Benjamin F. Chao

Impact of artificial reservoir water impoundment on global sea level.

By reconstructing the history of water impoundment in the worlds artificial reservoirs, we show that a total of ∼10,800 cubic kilometers of water has been impounded on land to date, reducing the magnitude of global sea level (GSL) rise by –30.0 millimeters, at an average rate of –0.55 millimeters per year during the past half century. This demands a considerably larger contribution to GSL rise from other (natural and anthropogenic) causes than otherwise required. The reconstructed GSL history, accounting for the impact of reservoirs by adding back the impounded water volume, shows an essentially constant rate of rise at +2.46 millimeters per year over at least the past 80 years. This value is contrary to the conventional view of apparently variable GSL rise, which is based on face values of observation.

Diurnal/semidiurnal polar motion excited by oceanic tidal angular momentum

The axial component of the oceanic tidal angular momentum (OTAM) has been demonstrated to be responsible for most of the diurnal and semidiurnal variations in Earths rotational rate. In this paper we study the equatorial components of OTAM and their corresponding effects on the orientation of Earths rotational axis, or polar motion. Three ocean tide models derived from TOPEX/Poseidon satellite altimetry are employed to predict the polar motion excited by eight major diurnal/semidiurnal tides (Q1,O1,P1, K1, N2, M2, S2, K2). The predictions are compared with geodetic measurements of polar motion from both long-term observations and during the intensive campaign Cont94. The prograde diurnal and prograde and retrograde semidiurnal periods are treated, whereas the retrograde diurnal polar motion is not treated (because it cannot be observed directly and uniquely.) The comparison shows generally good agreement, with discrepancies typically within 10–30 micro-arc- seconds for the largest tides. The eight tides collectively explain nearly 60% of the total variance in subdaily polar motion during Cont94. This establishes the dominant role of OTAM in exciting the diurnal/semidiurnal polar motion and paves the way for detailed studies of short-period nontidal polar motion. The present accuracy, however, is inadequate to shed light on the prograde diurnal polar libration.

Diurnal and Semidiurnal Variations in the Earth's Rotation Rate Induced by Oceanic Tides

Recent space-geodetic observations have revealed daily and subdaily variations in the Earths rotation rate. Although spectral analysis suggests that the variations are primarily of tidal origin, comparisons to previous theoretical predictions based on various ocean models have been less than satisfactory. This disagreement is partly caused by deficiencies in physical modeling. Rotation predictions based on a reliable tidal-height model, with corresponding tidal currents inferred from a modified form of Laplaces momentum equations, yield predictions of tidal variations in Universal Time that agree with very long baseline interferometer observations to 2 microseconds. This agreement resolves a major discrepancy between theory and observation and establishes the dominant role of oceanic tides for inducing variation in the Earths rotation at these frequencies.

Temporal variations of the earth's gravitational field from satellite laser ranging to LAGEOS

We have estimated monthly values of the J2 and J3 Earth gravitational coefficients using LAGEOS satellite laser ranging (SLR) data collected between 1980 and 1989. For the same time period, we have also computed corresponding estimates of the variations in these coefficients caused by atmospheric mass redistribution using surface atmospheric pressure estimates from the European Center for Medium Range Weather Forecasts (ECMWF). These data were processed both with and without a correction for the “inverted barometer effect,” the oceans isostatic response to atmospheric loading. While the estimated zonal harmonics in the orbit analysis accommodate gravitational changes at a reduced level arising from all other higher degree zonal effects, the LAGEOS and atmospheric time series for J2 compare quite well and it appears that the non-secular variation in J2 can be largely attributed to the redistribution of the atmospheric mass. While the observed changes in the “effective” J3 parameters are not well predicted by the third degree zonal harmonic changes in the atmosphere, both odd zonal time series display strong seasonality. The LAGEOS J3 estimates are very sensitive to as yet unmodeled forces acting on the satellite and these effects must be better understood before determining the dominant geophysical signals contributing to the estimate of this time series.

Oceanic angular momentum variability estimated from the Parallel Ocean Climate Model, 1988–1998

This paper describes the use of a global numerical model of the oceans to estimate variations in both the equatorial and axial components of angular momentum resulting from oceanic mass redistribution and circulation. The Parallel Ocean Climate Model, driven by daily wind fields and monthly heat fluxes from the European Centre for Medium-Range Weather Forecasts for 1988–1998, provides insight into variations in the transport and exchange of angular momentum, a quantity which is essentially conserved within the Earth system. Exchange of angular momentum between the oceans and the solid Earth should be manifest as changes in the Earths rotation (both polar motion and length of day), and it is possible to compare predicted Earth rotation changes with actual geodetic observations. Using an inverted barometer assumption for oceans, the numerical model predictions of rotation change agree in sign and magnitude and are significantly correlated with observed polar motion and length of day variations after subtracting the dominant atmospheric contributions. The correlation has a seasonal variation which suggests that the role of the oceans in the excitation polar motion is more important during the Northern Hemisphere summer. Our results indicate that the oceans, to a different extent, account for a significant part of the nonatmospheric angular momentum budget for the Earth, and its fluid envelopes on interannual to submonthly timescales.

