Length Of Day Research Articles

Evaluation of MODIS Land Surface Temperature Data to Estimate Near-Surface Air Temperature in Northeast China

Air temperature (Tair) near the ground surface is a fundamental descriptor of terrestrial environment conditions and one of the most widely used climatic variables in global change studies. The main objective of this study was to explore the possibility of retrieving high-resolution Tair from the Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) products, covering complex terrain in Northeast China. The All Subsets Regression (ASR) method was adopted to select the predictors and build optimal multiple linear regression models for estimating maximum (Tmax), minimum (Tmin), and mean (Tmean) air temperatures. The relative importance of predictors in these models was evaluated via the Standardized Regression Coefficients (SRCs) method. The results indicated that the optimal models could estimate the Tmax, Tmin, and Tmean with relatively high accuracies (Model Efficiency ≥ 0.90). Both LST and day length (DL) predictors were important in estimating Tmax (SRCs: daytime LST = 0.53, DL = 0.35), Tmin (SRCs: nighttime LST = 0.74, DL = 0.23), and Tmean (SRCs: nighttime LST = 0.72, DL = 0.28). Models predicting Tmin and Tmean had better performance than the one predicting Tmax. Nighttime LST was better at predicting Tmin and Tmean than daytime LST data at predicting Tmax. Land covers had noticeable influences on estimating Tair, and even seasonal vegetation greening could result in temporal variations of model performance. Air temperature could be accurately estimated using remote sensing, but the model performance was varied across different spatial and temporal scales. More predictors should be incorporated for the purpose of improving the estimation of near surface Tair from the MODIS LST production.

Diurnal atmosphere‐ocean signals in Earth's rotation rate and a possible modulation through ENSO

AbstractSpace 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 data sets yields an LOD in‐phase and quadrature estimate of (5.91, −0.22) 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.

Atmospheric acceleration and Earth-expansion deceleration of the Earth rotation

Previous studies suggest that tidal friction gives rise to the secular deceleration of the Earth rotation by a quantity of about 2.25 ms/cy. Here we just consider additional contributions to the secular Earth rotation deceleration. Atmospheric solar semi-diurnal tide has a small amplitude and certain amount of phase lead. This periodic global air-mass excess distribution exerts a quasi-constant torque to accelerate the Earth's spin rotation. Using an updated atmospheric tide model, we re-estimate the amounts of this atmospheric acceleration torque and corresponding energy input, of which the associated change rate in LOD (length of day) is −0.1 ms/cy. In another aspect, evidences from space-geodesy and sea level rise observations suggest that Earth expands at a rate of 0.35 mm/yr in recent decades, which gives rise to the increase of LOD at rate of 1.0 ms/cy. Hence, if the previous estimate due to the tidal friction is correct, the secular Earth rotation deceleration due to tidal friction and Earth expansion should be 3.15 ms/cy.

Possible damping model of the 6 year oscillation signal in length of day

A significant 6year oscillation signal exists in the observed length-of-day (LOD) series. We recently used normal Morlet wavelet transform method to recover this oscillation signal. The result indicates that the amplitude of this oscillation has been decreasing for the over past 50years. Using the simulation analysis, this study further demonstrates that the above result is reliable. However, the geophysical mechanism responsible for this decrease is less clear. Here, we develop a temporal-decaying function to characterize the secular attenuation of the oscillation signal. Using the least squared method, we obtain the corresponding quality factor Q value (51.6±0.4) and the damping relaxation time τ (99.2±0.8years). We find the attenuation of the 6year oscillation signal and its Q value can be explained by previous theoretical prediction, providing constraints on the related physical parameters of the lowermost mantle. The dissipative effect of electromagnetic coupling at the core-mantle boundary is likely to be a primary factor to cause the decaying of the 6year oscillation signal.

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.

