JPGM10a

Tidally induced changes of Earth rotation

Peter Brosche

University of Bonn, GERMANY

The three major components of the Earth (solid part, oceans, atmosphere) are subject to tidal forces from the Moon and Sun. Their interactions with the tide-generating bodies and amongst each other cause transfers of angular momentum and energy. We describe the achievements and the problems in the theoretical understanding of the balance. The theoretical predictions are compared with the observations of the rotation of the solid Earth. Those data are augmented by observations of the concomitant changes in the lunar orbit. The latter are especially important for the secular effect of tidal friction in the Earth-moon-system.

 

JPGM10b

Response of an OGCM to real time forcing and implications to Earth’s rotation

J. Segschneider, J. S¸ndermann and P. Brosche

University of Hamburg, GERMANY,
University of Bonn, GERMANY

The ocean’s contribution to interannual variations in length of day (l.o.d.) is investigated by means of the global Hamburg large scale ocean circulation model (LSG) forced with observed windstress and air temperature fields. The horizontal resolution of the model is 3.5o in latitude and longitude, and eleven layers exist in the vertical; the timestep used is one month. The atmospheric forcing is obtained by adding ECMWF 1000 mb monthly anomalies of windstress and air temperature to climatological values of Hellerman and Rosenstein and COADS, respectively. The data extend from November 1979 to November 1993. Within this period three El NiŇo events (1982/83, 1986/87 and 1991/92) and two La NiŇa events (1984, 1988) were observed. Variations in the pressure torque, the inertia tensor of the ocean and the momentum connected with the currents are calculated diagnostically from the OGCM output. Model results show that the ocean works mainly as a transmitter of momentum from the atmosphere to the solid earth and that contributions to interannual variations of l.o.d. from the mass distribution term amount to four times the effect of the motion term. Contributions to interannual variability of l.o.d. can mainly be attributed to the pressure torque and the matter term whereas the contribution by the ocean currents varies on shorter timescales up to one year. The calculated total changes in l.o.d. are in the order of 0.1 ms. This is the right order of magnitude to close the imbalance between observed changes and results from atmospheric circulation models.

 

JPGM10C

Parameters of Large-Scale Circulation Over Oceans, Their InterannualVariability and Relation to the Earth Rotation Velocity

G.A. Chernega and A.M. Sirota

Atlantic Scientific Research Institute of Marine Fisheries & Oceanography, Kaliningrad, RUSSIA.

A database of monthly average pressure and its anomalies on a regular grid of 5 by 10 degrees was analysed in the Northern hemisphere for the period 1900-1995 and in the Southern hemisphere for the period of 1970-1995. Research of interannual variability of large-scale circulation over Atlantic and Pacific Oceans depending on the Earth rotation velocity and resultant air-mass transport in equatorial stratosphere was carried out. The following parameters of circulation were chosen: - Gradients of pressure between atmospheric centers of action, characterizing intensity of transports in the basic circulation (west-eastern transport in moderate latitudes and trades in tropical); - The Kats and Pogosyan-Pavlovskaya Indexes, reveal intensity and abnormality of zonal and meridional processes. It was established, that the changes of the century background of atmospheric circulation intensity over the oceans are well coordinated with a century course of the Earth rotation velocity. This variability of circulation reflects intrasecular (quasi-forty-year) changes of velocity in the Atlantic and intra-epochal (quasi-fifteen-year) changes in the Pacific Ocean. Possible mechanisms of relationship are offered. The current epoch of acceleration of the Earth rotation is characterized by strengthening of circulation in systems of zonal transport over all oceans and the trades over the Southern Atlantic. At the same time, the trades in the Northern Atlantic and Pacific Ocean are weakened. In the Southern hemisphere and the Northern Atlantic the tendency of abnormal development of zonal processes is revealed. Meridional processes of atmospheric circulation amplify over the Pacific Ocean in the Northern hemisphere. A quasi-bi-annual cyclic signal, connected with a quasi-bi-annual cycle of equatorial stratospheric winds, prevails in the interannual variability of circulation parameters. The relationship, as a rule, depends on epochal trends of the circulation parameters. In this work the forecasts appropriateness of the relationship, are discussed, and some statistical characteristics of low-frequency variability of atmospheric circulation over oceans are considered.

 

JPGM10d

Angular momentum budgets of hydrodynamic global ocean tide models with variational data assimilation procedure

Wilfred Zahel

Institut f¸r Meereskunde, Universit”t Hamburg, Hamburg, Germany

It has been demonstrated that assimilation of ocean tide data of different kinds into hydrodynamic models by minimising a discrete least squares functional, which depends on dynamical and data residuals, yields increasingly realistic results. The agreement of the computed tidal fields with independent pelagic and coastal tidal elevation data and with loading gravity data has been improved considerably, firstly assimilating pelagic tide-gauge data only, then including loading gravity data, and finally introducing also Topex/Poseidon data leading to a uniform coverage of the global ocean area data.

