GRACE Science Team Meeting

Session B.4-a: GRACE & the Arctic


(Arctic Ocean Tides from GRACE Satellite Accelerations)
Bryan Killett

(Variations of ocean bottom pressure in the Arctic from weekly to seasonal time scales)
Cecilia Peralta-Ferriz

(Interannual Trends in Ocean Bottom Pressure in the Arctic Ocean 2002-2009 and Their Relation to Circulation and Water Mass Changes )
Morison James

(Average and interannual dynamic topography from altimetry and the GRACE mean geoid )
Ron Kwok

B.4.b: Sea Level Workshop

(An Overview of the 2009 NASA Sea Level Workshop)
R. Steven Nerem

Session B.4-d: Oceanography

(How well can we estimate rapid variability in ocean bottom pressure?)
Rui Ponte

(Uncertainty in ocean mass trends from GRACE)
Katherine Quinn

(An Overview of OceanObs09 and GRACE)
Don Chambers

(Recent research in tides in regards to GRACE)
Richard Ray

(A Global Evaluation of Ocean Bottom Pressure from GRACE, OMCT, and Steric-Corrected Altimetry)
Don Chambers

Session: B.4.a - Theme: GRACE & the Arctic
Title: Arctic Ocean Tides from GRACE Satellite Accelerations
First Author: Bryan Killett
Presenter: Bryan Killett
Co-Authors: John M Wahr, Shailen D Desai, Dah-Ning Yuan, Michael M Watkins

Abstract: Because missions such as TOPEX/POSEIDON don't extend to high latitudes, Arctic ocean tidal solutions aren't constrained by altimetry data. The resulting errors in tidal models alias into monthly GRACE gravity field solutions at all latitudes. Fortunately, GRACE inter-satellite ranging data can be used to solve for these tides directly. Five years of GRACE inter-satellite acceleration data are inverted using a mascon approach to solve for amplitudes and phases of major solar and lunar tides in the Arctic ocean. The resulting tidal amplitudes are compared to existing tidal models using in-situ data from coastal tide gauges and deep sea bottom pressure recorders.

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Session: B.4.a - Theme: GRACE & the Arctic
Title: Variations of ocean bottom pressure in the Arctic from weekly to seasonal time scales
First Author: Cecilia Peralta-Ferriz
Presenter: Cecilia Peralta-Ferriz
Co-Authors: J. H. Morison

Abstract: The ocean bottom pressure (OBP) was measured at the North Pole from 2005 to 2008, as part of the North Pole Environmental Observatory. OBP shows a spectral peak at a period of about 19 days, which is consistent with modeling results of OBP from the PanArctic Ice-Ocean Model Assimilation System, PIOMAS. This signal is also detected in the sea level pressure (SLP) records from the NCEP/NCAR re-analysis for the same time as the observations of OBP. Similarly, Morison [1990] found a spectral peak at 19 days in OBP observations across the West Spitsbergen Current, in Fram Strait. Here we explore two questions: What is the source of the 19-day period atmospheric signal and how is this signal transferred to the ocean. Comparison of OBP from PIOMAS, which assumes a perfect inverted barometer, with observed OBP suggests that departures from the inverted barometer response are small. The fact that the PIOMAS OBP without direct atmospheric pressure loading shows a spectral peak that is similar to observed OBP, suggests that these oscillations are wind (pressure gradient) driven rather than due to direct atmospheric loading. The basin-averaged OBP variations from PIOMAS are well correlated with southerly winds in Fram Strait and the basin-averaged OBP, with the pressure lagging the wind by 1-2 days. Through examination of atmospheric pressure data and ice-ocean model results, we investigate the hypotheses that the SLP variation is related to the passage of planetary waves across the North Atlantic, and that an Ekman slope current through Fram Strait is driving the ocean bottom pressure change. OBP observations from GRACE averaged over the Arctic Ocean reveal an annual cycle with amplitude of 2 cm peak to peak. The maximum occurs in late summer to early fall, and the minimum in late winter to early spring. We explore the process with a simple model of OBP response to the combined forcing of runoff, net precipitation and atmospheric pressure that agrees with the observations.

Morison, J.H., (1990), Seasonal fluctuations in the West Spitsbergen Current estimated from bottom pressure measurements. J. Geophys. Res., 96 (C10), 18,381-18,395.