Improving the modeling of the atmospheric delay in the data analysis of the Intensive VLBI sessions and the impact on the UT1 estimates

The very long baseline interferometry (VLBI) Intensive sessions are typically 1-h and single-baseline VLBI sessions, specifically designed to yield low-latency estimates of UT1-UTC. In this work, we investigate what accuracy is obtained from these sessions and how it can be improved. In particular, we study the modeling of the troposphere in the data analysis. The impact of including external information on the zenith wet delays (ZWD) and tropospheric gradients from GPS or numerical weather prediction models is studied. Additionally, we test estimating tropospheric gradients in the data analysis, which is normally not done. To evaluate the results, we compared the UT1-UTC values from the Intensives to those from simultaneous 24-h VLBI session. Furthermore, we calculated length of day (LOD) estimates using the UT1-UTC values from consecutive Intensives and compared these to the LOD estimated by GPS. We find that there is not much benefit in using external ZWD; however, including external information on the gradients improves the agreement with the reference data. If gradients are estimated in the data analysis, and appropriate constraints are applied, the WRMS difference w.r.t. UT1-UTC from 24-h sessions is reduced by 5% and the WRMS difference w.r.t. the LOD from GPS by up to 12%. The best agreement between Intensives and the reference time series is obtained when using both external gradients from GPS and additionally estimating gradients in the data analysis.

Winter NH low-frequency variability in a hierarchy of low-order stochastic dynamical models of earth-atmosphere system

The origin of winter Northern Hemispheric low-frequency variability (hereafter, LFV) is regarded to be related to the coupled earth-atmosphere system characterized by the interaction of the jet stream with mid-latitude mountain ranges. On the other hand, observed LFV usually appears as transitions among multiple planetary-scale flow regimes of Northern Hemisphere like NAO + , AO +, AO − and NAO − . Moreover, the interaction between synoptic-scale eddies and the planetary-scale disturbance is also inevitable in the origin of LFV. These raise a question regarding how to incorporate all these aspects into just one framework to demonstrate (1) a planetary-scale dynamics of interaction of the jet stream with mid-latitude mountain ranges can really produce LFV, (2) such a dynamics can be responsible for the existence of above multiple flow regimes, and (3) the role of interaction with eddy is also clarified. For this purpose, a hierarchy of low-order stochastic dynamical models of the coupled earth-atmosphere system derived empirically from different timescale ranges of indices of Arctic Oscillation (AO), North Atlantic Oscillation (NAO), Pacific/North American (PNA), and length of day (LOD) and related probability density function (PDF) analysis are employed in this study. The results seem to suggest that the origin of LFV cannot be understood completely within the planetary-scale dynamics of the interaction of the jet stream with mid-latitude mountain ranges, because (1) the existence of multiple flow regimes such as NAO+, AO+, AO− and NAO− resulted from processes with timescales much longer than LFV itself, which may have underlying dynamics other than topography-jet stream interaction, and (2) we find LFV seems not necessarily to come directly from the planetary-scale dynamics of the interaction of the jet stream with mid-latitude mountain, although it can produce similar oscillatory behavior. The feedback/forcing of synoptic-scale eddies on the planetary-scale dynamics seems to play a more essential role in its origin.

Updating a simple model of lunar recession

A classical model of lunar recession is reviewed and updated, where input parameters are taken or inferred from earlier astronomical computation of insolation quantities on Earth back to 0.25 Gyr. Free parameters are (i) the transition age, where mean lunar recession is suddenly lowered, and (ii) the merging age, where Earth-Moon distance (EMD), drops to zero. Predicted mean EMD and length of day (LOD) slightly overstimate their counterparts from the above mentioned astronomical computation. Predicted LOD, where the effect of atmospheric tides and nontidal processes is considered and supposed to be time independent, slightly understimates a linear interpolation from paleontological data related to Phanerozoic, inferred in earlier investigations. If short-period ($\Delta t\approx10^{-3}$ Gyr) EMD fluctuations ($\Delta a\approx\mp0.5R_\oplus$), resulting from the above mentioned astronomical computation back to 0.25 Gyr, occur along the whole evolution of Earth-Moon system (EMS), then the predicted mean EMD is consistent with values inferred from three well studied paleontological data sets: Elatina-Reynella (ER), Big Cottonwood (BC), Weeli Wolli (WW), for a suitable choice of input parameters. The same holds for LOD where, in addition, computed values are consistent with a linear interpolation from paleontological data related to Proterozoic, inferred in earlier investigations. The effect of atmospheric tides and nontidal processes is also discussed. In conclusion, the current model can be considered as a useful zeroth-order approximation to the evaluation of lunar recession and LOD. More accurate paleontological data, expecially in connection with error evaluation, would be desirable in view of improved results and further constraints involving both Astronomy and Paleontology.