The global hydrodynamic model having been applied together with a variational data assimilation procedure is defined by sea surface elevations relative to the ocean bottom and by current velocity components as dependent variables. The model includes the full tidal loading and self attraction effect among all the dynamical constituents known as being relevant. Consistent tidal fields of elevation and current velocity have been computed for the eight most important semidiurnal and diurnal constituents, the vast majority of the assimilated data having been derived from Topex/Poseidon altimetry.

The angular momentum and energy budgets are evaluated and the dependence of the rates on different assumptions concerning data errors and dynamical error covariances and on data sets assimilated is studied. Dynamical residuals are regarded as implicitly since the taking into account physical effects unresolved in the model, in particular the role these residuals are play in the computed budgets is investigated. From the instantaneous angular momentum budgets Earth rotation parameters, like UT1 variations and periodic polar motion terms are derived and compared with corresponding recent measurements. Properly using data information in combination with dynamical models is contributed to decisively improved geophysical quantities, as derived from the computed model fields, and to a better understanding of the physical mechanisms taking effect.

 

JPGM10E

Interannual Variations of Oceanic Angular Momentum in an OGCM Forced by ECMWF Reanalysis

Joachim Segschneider and J¸rgen S¸ndermann

University of Hamburg, Institute of Oceanography, Hamburg, Germany

The Hamburg large scale ocean general circulation model is forced by wind stress and air temperatures from the ECMWF-reanalysis for the period of 1979 to 1995 to study interannual variations of oceanic angular momentum. Variations in the pressure torque, the inertia tensor of the ocean and the momentum stored in ocean currents are calculated diagnostically from the computed density, velocity and sea level distributions. These variations are compared to results of a former experiment where the same ocean model has been forced with the older version of the ECMWF data. The new results are also compared to residuals between observed angular momentum of the solid earth (l.o.d.) and atmospheric angular momentum as calculated with atmospheric general circulation models. In the model experiment with the original ECMWF dataset, a spurious l.o.d. anomaly was computed for 1980 due to unrealistic air temperatures in the forcing data. Also, the simulated 1986/87 El NiŇo was too weak compared to observations. The now available ECMWF-reanalysis allow to be forced the ocean model with a consistent data set. Thus, it can be investigated, whether deviations between computed oceanic angular momentum and residuals can be attributed to the forcing data or the internal dynamics of the ocean model.

 

JPGM10f

Oceanic Angular Momentum Variability Estimated From The Parallel Ocean Circulation Model

Thomas J. Johnson and Clark R. Wilson

The University of Texas at Austin, Center for Space Research, Austin, Texas, USA

Although most length of day variations are explained by exchange of atmospheric angular momentum with the solid earth, the oceans are likely to be important, both as an intermediary (to transfer angular momentum) and as the source of small but significant discrepancies in the global angular momentum budget. Since the required observations of the real ocean do not exist, Parallel Ocean Circulation Model data products were used to examine the role of the oceans in the length of day changes. Unlike earlier rigid-lid models, this free-surface multi-layer ocean model allows the estimation of angular momentum resulting from mass redistribution and relative motion that is more representative of the real ocean. This study compares the oceanic angular momentum variability to the variability observed in the geodetic observations not explained by the atmospheric angular momentum. Semi-annual, annual, and interannual variability are observable in the angular momentum products computed from the ocean model data. The inertial terms of the oceanic angular momentum have most of their variability in the upper most layers. The relative angular momentum variability is distributed throughout the full range of ocean depths. This implies that all levels are important in determining the role of the oceans in the planetary angular momentum budget. Finally, the computed oceanic angular momentum has the correct sign and magnitude to account for some of the unexplained variability in both length of day and polar motion.

 

JPGM10g

Ocean tides and angular momentum since the last glacial maximum

J¸rgen S¸ndermann and Maik Thomas

University of Hamburg, GERMANY

The oscillation system of the World Ocean is strongly dependent on its topography and coastal shape. Both geometrical properties have changed substantially during Earth’s history, most recently after the last glaciation.

In the paper the evolution of oceanic tides for the past 21000 years is investigated. The characteristics of the major tidal constituents M2 and O1 are calculated by a hydrodynamic model in time slices of 1000 years. Further the variation in the angular momentum budget of the ocean and its consequences for the Earth’s rotation and the Moon’s orbit is discussed. It turns out that the ocean tides respond very sensitively to changes in topography even on the scale of a few thousand years.