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Session: B.4.a - Theme: GRACE & the Arctic
Title: Interannual Trends in Ocean Bottom Pressure in the Arctic Ocean 2002-2009 and Their Relation to Circulation and Water Mass Changes
First Author: Morison James
Presenter: Morison James
Co-Authors: Cecilia Peralta-Ferriz

Abstract: Annual average Arctic Ocean bottom pressure from GRACE Release 4 through 2009 shows a general decrease from 2002 to 2006. The decrease was particular strong in the Makarov Basin and on the Eurasian side of the Lomonosov Ridge. This is consistent with the observed decrease in salinity in the Makarov Basin associated with an anticyclonic shift in the front between Pacific-derived and Atlantic-derived upper ocean waters (Morison et al., 2007). From 2005 to 2009, the pattern and its relation to hydrography was more complicated. Bottom pressure continued to drop in the Beaufort Sea due to unprecedented freshening. Ocean bottom pressure increased in the Eurasian and Makarov basins in a seeming reversal of the 2002-2005 shift, but in some regions this was opposite to what would be expected from observations of freshening. In part, bottom pressure trends in the later period are clouded by a strong central Arctic Ocean bottom pressure increase in 2007. However, the pattern of increasing bottom pressure and decreasing upper ocean salinity in the northern Beaufort Sea reflects the building of a dome of fresh water in that region in 2007. This was due to Ekman pumping under and unusually strong Beaufort High pressure system in the summer of 2007 that was also partly responsible for the record minimum sea ice extent in September 2007. The overall 2005-2009 pattern may be best explained as a combination of intense anticyclonic wind forcing in the Beaufort Sea, embedded in a large but more diffuse cyclonic pattern of atmospheric forcing.

Morison, J., J. Wahr, R. Kwok, and C. Peralta-Ferriz, 2007, Recent trends in Arctic Ocean mass distribution revealed by GRACE, Geophys. Res. Lett., 34, L07602, doi:10.1029/2006GL029016.

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Session: B.4.a - Theme: GRACE & the Arctic
Title: Average and interannual dynamic topography from altimetry and the GRACE mean geoid
First Author: Ron Kwok
Presenter: Ron Kwok
Co-Authors: J. Morison

Abstract: We present our first estimates of the dynamic topography of the Arctic Ocean derived from ICESat and EGM2008. EGM2008 has benefited from the latest GRACE based satellite-only solutions, but is mixed with altimetric estimates to the best of our knowledge. In ICESat processing, we retrieve the ice and sea surface heights (ISH, SSH) by separating the elevation returns from Arctic sea ice and open water. Differencing the ISH with the local SSH gives the sea ice freeboard used in ice thickness calculations. The sparsely sampled SSHs within the ice cover are used to estimate dynamic topography less than a few percent of the area of ice cover is open water. An accurate geoid is clearly useful in minimizing the uncertainties in our derived quantities at all length scales. We show the contributions of GRACE data in improving the Arctic Gravity Project geoid (ArcGP-geoid) - what looks to be long wavelength anomalies across the Arctic Basin are not longer present. The resulting dynamic topography shows the expected relief but the interpretation of the fields of interannual variability remains. There are residuals in the field that may be due to uncertainties in EGM2008.

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Session: B.4.b - Theme: Sea Level Workshop
Title: An Overview of the 2009 NASA Sea Level Workshop
First Author: R. Steven Nerem
Presenter: R. Steven Nerem

Abstract: An overview will be presented on the results of the NASA Sea Level Workshop that was held just prior to the GRACE Science Team Meeting in Austin, Texas. The purpose of the workshop was: 1) To review the status of scientific observations and understanding of present day sea level change and its contributing factors, including its relation to past sea level change, 2) To assess the suitability of the present observing system for capturing ongoing changes and predicting future ones, and 3) To determine what type of observations and research activities should be encouraged in order to improve sea level projections for the 21st century. Highlights and recommendations of the workshop will be presented. The importance of the GRACE measurements to sea level research will be emphasized.

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Session: B.4.d - Ocean Processes & Validation
Title: How well can we estimate rapid variability in ocean bottom pressure?
First Author: Rui Ponte
Presenter: Rui Ponte
Co-Authors: K.J. Quinn; N.T. Vinogradova

Abstract: Non-tidal ocean bottom pressure (OBP) signals at periods shorter than 60 days are not well resolved by nominal GRACE sampling. To minimize potential aliasing into the monthly mean GRACE fields, such rapid OBP variability is simulated by the Ocean Model for Circulation and Tides (OMCT) and its impact on the range times is removed during data processing. The efficacy of dealiasing methods depends critically on the ability to estimate the rapid OBP signals. Here we compare OMCT fields with over 100 OBP in situ records distributed over several ocean regions. Independent OBP fields based on data-constrained estimates from ECCO (Estimating the Circulation and Climate of the Oceans) are also used. All analyses are based on daily mean series that have been high-pass filtered with a cutoff period at 60 days. The observed high-frequency variance explained by either OMCT or ECCO fields is typically less than 50%, and both OMCT and ECCO values considerably underestimate the variability in the data over most sites. Averaged over all stations, BPR residual variance is > 10 cm**2 after OMCT and ECCO estimates are removed. Agreement between OMCT and ECCO fields also varies, with best correlations seen at mid latitudes and away from coastal regions, and weaker correlations at low latitudes. Poor knowledge of the spatial scales involved in OBP variability at the short periods of interest remains an issue in attempts to understand how much non-tidal variance is still being aliased in the GRACE records, but the need for continuing improvements in the dealiasing procedures is clear.

