Steven L. Marcus

The Earth's angular momentum budget on subseasonal time scales

Irregular length of day (LOD) fluctuations on time scales of less than a few years are largely produced by atmospheric torques on the underlying planet. Significant coherence is found between the respective time series of LOD and atmospheric angular momentum (AAM) determinations at periods down to 8 days, with lack of coherence at shorter periods caused by the declining signal-to-measurement noise ratios of both data types. Refinements to the currently accepted model of tidal Earth rotation variations are required, incorporating in particular the nonequilibrium effect of the oceans. The remaining discrepancies between LOD and AAM in the 100- to 10-day period range may be due to either a common error in the AAM data sets from different meteorological centers, or another component of the angular momentum budget.

The oceanic contribution to the Earth's seasonal angular momentum budget

Seasonal variations in the speed of the Earths rotation manifest themselves as fluctuations in the length of the day (LOD) with an amplitude of about 1000 microseconds (µs). We know from previous work that at least 95% of these variations can be accounted for in terms of angular momentum exchanged between the atmosphere and the solid Earth. Here we examine the respective contributions of the Antarctic Circumpolar Current (ACC) and the global oceans to the Earths seasonal angular momentum budget, using in situ data from the Drake Passage and results from both the oceanic regional model (Fine Resolution Antarctic Model—FRAM) of Webb et al. [1991] and the global oceanic model of Maier-Reimer et al. [1993] as analyzed by Brosche et al. [1990]. The estimated annual contribution of the ACC (2–4 µs) is much smaller than the total variation in the oceanic models or the existing LOD-AAM residual (both ∼15–20 µs). The estimated semi-annual ACC contribution (3–8 µs) is offset by counter-currents further north in both oceanic models, which exhibit larger semi-annual variations in planetary angular momentum. Further refinements in the Earths seasonal angular momentum budget, therefore, will require the full (planetary plus relative) contribution of the global oceans in addition to that of the ACC.

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 Nino/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.

Sub‐daily resolution of Earth rotation variations wtth global positioning system measurements

Data from a worldwide Global Positioning System (GPS) tracking experiment have been used to determine variations in Earth rotation (UT1-UTC) over a time period of three weeks. Kalman filtering and smoothing enabled changes in UT1-UTC over intervals of 2 to 24 hrs to be detected with the GPS data. Internal consistency checks and comparisons with other solutions from very long baseline interferometry (VLBI) and satellite laser ranging (SLR) indicate that the GPS UT1-UTC estimates are accurate to about 2 cm. Comparison of GPS-estimated variations in UT1-UTC with 2-hr time resolution over 4 days with predicted variations computed from diurnal and semi-diurnal oceanic tidal contributions strongly suggests that the observed periodic sub-daily variations ∼0.1 msec (5 cm) are largely of tidal origin.

Angular momentum exchange among the solid Earth, atmosphere, and oceans: A case study of the 1982–1983 El Niño event

The 1982-1983 El Nino/Southern Oscillation (ENSO) event was accompanied by the largest interannual variation in the Earths rotation rate on record. In this study we demonstrate that atmospheric forcing was the dominant cause for this rotational anomaly, with atmospheric angular momentum (AAM) integrated from 1000 to 1 mbar (troposphere plus stratosphere) accounting for up to 92% of the interannual variance in the length of day (LOD). Winds between 100 and 1 mbar contributed nearly 20% of the variance explained, indicating that the stratosphere can play a significant role in the Earths angular momentum budget on interannual time scales. Examination of LOD, AAM, and Southern Oscillation Index (SOI) data for a 15-year span surrounding the 1982-1983 event suggests that the strong rotational response resulted from constructive interference between the low-frequency (approximately 4-6 year) and quasi-biennial (approximately 2-3 year) components of the ENSO phenomenon, as well as the stratospheric Quasi-Biennial Oscillation (QBO). Sources of the remaining LOD discrepancy (approximately 55 and 64 microseconds rms residual for the European Centre for Medium-Range Forecasting (EC) and U.S. National Meteorological Center (NMC) analyses) are explored; noise and systematic errors in the AAM data are estimated to contribute 18 and 33 microseconds, respectively, leaving a residual (rms) of 40 (52) microseconds unaccounted for by the EC (NMC) analysis. Oceanic angular momentum contributions (both moment of inertia changes associated with baroclinic waves and motion terms) are shown to be candidates in closing the interannual axial angular momentum budget.