 

 

JPGM10h

The Influence of Nontidal Oceanic Current and Density Changes on the Earth's Rotation and Polar Motion

R. S. Gross, S. L. Marcus, Y. Chao, J. O. Dickey

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA

Atmospheric wind and pressures changes are the dominant mechanisms causing the Earth's rotation rate to change on time scales of a few days to a few years, and are a major source of polar motion excitation. However, upon removing the modeled effects of the atmosphere from Earth rotation and polar motion measurements, non-negligible signals remain. The effect on the Earth's rotation and polar motion of nontidal oceanic current and density fluctuations is estimated here from the products of an oceanic general circulation model in order to ascertain the degree to which they contribute to the observed residual Earth rotation and polar motion signals.

In a preliminary study of the influence of nontidal ocean processes on the Earth's rotation and polar motion, a version of the Miami isopycnal-coordinate global ocean general circulation model (OGCM) adapted by D. Hu at the Joint Institute for Studies of the Atmosphere and Ocean was run at JPL using forcing by observed winds determined from the National Centers for Environmental Prediction (NCEP; formerly the National Meteorological Center) operational analysis. This OGCM has a free surface, 11 vertical layers plus a mixed layer, realistic bottom topography, and a 2 degree longitude by 1 degree latitude grid spanning 80 S to 80 N latitude. The model was run in spinup mode for 10 years with climatological air-sea fluxes followed by a simulation spanning 1992-1994 with daily wind and heat flux from the NCEP operational analysis and sea surface salinity restoring to Levitus climatology. The axial and equatorial components of the angular momentum due to oceanic currents and density fluctuations were computed and saved at 3-day intervals. Comparisons between these ocean angular momentum estimates and Earth rotation and polar motion measurements (from which atmospheric effects have been removed) will be shown.

 

JPGM10i

Oceanic Excitation of Variations in Earth Rotation Based on Numerical Simulations of the Ocean General Circulation

Frank O. Bryan and Dailin Wang

National Center for Atmospheric Research, USA

Estimation of the oceanic excitation of variations in Earth rotation requires knowledge of the global, time dependent distributions of bottom pressure and vertically integrated mass transport. These quantities are not directly available from current ocean observing systems. In this study we use results from a free-surface, primitive equation, global ocean general circulation model as surrogate data to compute the variability of both the axial and equatorial components of oceanic angular momentum (OAM), and to examine the torques responsible for the exchanges of angular momentum between the ocean, atmosphere and solid Earth. The model is forced with wind stress, atmospheric pressure loading and thermohaline fluxes derived from ECMWF operational analyses for the period 1985 through 1995.

The inclusion of atmospheric pressure loading allows us to compute the net excitation from atmospheric and oceanic mass redistribution, avoiding assumptions about the degree to which the ocean responds as an inverted barometer. Comparison of the simulated sea surface height with altimeter observations shows that the model is able to realistically capture large-scale ocean variability on a broad range of time scales, lending credence to the OAM analysis. Results from several simulations, differing in the combinations of forcing (e.g., with and without pressure loading) and frequency content of the forcing (from 6 hourly to annual mean) are compared to provide insight into the oceanographic processes involved in the variability of OAM and provide a measure of reliability of the results. The resulting excitation functions are compared to those obtained from geodetic observations and from atmospheric analyses. Consistent with previous studies, oceanic excitation of length-of-day variability on seasonal to interannual time scales is small compared to that of the atmosphere. By contrast, oceanic excitation of polar motion may be comparable in magnitude to the atmospheric excitation. However, the net oceanic excitation involves significant cancellation between the contributions of individual ocean basins and between terms, e.g., between the sea level and density contributions to the mass term. This suggest that quantitatively useful estimates of oceanic excitation will require very accurate ocean models and data assimilation systems.

 

JPGM10j

On the Angular Momentum Transfer between the Earth and the Antarctic circumpolar current

Roberto Devoti, Gianluca Borzelli and Francesco Vespe

Nuova Telespazio, Via Tiburtina 965, 00156 Roma,ITALY,
Agenzia Spaziale Italiana,Centro di Geodesia "G.Colombo", Matera, ITALY.

The Antarctic Circumpolar Current (ACC) is the only simply connected current around the world. Research has been undertaken to study effects of the ACC variability on the Earth Rotation (ER) and it has been shown that seasonal variations of currents through the Drake Passage (DP) can be related to seasonal changes of Length of the Day (LOD). However this analysis is based on the assumption that flow conservation along the ACC is ensured by velocity changes along the current rather than meridional shifts of Polar Fronts (PF), thus assuming current meters profiles in the DP are representative of velocity changes along the whole current. More recent studies of the ACC indicate that velocity variations along the current, far from DP, are dominated by mesoscale variability without any evidence of a seasonal signal. In this work a two-year-long series of ERS-1 altimeter data is used to study meridional shifts of PFs far from DP. The ACC angular momentum variations are evaluated on the basis of PFs shifts. Assuming that the sum of the Earth and ACC angular momenta is constant, LOD variations and Polar Motions, estimated from Satellite Laser Ranging Data at the ASI Center for Space Geodesy, are discussed in terms of angular momentum variations of the ACC associated with PFs displacements. The relative influence of ACC and atmospheric angular momentum on LOD is discussed as well.

