds to the Arctic Oscillation. The relationships with AAM of the latter essentially occurs through the mass term. Mountain torques are, furthermore, nearly in phase with dominant patterns of low-frequency variability that exhibit substantial pressure gradients across the Rockies and the Tibetan Plateau.

Low‐frequency intraseasonal variations of the wintertime very high latitude mesopause regions

We have performed a spectral analysis of three fall and winter airglow data sets each for South Pole Station and for Eureka, Canada (80°N). These analyses show significant spectral features in the mesopause region at very high latitudes that are similar in period to low‐frequency intraseasonal (IS) variations seen in analysis of length of day, atmospheric angular momentum variations, and outgoing long wave radiation, as well as in height or wind fields up to 10 hPa (∼32 km). These are fairly broadband spectral features centered approximately at 17, 23, and 45 days. The 17‐day oscillation we find is more prominent than that found in the data sets reflecting variations mainly in the troposphere and stratosphere. This oscillation is most likely associated with the second symmetric s = 1 free Rossby mode. The data from South Pole Station should not reflect this mode, as such, because the spectral components that produce airglow variations should vanish at the pole unless they are zonally symmetric; however, free oscillations having zonal wave number zero with such long periods should not exist. We suggest that, at least at South Pole Station, the 17‐day oscillation is driven by interactions between the global Rossby mode and stationary wave number one planetary waves. In addition to the low‐frequency oscillations that have been reported in the IS literature, we see a wave in the 12–14 day period range. At South Pole Station that wave may also reflect forcing by a global free mode.

Lunisolar torque on the atmosphere and Earth's rotation

The atmospheric effect on Earth's rotation is classically computed from atmospheric angular momentum (AAM) series by considering that the whole AAM variations are transmitted to the solid Earth. In fact, such a procedure is an approximation because it ignores that the atmosphere exchanges also angular momentum with the moon and the sun. The computation of the lunisolar gravitational torque on the atmosphere is the core of the present study. It is derived analytically and then estimated from pressure data over the period 1975–2000. The mean surface pressure is the main contributor, at the level of 10 17 N m for the equatorial component, 10 16 N m for the axial one, whereas the thermally driven pressure variations provide about 10% of the variance. This torque plays a significant role in the budget of the atmospheric angular momentum, but does not account for the inconsistency between the present series of the atmospheric angular momentum and the interaction torque between the solid Earth and the atmosphere. From the tidal waves of this torque we derive accurately the contribution of the atmosphere to the lunisolar precession–nutation of the Earth ( 0.31 marcsec/year for the precession in longitude, 46 microarcseconds for the nutation in 18.6 years).

Refined correlations between atmospheric and rapid polar motion excitations

SUMMARY In this work we employ Monte Carlo experiments to explore reports by others of a statistically significant correlation between atmospheric angular momentum variations and polar motion on timescales of days to months. Our experiments verify that the correlation between atmospheric and geodetic excitation is statistically diVerent from zero at the 0.997 confidence level, and demonstrate that the correlations improve with more recent recreations of the older data sets. Additional Monte Carlo experiments reveal that, during the previous decade, about 60 per cent of the atmospheric excitation was eVective in exciting rapid polar motion, and nearly 80 per cent of the geodetic excitation was atmospheric in origin on these timescales; with the older data sets, barely 50 per cent of the geodetic excitation could be ascribed to an atmospheric origin. Possible sources of the remaining polar motion excitation are briefly discussed. Our work implies that simply subtracting atmospheric angular momentum from geodetic data may not be the best way to remove the atmospheric contribution. We also present the first correlation results employing atmospheric excitation data corrected for the dynamic response of the oceans to barometric pressure forcing.

The influence of global warming in Earth rotation speed

Abstract. The tendency of the atmospheric angular momentum (AAM) is investigated using a 49-year set of monthly AAM data for the period January 1949-December 1997. This data set is constructed with zonal wind values from the reanalyses of NCEP/NCAR, used in conjunction with a variety of operationally produced AAM time series with different independent sources and lengths over 1976-1997. In all the analyzed AAM series the linear trend is found to be positive. Since the angular momentum of the atmosphere-earth system is conserved this corresponds to a net loss of angular momentum by the solid earth, therefore decreasing the Earth rotation speed and increasing the length of day (LOD). The AAM rise is significant to the budget of angular momentum of the global atmosphere-earth system; its value in milliseconds/century (ms/cy) is +0.56 ms/cy, corresponding to one-third of the estimated increase in LOD (+1.7 ms/cy). The major contribution to this secular trend in AAM comes from the equatorial Tropopause. This is consistent with results from a previous study using a simplified aqua-planet model to investigate the AAM variations due to near equatorial warming conditions. During the same time interval, 1949-1997, the global marine + land-surface temperature increases by about 0.79 °C/cy, showing a linear correspondence between surface temperature increase and global AAM of about 0.07 ms per 0.1 °C. These results imply that atmospheric angular momentum may be used as an independent index of the global atmosphere's dynamical response to the greenhouse forcing, and as such, the length of day may be used as an indirect indicator of global warming.Key words. Meteorology and atmospheric dynamics (general circulation) · Geodesy

Atmospheric torque on the Earth and comparison with atmospheric angular momentum variations

The purpose of this paper is to compute atmospheric torques on the Earth, including the oceans, with an emphasis on the equatorial components. This dynamic approach is an alternative method to the classical budget‐based angular momentum method for viewing atmospheric effects on Earth's orientation in space. The expression of the total torque interaction between the atmosphere and the Earth is derived from the angular momentum balance equation. Such a torque is composed of three parts due to pressure, gravitation, and friction. Each of these torque components is evaluated numerically by a semi‐analytical approach involving spherical harmonic approximations, and their orders of magnitude are intercompared. For the equatorial components the pressure and gravitational torques have far larger amplitudes than that of the friction torque; these two major torques have the same order of magnitude but opposite signs, and the value of the sum of the torques is shown to be close to the equatorial components of the atmospheric angular momentum time derivative s, as would be expected in a consistent model‐based analysis system. The correlation between the two time series is shown to be very good at low frequency and decrease slowly with increasing frequency. The correlation is still significant (≥ 0.7) up to 0.5 cycle per day, but the correlation coefficient reduces to 0.5 at the diurnal frequency band, indicating the difficulty of calculating rapidly changing model‐based torques within an atmospheric analysis system.

Detection and modeling of NonTidal oceanic effects on Earth's rotation rate

Subdecadal changes in Earth's rotation rate, and hence in the length of day (LOD), are largely controlled by variations in atmospheric angular momentum. Results from two oceanic general circulation models (OGCMs), forced by observed wind stress and heat flux for the years 1992 through 1994, show that ocean current and mass distribution changes also induce detectable LOD variations. The close similarity of axial oceanic angular momentum (OAM) results from two independent OGCMs, and their coherence with LOD, demonstrate that global ocean models can successfully capture the large-scale circulation changes that drive OAM variability on seasonal and shorter time scales.