Interannual variations in length of day and atmospheric angular momentum, and their seasonal associations with El Niño/Southern Oscillation-like sea surface temperature patterns

This study examines the seasonal connections between the interannual variations in LOD (length of day)/AAMglobe (the relative atmospheric angular momentum for the whole globe) and the ENSO-like SST (El Nino/Southern Oscillation-like sea surface temperature) pattern and corresponding zonal and vertical circulations. Consistent with previous studies, the ENSO-like SST impact the following season LOD/AAMglobe, with the strongest correlations in DJF (December, January, and February), when it is likely to be the peak El Nino/La Nina period. Lag correlations between the interannual variations in LOD/AAMglobe and surface temperature, and the interannual variations in LOD and both zonal circulation and vertical airflow around the equator, consistently indicate that the LOD/AAMglobe reflect the potential impacts of variations in the Earth’s rotation rate on the following season’s sea surface temperatures (SST) over the tropical central and eastern Pacific (where the ENSO-like SST pattern is located). Moreover, the centers of strongest variation in the AAMcolumn (the relative atmospheric angular momentum for an air column and the unit mass over a square meter) are located over the mid-latitudinal North Pacific in DJF and MAM (March, April, and May), and over the mid-latitudinal South Pacific in JJA (June, July, and August) and SON (September, October, and November). This suggests that the AAMcolumn over the mid-latitudinal Pacific around 30°N (30°S) dominate the modulation of Earth’s rotation rate, and then impact the variations in LOD during DJF and MAM (JJA and SON).

Earth rotation changes since −500 CE driven by ice mass variations

We predict the perturbation to the Earth's length-of-day (LOD) over the Common Era using a recently derived estimate of global sea-level change for this time period. We use this estimate to derive a time series of “clock error”, defined as the difference in timing of two clocks, one based on a theoretically invariant time scale (terrestrial time) and one fixed to Earth rotation (universal time), and compare this time series to millennial scale variability in clock error inferred from ancient eclipse records. Under the assumption that global sea-level change over the Common Era is driven by ice mass flux alone, we find that this flux can reconcile a significant fraction of the discrepancies between clock error computed assuming constant slowing of Earth's rotation and that inferred from eclipse records since 700 CE. In contrast, ice mass flux cannot reconcile the temporal variability prior to 700 CE.

Detection of different-time-scale signals in the length of day variation based on EEMD analysis technique

Scientists pay great attention to different-time-scale signals in the length of day (LOD) variations ΔLOD, which provide signatures of the Earth's interior structure, couplings among different layers, and potential excitations of ocean and atmosphere. In this study, based on the ensemble empirical mode decomposition (EEMD), we analyzed the latest time series of ΔLOD data spanning from January 1962 to March 2015. We observed the signals with periods and amplitudes of about 0.5 month and 0.19 ms, 1.0 month and 0.19 ms, 0.5 yr and 0.22 ms, 1.0 yr and 0.18 ms, 2.28 yr and 0.03 ms, 5.48 yr and 0.05 ms, respectively, in coincidence with the results of predecessors. In addition, some signals that were previously not definitely observed by predecessors were detected in this study, with periods and amplitudes of 9.13 d and 0.12 ms, 13.69 yr and 0.10 ms, respectively. The mechanisms of the LOD fluctuations of these two signals are still open.