 

JPGM10k

The Effect of the Pacific Ocean on the Earth's Seasonal Wobble As Inferred from NCEP Ocean Analysis Data

Masato Furuya

Department of Earth and Planetary Physics, University of Tokyo, JAPAN

Under the rule of angular momentum (AM) conservation, global mass redistribution and momentum fluctuation within the atmosphere and hydrosphere are integrated into the Earth's variable rotation. Based on the output of global atmospheric data assimilation systems, the length of day change on the time scale of weeks to a few years has been well-explained only by the atmospheric angular momentum(AAM) fluctuation; there appears to be very small ocean contribution for the axial AM budget. Meanwhile, the equatorial AM budget as reflected in the Earth's wobble excitation is much less understood; for instance, a considerable discrepancy between the AAM and wobble excitation remains even for seasonal frequencies. Thus, the oceanic angular momentum may potentially play an important role in the excitation of Earth's wobble. However, the lack of realistic global ocean data hampers reliable estimates of the ocean effect. Ocean data assimilation, in contrast with that of atmosphere, is still in its infancy due, largely, to the lack of observation. Nevertheless, basin-scale analysis is presently being carried out at NCEP (Ji et al. 1995, MWR). We analyze the atmospheric and Pacific Ocean influence on the Earth's seasonal wobble excitation, employing the NCEP Pacific Ocean analysis data as well as the NCEP reanalysis atmospheric data. SPACE95 is used for the wobble data set. Two orthogonal axes are necessary to discuss the equatorial AM budget; X-axis orients toward Greenwich, and Y-axis toward 90 deg W longitude. Our results indicate that the AAM can trace the excitation around the X-axis well but hardly affects that around the Y-axis. The POAM can generally explain the excitation around Y-axis, and has less contribution around X-axis. However, since the development of the ocean data assimilation system is still ongoing and subject to further improvement, the Pacific Ocean angular momentum (POAM) result should be regarded as preliminary.

 

JPGM10L

Oceanic Excitation of Length-of-Day Fluctuations from TOPEX/Poseidon

Herman van Gysen, Richard Coleman, Nathan Bindoff and J–rg Olaf Wolff

Department of Surveying and Mapping, University of Natal, Durban ,SOUTH AFRICA,
Department of Surveying and Spatial Information Science,
The University of Tasmania, Hobart, Tasmania, AUSTRALIA
Antarctic Cooperative Research Centre, The University of Tasmania, Hobart, Tasmania, AUSTRALIA

Sea surface height variations, as revealed by TOPEX/Poseidon satellite altimetry, provide a complex signal, part of which is due to non-tidal gravitational changes in the oceans and due to changes in the strength of the zonal component of ocean currents, which are the two ways in which changes in the oceans contribute to fluctuations in the length-of-day. For this redistribution in mass and change in torque to be determined, the non-contributing steric component of the sea surface height signal needs to be removed. We have done this using three models - two simple models for the mixed layer, and the Hamburg Large Scale Circulation Model. The results we infer for the oceanic contribution to fluctuations in the length-of-day are compared with LOD fluctuations determined by the IERS, with the (dominant) atmospheric contribution removed.

 

JPGM10M

earth-moon tidal evolution: model results and observational evidence

Boris A. Kagan

P.P. Shirshov Institute of Oceanology, RUSSIA

There are five datasets which can be used to verify models of the Earth-Moon tidal evolution. These are: 1) geological evidence of the age of earth and lunar rocks; 2) paleontological and sedimentological data of growth increments in marine invertebrate fossils, stromatolite growth patterns and cyclically laminated, thin-banded sediments; 3) astronomical data of the secular lunar orbit motion acceleration; 4) lunar laser ranging and satellite tracking data; and 5) estimates of tidal energy dissipation derived from global paleotide models. It is shown that models of the Earth-Moon tidal evolution come into conflict with one or the other dataset if they do not take into account the fluctuating effects of continental drift and their associatged changes in resonance properties of the ocean on geological time scales. The simplest of all possible ways out is to parameterise the response of the ocean to the tide-generating force by a multi-mode resonance approximation with time-dependent frequencies of resonance modes and to assume that the ocean resonance lifetime is much shorter than the duration of the cycle of consolidation-disintegration of continents.