Don P. Chambers

Estimating geocenter variations from a combination of GRACE and ocean model output

[1] In this study, we estimate a time series of geocenter anomalies from a combination of data from the GravityRecoveryandClimateExperiment(GRACE)satellitemissionandthe outputfromoceanmodels.Amatrixequationisderivedrelating totalgeocentervariations to the GRACE coefficients of degrees two and higher and to the oceanic component of the degree one coefficients. We estimate the oceanic component from two state-of-the-art ocean models. Results are compared to independent estimates of geocenter derived from other satellite data, such as satellite laser ranging and GPS. Finally, we compute degree one coefficients that are consistent with the processing applied to the GRACE Level-2 gravity field coefficients. The estimated degree one coefficients can be used to improve estimates of mass variability from GRACE, which alone cannot provide them directly.

Estimating Mean Sea Level Change from the TOPEX and Jason Altimeter Missions

The Jason-2 satellite altimeter mission was launched in June 2008, extending the record of precision sea level measurements that was initiated with the launch of TOPEX/Poseidon in 1992 and continued with the launch of Jason-1 in December 2001. We have used the measurements from these three missions to construct a seamless record of global mean sea level change from 1993 to the present. We present the results of our calibration activities, including data comparisons during the “tandem period” of the missions, during which we solve for biases between the missions, as well as comparisons to independent tide gauge sea level measurements. When the entire record is assembled, the average rate of sea level rise from 1993–2009 is 3.4 ± 0.4 mm/year. There is considerable interannual variation due to ENSO-related processes, which include the period of lower sea level rise over the last three years of the time series during the recent La Nina event.

GRACE observes small‐scale mass loss in Greenland

Using satellite gravity data between February 2003 and January 2008, we examine changes in Greenlands mass distribution on a regional scale. During this period, Greenland lost mass at a mean rate of 179 ± 25 Gt/yr, equivalent to a global mean sea level change of 0.5 ± 0.1 mm/yr. Rates increase over time, suggesting an acceleration of the mass loss, driven by mass loss during summer. The largest mass losses occurred along the southeastern and northwestern coast in the summers of 2005 and 2007, when the ice sheet lost 279 Gt and 328 Gt of ice respectively within 2 months. In 2007, a strong mass loss is observed during summer at elevations above 2000 m, for the first time since the start of the observations.

Mean Dynamic Topography of the Ocean Derived from Satellite and Drifting Buoy Data Using Three Different Techniques

Abstract Presented here are three mean dynamic topography maps derived with different methodologies. The first method combines sea level observed by the high-accuracy satellite radar altimetry with the geoid model of the Gravity Recovery and Climate Experiment (GRACE), which has recently measured the earth’s gravity with unprecedented spatial resolution and accuracy. The second one synthesizes near-surface velocities from a network of ocean drifters, hydrographic profiles, and ocean winds sorted according to the horizontal scales. In the third method, these global datasets are used in the context of the ocean surface momentum balance. The second and third methods are used to improve accuracy of the dynamic topography on fine space scales poorly resolved in the first method. When they are used to compute a multiyear time-mean global ocean surface circulation on a 0.5° horizontal resolution, both contain very similar, new small-scale midocean current patterns. In particular, extensions of western boundary c...

Anomalous warming in the Indian Ocean coincident with El Niño

The TOPEX/POSEIDON altimeter has provided further evidence that interannual warming occurs in the Indian Ocean with a frequency similar to that of El Nino in the Pacific and has yielded important clues to the dynamics driving the warming. The signal is especially strong during the 1997 El Nino. The altimeter observes long waves which move westward from the southeastern Indian Ocean at about the same time as westwardly wind anomalies appear in the east-central portion of the basin. The sea level peaks in the southwestern Indian Ocean and causes a sea level variation signal that is a near mirror image of El Nino in the eastern Pacific. Sea surface temperature data also show a similar correlation. An analysis of the altimeter data indicates significant variability in the Indian Ocean during the 1994 and 1997 El Nino events at the first and second baroclinic Rossby wave modes. Sea surface temperature and wind data suggest that the Indian Ocean warming has occurred during several previous El Nino events, particularly during the large events of 1982 and 1987. Based on these observations, it is suggested that the warming begins with wind-forced Rossby waves in the southeastern Indian Ocean associated with the Southern Oscillation, similar to the forcing of Kelvin waves which precede El Nino in the Pacific.