Effects of seasonal change and experimental warming on the temperature dependence of photosynthesis in the canopy leaves of Quercus serrata.

The effects of warming on the temperature response of leaf photosynthesis have become an area of major concern in recent decades. Although growth temperature (GT) and day length (DL) affect leaf gas exchange characteristics, the way in which these factors influence the temperature dependence of photosynthesis remains uncertain. We established open-top canopy chambers at the canopy top of a deciduous forest, in which average daytime leaf temperature was increased by 1.0 °C. We conducted gas exchange measurements for the canopy leaves of deciduous trees exposed to artificial warming during different seasons. The carbon dioxide assimilation rate at 20 °C (A20) was not affected by warming, whereas that at 25 °C (A25) tended to be higher in leaves exposed to warming. Warming increased the optimal temperature of photosynthesis by increasing the activation energy for the maximum rate of carboxylation. Regression analysis indicated that both GT and DL strongly influenced gas exchange characteristics. Sensitivity analysis revealed that DL affected A without obvious effects on the temperature dependence of A, whereas GT almost maintained constant A20 and strongly influenced the temperature dependence. These results indicate that GT and DL have different influences on photosynthesis; GT and DL affect the 'slope' and intercept' of the temperature dependence of photosynthesis, respectively.

Multifractal analysis of the earth oblateness and Length-Of-Day fluctuations

Geodynamic processes acting on the Earth’s system continuously modify the redistribution of the Earth’s mass. As a result, various inherent global physical parameters, including the gravitational field of the Earth and its variability, as well as the Earth’s rotation rate and variations often observed in Length-Of-Day (LOD), are affected. The Earth’s gravity field is described by a set of spherical harmonic functions derived from a combination of various satellite data as well as terrestrial observations. These functions act as proxy parameters that contribute towards our understanding of the dynamics of the Earth’s system. One of the most important spherical harmonic coefficients of the gravity field of the Earth is the even zonal harmonic coefficient J 2 . This coefficient is used to describe the Earth’s flattening due to mass distribution between the Earth’s Equator and the polar regions, and is therefore a measure of the Earth oblateness. The J 2 coefficient exhibits spatial-temporal variations which are often correlated with LOD variations. The aim of this contribution is to use the fractal nature of J 2 and LOD to demonstrate that their variability is not characterized by well-defined scales (of time) that play dominant roles, but rather involve all scales equally. Characterizing the variability of J 2 and LOD using the fractal paradigm will enhance our understanding of the complex geophysical processes of the Earth system.

Evidence for MAC waves at the top of Earth's core and implications for variations in length of day

Author(s): Buffett, B; Knezek, N; Holme, R | Abstract: Earth's liquid core hosts a diverse set of waves with periods ranging from days to thousands of years. One class of waves with periods of several decades is known to arise from an interplay between magnetic, Archimedes and Coriolis forces. These so-called MAC waves are thought to be relevant for interpreting historical fluctuations in the geomagnetic field. In this study, we show that MAC waves provide a good description of time-dependent zonal flow at the top of the core. The same collection of waves also offers a simple explanation for observed fluctuations in the dipole field. Both of these predictions require a stratified layer at the top of the core with a thickness of 130-140 km and a buoyancy frequency comparable to Earth's rotation rate. We extend these predictions to include changes in the length of day (LOD) and find that MAC waves can account for about half of the observed fluctuation at decadal periods. Larger fluctuations are possible when electromagnetic stresses couple MAC waves to flow in the interior of the core. In fact, an idealized model for the coupled motion overestimates the LOD fluctuations, probably reflecting limitations in this idealized model. Our results offer support for stable stratification at the top of the core and suggest a common origin for decadal fluctuations in the geomagnetic field and the LOD.

Does an Intrinsic Source Generate a Shared Low-Frequency Signature in Earth’s Climate and Rotation Rate?

