Jolanta Nastula

Regional atmospheric angular momentum contributions to polar motion excitation

We focus on a regional analysis of the equatorial components of the Effective Atmospheric Angular Momentum (EAAM) functions that are responsible for excitation of polar motion. These functions are computed from the NCEP/NCAR 40-Year Reanalysis Project data both globally and in 108 geographic sectors for the period from 1968 to 1997. We investigate regional contributions of these atmospheric angular momentum functions to the short period oscillations of geodetically-determined polar motion directly and to the global EAAM excitation functions themselves. We examine two excitation terms in parallel, both excluding and including the inverted barometer (IB) formulation which adjusts the atmosphere to account for an isostatic equilibrium response from the ocean to overlying pressure; the IB formulation tends to decrease effective atmospheric variability. In the case of pressure terms without IB the largest contributions to the equatorial components of EAAM functions originate in the South Pacific, North Atlantic and North Pacific regions. However, application of the IB correction may result in the dominance of Eurasia and North America instead, with nearly all southern hemisphere contributions disappearing. Oscillations of the polar motion excitation function are mainly coherent with variations of the pressure term of the EAAM excitation functions over northern mid-latitude land areas. Distinct oscillations appear to occur in two frequency bands: 25 – 75 days and 75 – 125 days, in both prograde and retrograde directions which correspond to counterclockwise and clockwise polar motion, respectively. Coherence and cross-spectral analyses are performed to determined degree of agreement and common amplitude, respectively. We examine the atmospheric functions with respect to one important region in Eurasia and note a propagating horizontal influence within the atmosphere.

Oceanic excitation of the chandler wobble

We estimate the oceanic contribution to the excitation of the Chandler wobble using an 11-year time series of ocean angular momentum. The time domain comparison of this series with the non-atmospheric excitation inferred from the polar motion and atmospheric angular momentum data shows a high correlation over most of the period of 1985 to 1996 when they overlap. The frequency domain comparison yields a high coherence in the vicinity of the resonant Chandler frequency. In terms of the free wobble excitation power, we compute the ocean contribution to be 11.9 mas2/cpy, which is approximately the same amount as supplied by the atmosphere. The combined action of the atmosphere and oceans provides about 80% of the power needed to maintain the wobble observed during 1985 to 1996, with the remaining deficit being at the level of one standard deviation of the excitation power estimated from a 100-year record of polar motion data.

Variability of polar motion oscillations with periods from 20 to 150 days in 1979-1991

Variability of short period oscillations of polar motion with periods ranging from 20 to 150 days were investigated in the period 1979–1991. The new computation method of time variable band pass filter spectra and the Wavelet Transform method were applied. These oscillations are elliptical with variable amplitudes. Modulation periods of amplitude variations of these oscillations of about two and three years were found. Correlations of short period oscillations of polar motion and of effective atmospheric angular momentum (EAAM) excitation functions show annual variations and connections of their increases with El Niño phenomena.

Oceanic excitation of the Chandler wobble using a 50-year time series of ocean angular momentum

A 50-year time series of ocean angular momentum (OAM) is used to estimate the oceanic contribution to the excitation of the Chandler wobble. Our estimate of the oceanic excitation power, 18 mas2/cpy, is in good agreement with the residual excitation derived from a simultaneous use of polar motion and atmospheric angular momentum data, 21 mas2/cpy. Direct comparison of the OAM series and the inferred non-atmospheric excitation yields lower correlation and lower coherence at the Chandler frequency than in the study of Brzezinski and Nastula (2001) based on the shorter OAM series of Ponte et al. (1998). Differences in coherence levels are partly related to significant differences found in the two OAM series, indicating substantial dependence of OAM results on model and data assimilation procedures.

Oceanic excitation of polar motion from intraseasonal to decadal periods

We study the oceanic excitation of polar motion by using six different time series of the non-tidal oceanic angular momentum which are either publicly available or more experimental in nature. The oceanic excitation series are compared to each other and to the difference between the excitation inferred from the polar motion data and the corresponding atmospheric contributions. Comparisons show good agreement between the observed residual excitation and most of the ocean series. By performing computations separately for the seasonal sinusoids and four non-overlapping spectral components decadall, interannual, seasonal and intraseasonal) we demonstrate that the quality of the ocean products depends on the considered frequency band. We also discuss the importance of using data-constrained ocean models in future efforts to improve ocean angular momentum estimates.

Regional Signals in Atmospheric and Oceanic Excitation of Polar Motion

Atmospheric and oceanic variability have been shown to play a role in the excitation of polar motion. Regional patterns of atmospheric and oceanic excitation are analysed and compared. The equatorial excitation functions, χ 1 an χ 2 , for the ocean are computed using velocity and mass fields from a near-global ocean model, driven by observed surface winds stresses, surface heat and freshwater fluxes, for the period from January 1985 to June 1997. To understand the relative role of the ocean versus the atmosphere, we used atmospheric excitation functions computed from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalyses. We consider regional mass terms [bottom pressure and atmospheric surface pressure with the inverted barometer (IB) correction] and regional motion terms as well (currents and winds). Results here confirm recent findings that oceans supplement the atmosphere as an important source for polar motion excitation. Regional signals in the oceanic bottom pressure terms have comparable amplitudes to those in the atmospheric pressure-IB terms. The regional wind term amplitudes, however, are several times larger than the values for both regional oceanic currents term and atmospheric pressure-IB term. Power in regional oceanic excitation is distributed between seasonal and subseasonal timescales while in the case of atmospheric excitation it is concentrated rather at seasonal timescales.