Earth‐atmosphere angular momentum exchange and ENSO: The rotational signature of the 1997–98 Event

The impact of the 1997-1998 ENSO event is presented in context of axial Earth-atmosphere angular momentum exchange utilizing length of day (LOD), Southern Oscillation Index (SOI) and atmospheric angular momentum (AAM) data from 1970 to 1998; comparisons are made with previous events. The analysis of equal-area latitudinally belted AAM from the NCEP reanalysis (1958-98) reveals slow global coherent poleward propagation of angular momentum. These structures originate in the equatorial regions, penetrate into high latitudes and are bimodal in nature with variations centered at low-frequency (LF - 4.7 yr) and quasi-biennial (QB ∼ 2.4 yr) periods. Analyses utilize both a recursive filter and multichannel singular spectrum analysis (M-SSA).

Intraseasonal Variability in a Two-Layer Model and Observations

Abstract A two-layer shallow-water model with R15 truncation and topographic forcing is used to study intraseasonal variability in the Northern Hemisphere’s (NH’s) extratropical atmosphere. The model’s variability is dominated by oscillations with average periods near 65–70 and 40–50 days. These periods are also found in 13.5 years of daily upper-air data from January 1980 to July 1993. The spatial variability associated with these oscillations is examined by compositing the streamfunction-anomaly fields of the model and the observations. The model’s 70-day oscillation is strongest in the Euro-Atlantic sector, where it bears a close resemblance to observed streamfunction composites of the North Atlantic oscillation. The observed 70-day mode exhibits similar features in the Euro-Atlantic sector, accompanied by a north–south “seesaw” over the Pacific and Eurasia. Previous authors, in their analyses of geopotential height observations, also found these features to be present in an empirical orthogonal functi...

Interannual fluctuations in atmospheric angular momentum simulated by the National Centers for Environmental Prediction medium range forecast model

An earlier study by Dickey et al. [1992] established the existence of globally coherent interannual fluctuations in atmospheric angular momentum (AAM), associated with the El Nino-Southern Oscillation (ENSO) cycle. In this paper, we pursue the origin and the structure of these fluctuations using an ensemble of experiments generated by the National Centers for Environmental Prediction, medium range forecast model version 9. In the control experiments, where the observed sea surface temperatures (SSTs) were used as the lower boundary conditions, the model captures the characteristic V-like structure in time-latitude plots of zonally averaged AAM found by Dickey et al., while experiments with climatological SSTs and those with either perpetual warm or cold ENSO conditions superimposed on the climatological SSTs failed to reproduce this structure. The numerical results indicate that these AAM structures are related to SST variations associated with transitions between different phases of the ENSO cycle and have both propagating and standing components. The largest zonal wind contribution from the levels studied (850, 500, and 200 hPa) is at 200 hPa, where the tropical convective outflow is the strongest. Composites of zonal wind and geopotential height show a clear relationship between the stages of the global AAM oscillation and the ENSO cycle. The strong similarity between the simulated and observed AAM series attests to the models ability to realistically simulate the interannual response of the atmosphere to ENSO SST anomalies.

Models of Solar Irradiance Variability and the Instrumental Temperature Record

The effects of decade-to-century (Dec-Cen) variations in total solar irradiance on global mean surface temperature Ts during the pre-Pinatubo instrumental era (1854-1991) are studied by using two different proxies for the irradiance and an energy-balance climate model. Irradiance anomalies based on solar-cycle length (CL) and solar-cycle decay rate (CD) proxies can account for most of the warming observed up to 1976, but anthropogenic forcing is needed to explain the subsequent sharp increase in Ts. The time series of CL-solar and anthropogenic radiative forcing resemble each other; this similarity makes it difficult to separate their effects in the instrumental Ts record. The CD-based irradiance values reflect heuristically intensity variations in photospheric turbulence; this proxy allows one to obtain more tightly constrained values of both solar variability and terrestrial climate sensitivity from the instrumental Ts record.

Air Temperature and Anthropogenic Forcing: Insights from the Solid Earth

Abstract Earth’s rotation rate [i.e., length of day (LOD)], the angular momentum of the core (CAM), and surface air temperature (SAT) all have decadal variability. Previous investigators have found that the LOD fluctuations are largely attributed to core–mantle interactions and that the SAT is strongly anticorrelated with the decadal LOD. It is shown here that 1) the correlation among these three quantities exists until 1930, at which time anthropogenic forcing becomes highly significant; 2) correcting for anthropogenic effects, the correlation is present for the full span with a broadband variability centered at 78 yr; and 3) this result underscores the reality of anthropogenic temperature change, its size, and its temporal growth. The cause of this common variability needs to be further investigated and studied. Since temperature cannot affect the CAM or LOD to a sufficient extent, the results favor either a direct effect of Earth’s core-generated magnetic field (e.g., through the modulation of charged-...