Abstract Previous studies have shown strong negative correlation between multidecadal signatures in length of day (LOD)—an inverse measure of Earth’s rotational rate—and various climate indices. Mechanisms remain elusive. Climate processes are insufficient to explain observed rotational variability, leading many to hypothesize external (astronomical) forcing as a common source for observed low-frequency signatures. Here, an internal source, a core-to-climate, one-way chain of causality, is hypothesized. To test hypothesis feasibility, a recently published, model-estimated forced component is removed from an observed dataset of Northern Hemisphere (NH) surface temperatures to isolate the intrinsic component of climate variability, enhancing its comparison with LOD. To further explore the rotational connection to climate indices, the LOD anomaly record is compared with sea surface temperatures (SSTs)—global and regional. Because climate variability is most intensely expressed in the North Atlantic sector, LOD is compared to the dominant oceanic pattern there—the Atlantic multidecadal oscillation (AMO). Results reveal that the LOD-related signal is more global than regional, being greater in the global SST record than in the AMO or in global-mean (land + ocean) or land-only surface temperatures. Furthermore, the strong (4σ) correlation of LOD with the estimated NH intrinsic component is consistent with the view proffered here, one of an internally generated, core-to-climate process imprinted on both the climate and Earth’s rotational rate. While the exact mechanism is not elucidated by this study’s results, reported correlations of geomagnetic and volcanic activity with LOD offer prospects to explain observations in the context of a core-to-climate chain of causality.

Possible oscillations in high-precision measurements of Newton's gravitational constant —A reassessment based on the generalized Lomb-Scargle periodogram

The presence of an oscillation with a period of 5.9 yr in measured values of Newton's gravitational constant G over more than three decades, and of a correlation with a 5.9 year oscillation in the length of day (LOD) variability, was recently reported by Anderson et al. (EPL, 110 (2015) 10002). A reanalysis based on an improved data set of measured G values was conducted by Schlamminger et al. (Phys. Rev. D, 91 (2015) 121101(R)) with the result of supporting the finding of a low-frequency oscillation present in the G measurements (with a period of ). A subsequent reanalysis by Anderson et al. (arXiv:1505.01774 [gr-qc]) using the improved data set of Schlamminger et al. confirmed the presence of the oscillation. However, the phase relationship changed (G and LOD not in phase anymore). In an additional analysis, Pitkin showed by Bayesian model selection that the oscillation is most probably due to chance since the data can be modelled at best with the assumption that the scattering of values is caused by measurement errors and an additional Gaussian noise term overlaid. In order to add to the analysis of possible oscillation in G data sets the aim of our work was to reanalyze the data based on the data sets compiled by Schlamminger et al. using the generalized Lomb-Scargle (GLS) periodogram (and the Lomb-Scargle (LS) periodogram, as a control) with additional bootstrapping-based statistical testing. We found periods of and in all the investigated data sets; however, the corresponding peaks in the spectra did not reach statistical significance. We therefore conclude that there is not enough statistical evidence that these oscillations are not due to chance —a finding in agreement with the work of Pitkin.

Performance of models for the beginning of sweet cherry blossom under current and changed climate conditions

Six phenological models, two simple forcing (F)-models and one sequential chilling/forcing (CF)-model, each with and without day length (DL)-term in the forcing approach were optimised (2001–2010) and validated (2011–2015) on very accurate blossoming data of an experimental sweet cherry orchard at Berlin-Dahlem (cultivar ‘Summit’). In parallel, in 3 seasons (2011/2012–2013/2014) climate chamber experiments were performed in order to determine the end of dormancy for ‘Summit’, which is usually an unknown or uncertain parameter in phenological modelling. Additionally, in the season 2013/2014 an in situ climate change experiment on three trees in the sweet cherry orchard were arranged, which was used to validate the phenological models for distinctly warmer climate conditions at the experimental site. On the basis of our climate chamber experiments we quantified the chilling requirement of ‘Summit’ trees. Thus, we were able to identify a CF-model for the beginning of sweet cherry blossom which is mostly physiologically based and works well for current and for future climate conditions at the experimental site. This paper also shows how phenological models can fail under warmer climates, if either the model is too simple or the model parameters are wrong. Additional, we confirmed that phenological models with DL-term in the forcing approach clearly surpassed the conventional phenological models without this parameter. The reason for this behaviour is extensively discussed.