El Niño impact on atmospheric polar motion excitation

We study the influences of El Nino phenomena on correlations between short-period variations of atmospheric and geodetic excitation functions of polar motion during 1980–1998. The correlation between these two excitation functions varies with time, with correlation coefficients computed for 1 year spans reaching maxima of 0.7–0.8. Series of these correlation coefficients themselves correlate with El Nino and Southern Oscillation Index (SOI) data, indicating the influence of El Nino and SOI on atmospheric and geodetic excitation functions of polar motion and on polar motion itself. The maximum correlation coefficients for Nino 4 sea surface temperature data is equal to 0.8 in the case of El Nino events in 1983 and 1987. Impacts of El Nino on polar motion have an impulsive character causing irregular variations of polar motion during the El Nino epochs.

Empirical patterns of variability in atmospheric and oceanic excitation of polar motion

Abstract Excitation of polar motion is related in large measure to the redistribution of atmospheric and oceanic mass and to circulation changes. The atmosphere and ocean exhibit spatial patterns that may be isolated by a principal-component type of analysis as fundamental modes explaining their variability. These patterns contribute to polar motion excitation. Here atmospheric excitation functions χ A are computed in equal area sectors (equivalent to 5°×10° boxes at the equator) for the period 1983–2000, based on the reanalyses of the US National Centers for Environmental Prediction–National Center for Atmospheric Research. Oceanic excitation functions χ O are calculated in sectors of comparable resolution, based on the near-global ocean model of Ponte and Stammer (1999) [Geophys. Res 104 23,293,409] for the period 1985–1997. To find patterns of variability in χ A and χ O we examined modes obtained using complex Empirical Orthogonal Function analysis designed to mathematically isolate independent types of variability. EOF analysis, commonly used in studies of climate, separates types of variability that often are caused by an underlying physical mechanism. The first mode is clearly significant in both χ A and χ O and their associated time series have strong annual oscillations with temporally varying amplitudes. The first mode of χ A has strong signals over central Asia, Greenland, Australia, and the southern tips of South America and Africa, and those of χ O are over the mid-latitude North Pacific and North Atlantic as well as areas of the Southern Ocean. Others modes, though only marginally significant, have elements of noted climate signals such as the North Atlantic Oscillation or Pacific-North American patterns.

Chandler wobble parameters from SLR and GRACE

The period and quality factor Q of the Chandler wobble are functions of the internal structure and dissipation processes of the Earth. Better estimates of the period and Q of the Chandler wobble can therefore be used to better understand these properties of the Earth. Here the period and Q of the Chandler wobble are estimated by finding those values that minimize the power in the Chandler frequency band of the difference between observed and modeled polar motion excitation functions. The observations of the polar motion excitation functions that we used are derived from both space-geodetic polar motion observations and from satellite laser ranging (SLR) and Gravity Recovery and Climate Experiment (GRACE) observations of the degree-2 coefficients of the Earths time-varying gravitational field. The models of the polar motion excitation functions that we used are derived from general circulation models of the atmosphere and oceans and from hydrologic models. Our preferred values for the period and Q of the Chandler wobble that we estimated using this approach are 430.9 ± 0.7 solar days and 127 (56, 255), respectively.

Regional Geophysical Excitation Functions of Polar Motion over Land Areas

Here we estimate hydrological polar motion excitation functions over land areas regionally from hydrological models and from Gravity and Climate Recovery Experiment (GRACE) gravity fields. The models include equivalent water heights fields determined from groundwater, soil moisture and snow estimates on continents. In this regard, we consider land data from the Climate Prediction Center (CPC) hydrological model and from the surface modelling system Global Land Data Assimilation System (GLDAS), both of which produce monthly estimates. Also we used satellite -gravimetry data, in the form of the GRACE RL04 equivalent water heights from Center for Space Research. The mass effects of the ocean and atmosphere and postglacial rebound are being removed, so in this way hydrological excitation of polar motion can be estimated from the gravimetric data. The monthly step of the data restricts our analysis to seasonal signals only. Large hydrological variability in equivalent water thickness occurs in the lower latitude Southeast Asia, South Asia, and the South American Amazon regions, and remain important in polar excitation even after multiplication by polar motion transfer functions, with the exception of the band very close to the equator. Differences among models and GRACE related values are still considerable, and need to be reconciled to form the best estimates of hydrological variability. Additionally, variations from the atmosphere are determined over land areas from NCEP/NCAR reanalyses; they are noted to be strongly dependent on variability over the high topography regions of Eurasia and North America.