Anthropogenic impact on global geodynamics due to reservoir water impoundment

Water impounded in artificial reservoirs since ∼1950 is by far the largest anthropogenic hydrological change in terms of the mass involved. This mass redistribution contributes to geodynamic changes in the Earths rotation and gravitational field that have been closely monitored by modern space geodetic techniques. We compute the effect of 88 major reservoirs on length-of-day, polar motion, and low-degree gravitational coefficients. On an individual basis much smaller than geophysical signals in scale and magnitude, these anthropogenic effects prove to be non-negligible cumulatively, especially when considering the fact that our results represent underestimates of the reality. In particular, reservoir water has contributed a significant fraction in the total observed polar drift over the last 40 years.

Wavelet analysis provides a new tool for studying Earth's rotation

The solid Earths rotation varies slightly with time due to geophysical processes that involve motions and redistributions of mass occurring on or within the Earth, as dictated by the conservation of angular momentum. In particular, these variations Δ LOD) in the atmosphere in terms of the axial atmospheric anglar momentum (AAM) are the primary cause for nontidal length-of-day variations on timescales of several days to several years [e.g., Rosen, 1993]. Here Δ LOD is a convenient measure of Earths rotational speed relative to the uniform time kept by atomic clocks.

Change of sea level trend in the Mediterranean and Black seas

Sea level anomaly (SLA) data in the Mediterranean and Black seas obtained by ocean radar altimetry missions (TOPEX/Poseidon, Jason, ERS-1/2 and ENVISAT) are studied in conjunction with corresponding sea-surface temperature (SST) data. The studied time span is 11 years long, 1993-2003. Besides confirming previously published results, we report a significant, but enigmatic, abrupt change in the SLA trend that took place in mid-1999 which has been corroborated by independent tide gauge data. Results obtained from an Empirical Orthogonal Function (EOF) analysis show that the change in 1999 is not uniform in the Mediterranean Sea, which can thus be divided into 6 sub-regions. This 1999 kink in the rate-of-change happened in four of these sub-regions as well as in the Black Sea. Upon splitting the time series at mid-1999, we see a good spatial correlation for the first period between SLA and SST trend maps in both the Mediterranean Sea and the Black Sea, while for the second period such correlation virtually disappeared in the Mediterranean and is greatly reduced in the Black Sea. It implies that prior to 1999 the steric effect was a major factor in interannual variability of sea level in the Mediterranean and Black seas, but after the time the SLA inverted its trend in mid-1999, this steric effects became less important as a forcing factor. It is premature to draw conclusions about the physical processes involved based on the data sets we study, but it appears that the Mediterranean Sea might be seeing a restoration of Adriatic as the main source of deep water in the eastern basin, while the Black Sea level has been largely controlled by an interannual or interdecadal steric effect.

Excitation of Earth’s polar motion by atmospheric angular momentum variations, 1980–1990

We compute the polar-motion excitation function due to the atmospheric angular momentum (AAM) for both IB (inverted-barometer) and non-IB cases, as well as the excitation function from geodetically observed Earth orientation data for the period 1980-1990. The two are then compared in studying the AAM contri- bution to the polar motion excitation. The polar drifts with periods longer than --2 years have similar characteristics, but the comparison is inconclusive because of data uncertainties. For the seasonal wobble excitation, the agreement is poor except for the prograde annual wobble, indicating the influence of other geophysical excitations than AAM. For the Chandler wobble excitation, a cor- relation coefficient of 0.53 for non-IB and 0.58 for IB are found for 1986-1990. Together with a coherence spectral analysis, they clearly demonstrate a strong contribution of AAM to the Chandler wobble excitation. The polar motion is a 2-dimensional quantity, conve- niently expressed in terms of the complex angular distance m -- m 1 + i m 2 in radians, where subscripts 1 and 2 refer to the x (along the Greenwich Meridian) and y (along the 90oE longitude) coordinates of the terrestrial frame. The equation of motion governing the excitation of the geode- tically observed m is (Barnes et al., 1983; Gross, 1992)

On the steric and mass‐induced contributions to the annual sea level variations in the Mediterranean Sea

The authors thank NASA Pathfinder project and Andrew Au for assistance in data processing. The altimeter products were produced by the CLS Space Oceanography Division as part of the Environment and Climate EU ENACT project (EVK2-CT2001-00117) and with support from CNES. GRACE data are provided by the NASA/DLR GRACE Project via the Center for Space Research website in the University of Texas at Austin. The ECCO model is a contribution of the Consortium for Estimating the Circulation and Climate of the Ocean funded by the National Oceanographic Partnership Program. This work, as part of the lead authors Ph.D. dissertation, is supported by the Spanish Science and Technology Ministry Projects ESP2001-4533/PE, ESP2005-02212 and REN2003- 01608/MAR, by the Valencian regional government grant ACOMP06-120, and by NASA’s Physical Oceanography Program.