Earth’s rotation variability triggers explosive eruptions in subduction zones

The uneven Earth’s spinning has been reported to affect geological processes, i.e. tectonism, seismicity and volcanism, on a planetary scale. Here, we show that changes of the length of day (LOD) influence eruptive activity at subduction margins. Statistical analysis indicates that eruptions with volcanic explosivity index (VEI) ≥3 alternate along oppositely directed subduction zones as a function of whether the LOD increases or decreases. In particular, eruptions in volcanic arcs along contractional subduction zones, which are mostly E- or NE-directed, occur when LOD increases, whereas they are more frequent when LOD decreases along the opposite W- or SW-directed subduction zones that are rather characterized by upper plate extension and back-arc spreading. We find that the LOD variability determines a modulation of the horizontal shear stresses acting on the crust up to 0.4 MPa. An increase of the horizontal maximum stress in compressive regimes during LOD increment may favour the rupture of the magma feeder system wall rocks. Similarly, a decrease of the minimum horizontal stress in extensional settings during LOD lowering generates a larger differential stress, which may enhance failure of the magma-confining rocks. This asymmetric behaviour of magmatism sheds new light on the role of astronomical forces in the dynamics of the solid Earth.

Recovery of the 6-year signal in length of day and its long-term decreasing trend

There is a significant 6-year oscillation signal (called 6-year signal in this paper) existing in the interannual variations of length of day (LOD). It is unclear to understand its nature variation features. This paper extracts quantitatively the 6-year signal, from 1962~2012, using normal Morlet wavelet (NMWT) method combining wavelet packet and Fourier analysis technique, for the first time, and we investigate it in both time and frequency domains. The results indicate that the amplitude of a 6-year signal shows a long-term decreasing trend and the total amplitude reduction is about 0.05 ms during the past 50 years. The ratio of above reduction to the mean amplitude of 0.124 ms reaches 40 %. For interpreting the phenomenon on the above long-term decreasing trend, this paper proposes two alternatives; however, there is still no firm conclusion and it is required to be further explored.

On magnetic estimation of Earth's core angular momentum variation

AbstractWe study systematically the estimation of Earth's core angular momentum (CAM) variation between 1962.0 and 2008.0 by using core surface flow models derived from the recent geomagnetic field model C3FM2. Various flow models are derived by changing four parameters that control the least squares flow inversion. The parameters include the spherical harmonic (SH) truncation degree of the flow models and two Lagrange multipliers that control the weights of two additional constraints. The first constraint forces the energy spectrum of the flow solution to follow a power law , where l is the SH degree and p is the fourth parameter. The second allows to modulate the solution continuously between the dynamical states of tangential geostrophy (TG) and tangential magnetostrophy (TM). The calculated CAM variations are examined in reference to two features of the observed length‐of‐day (LOD) variation, namely, its secular trend and 6 year oscillation. We find flow models in either TG or TM state for which the estimated CAM trends agree with the LOD trend. It is necessary for TM models to have their flows dominate at planetary scales, whereas TG models should not be of this scale; otherwise, their CAM trends are too steep. These two distinct types of flow model appear to correspond to the separate regimes of previous numerical dynamos that are thought to be applicable to the Earth's core. The phase of the subdecadal CAM variation is coherently determined from flow models obtained with extensively varying inversion settings. Multiple sources of model ambiguity need to be allowed for in discussing whether these phase estimates properly represent that of Earth's CAM as an origin of the observed 6 year LOD oscillation.