Evaluation of Release-05 GRACE time-variable gravity coefficients over the ocean

Abstract. The latest release of GRACE (Gravity Recovery and Climate Experiment) gravity field coefficients (Release-05, or RL05) are evaluated for ocean applications. Data have been processed using the current methodology for Release-04 (RL04) coefficients, and have been compared to output from two different ocean models. Results indicate that RL05 data from the three Science Data Centers – the Center for Space Research (CSR), GeoForschungsZentrum (GFZ), and Jet Propulsion Laboratory (JPL) – are more consistent among themselves than the previous RL04 data. Moreover, the variance of residuals with the output of an ocean model is 50–60% lower for RL05 data than for RL04 data. A more optimized destriping algorithm is also tested, which improves the results slightly. By comparing the GRACE maps with two different ocean models, we can better estimate the uncertainty in the RL05 maps. We find the standard error to be about 1 cm (equivalent water thickness) in the low- and mid-latitudes, and between 1.5 and 2 cm in the polar and subpolar oceans, which is comparable to estimated uncertainty for the output from the ocean models.

Evaluation of groundwater storage monitoring with the GRACE satellite: Case study of the High Plains aquifer, central United States

(450,000 km 2 area), which is subjected to intense irrigation. GRACE-derived terrestrial water storage (TWS) is highly correlated with the sum of soil moisture (SM) and groundwater storage (GWS) (R = 0.96 for in situ measured SM from 78 stations and R = 0.95 for simulated SM with the Noah land surface model with root-mean-square difference of 38 mm and 36 mm, respectively). Correlation between seasonal GWS changes calculated from GRACE TWS minus SM and measured GWS (� 1000 wells per season)isalsohigh(R=0.73forinsituSMandR=0.72forsimulatedSM).VariabilityinSM is mostly restricted to the upper 2 m of the soil. Monitored SM compared favorably with simulated SM (R = 0.82). Study results show the potential for using GRACE gravity measurements to monitor TWS and GWS over large semiarid regions subjected to intense irrigation.

Variations in global mean sea level associated with the 1997–1998 ENSO event: Implications for measuring long term sea level change

The TOPEX/POSEIDON satellite has observed variations in global mean sea level with a precision of 4 mm at 10-day intervals since late 1992. During the 1997–1998 ENSO event, a 20 mm rise, and subsequent fall, of mean sea level was observed. These changes are well correlated with global mean sea surface temperature anomalies, which exhibit a similar response for every major ENSO event since 1981, suggesting the observed mean sea level change is mostly caused by thermal expansion. An Empirical Orthogonal Function analysis of the altimeter-derived sea level maps also suggests a connection with ENSO. We observed the same signal in global mean dynamic heights of the MOM2 ocean model and in anomalies of global mean precipitable water vapor. The presence of ENSO-variability in global mean sea level suggests that detecting the much smaller sea level variations associated with climate change will require at least a decade of precise altimeter measurements.

GRACE-Based Estimates of Terrestrial Freshwater Discharge from Basin to Continental Scales

Abstract In this study, new estimates of monthly freshwater discharge from continents, drainage regions, and global land for the period of 2003–05 are presented. The method uses observed terrestrial water storage change estimates from the Gravity Recovery and Climate Experiment (GRACE) and reanalysis-based atmospheric moisture divergence and precipitable water tendency in a coupled land–atmosphere water mass balance. The estimates of freshwater discharge are analyzed within the context of global climate and compared with previously published estimates. Annual cycles of observed streamflow exhibit stronger correlations with the computed discharge compared to those with precipitation minus evapotranspiration (P − E) in several of the world’s largest river basins. The estimate presented herein of the mean monthly discharge from South America (∼846 km3 month−1) is the highest among the continents and that flowing into the Atlantic Ocean (∼1382 km3 month−1) is the highest among the drainage regions. The volume...

Long‐period ocean heat storage rates and basin‐scale heat fluxes from TOPEX

Over 3 years of TOPEX altimeter data have been used to estimate annual heat storage and long-period heat storage rates in 1° grids over the global oceans (65°S to 65°N). The mean annual heat storage rates computed from the TOPEX data agree with those computed from monthly-mean temperatures to within 30 W m2. The accuracy of long-term heat storage rates is estimated to be less than 10 W m2 over mid and low latitudes. Regional heat storage rates are larger than this error in many regions, most notably in the tropical Pacific and Indian Oceans and in the northern Atlantic Ocean. The heat storage changes associated with the 1991–1993 El Nino event are evident and agree with values computed from in situ measurements collected by the Tropical Ocean-Global Atmosphere program Tropical Atmosphere Ocean array moorings. The heat storage rates inferred from TOPEX data are also shown to qualitatively agree with sea surface temperature rates computed over the same time period. Finally, heat storage rates are integrated over ocean gyres to estimate atmosphere-ocean heat fluxes. Results suggest that from 1993 to 1995 the North Atlantic and the southern hemisphere gained heat from the atmosphere at a rate of between 1.5 and 2 W m2, while the North Pacific either lost heat or maintained zero net flux. However, the estimated error on this measurement is of the order of 1.5 W m2 indicating the true flux could be from 0 to 3 W m2. Both the TOPEX and temperature measurements suggest that the North Atlantic gained more heat per unit area than the North Pacific between 1992 and 1995.