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Session: B.4.d - Ocean Processes and Validation
Title: Uncertainty in ocean mass trends from GRACE
First Author: Katherine Quinn
Presenter: Rui Ponte
Co-Authors: R. M. Ponte

Abstract: Ocean mass, together with steric sea level, are the key components of total observed sea level change. Monthly observations from the Gravity Recovery and Climate Experiment (GRACE) can provide estimates of the ocean mass component of the sea level budget, but full use of the data requires a detailed understanding of its errors and biases. We have examined trends in ocean mass calculated from six years of GRACE data and found differences of up to 1~mm/yr between estimates derived from different GRACE processing center solutions. In addition, variations in post-processing masking and filtering procedures required to convert the GRACE data into ocean mass lead to trend differences of up to 0.5~mm/yr. Necessary external model adjustments add to these uncertainties, with reported post-glacial rebound corrections differing by as much as 1~mm/yr. Disagreement in the regional trends between the GRACE processing centers is most noticeably in areas south of Greenland, and in the southeast and northwest Pacific Ocean. Non-ocean signals, such as in the Indian Ocean due to the 2004 Sumatran-Andean earthquake, and near Greenland and West Antarctica due to land signal leakage, can also corrupt the ocean trend estimates. Based on our analyses, formal errors may not capture the true uncertainty in either regional or global ocean mass trends derived from GRACE.

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Session: B.4.c - Theme: GRACE & Oceanobs '09
Title: An Overview of OceanObs09 and GRACE
First Author: Don Chambers
Presenter: Don Chambers
Co-Authors: R. Steven Nerem; Josh Willis

Abstract: The second OceanObs conference was recently held in Venice, Italy. The goals of the conference were to celebrate the success of the global ocean observing system proposed at the original OceanObs conference in 1999 and to recommend plans for sustaining and evolving the network over the next 10 years. Although GRACE was not envisioned as an important part of the network 10 years ago before its launch, numerous posters and talks at the meeting discussed the important contributions made by GRACE over the last 7 years. Here, we will summarize the discussions of the current contributions of GRACE to the global ocean observing system, as well as the recommendations given for the future usefulness of time-variable gravity.

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Session: B.4.d - Ocean Processes & Validation
Title: Recent research in tides in regards to GRACE
First Author: Richard Ray
Presenter: Richard Ray
Co-Authors: G Egbert, S Erofeeva, S Han, S Luthcke

Abstract: We review several examples of tide modeling errors in current GRACE processing (based on FES2004 as well as other models). These include (a) tidally coherent anomalies in GRACE range-rate residuals, (b) aliased signals in monthly gravity (e.g. from S2), and (c) aliased signals in monthly gravity produced by analyzing separately data from ascending and descending passes (which highlights errors in diurnal tides). We further review some recent efforts to extract residual tidal signals from GRACE range-rate data and how such estimates should be incorporated, or assimilated, into hydrodynamic models. Emphasis is given to our group's recent work on Antarctic tides, where the results are tested against independent tide-gauge data and independent ICESat laser altimeter data.

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Session: B.4.d - Ocean Processes & Validation
Title: A Global Evaluation of Ocean Bottom Pressure from GRACE, OMCT, and Steric-Corrected Altimetry
First Author: Don Chambers
Presenter: Don Chambers
Co-Authors: Josh Willis

Abstract: Ocean bottom pressure (OBP) from the Gravity Recovery and Climate Experiment (GRACE) and the Ocean Model for Circulation and Tides (OMCT) are compared globally with OBP computed from altimetry corrected for steric variations from Argo floats from January 2005 to December 2007. Two methods of smoothing the GRACE data are examined. The first uses a standard Gaussian smoother with a radius of 300 km. The second method projects those smoothed maps onto Empirical Orthogonal Functions derived from OMCT in a least squares estimation in order to produce maps that better agree with the physical processes embodied by the model. We find that these new maps agree significantly better with estimates from the steric-corrected altimetry, reducing the variance on average by 30% over 70% of the ocean. This is compared to smaller reductions over only 14% of the ocean using the 300 km Gaussian maps, and 56% of the ocean using OMCT maps. The OMCT maps do not reduce variance as much in the Southern Ocean where OBP variations are largest, whereas the GRACE maps do. Based on this analysis, we estimate that the local, or point-to-point uncertainty of new EOF filtered maps of GRACE OBP is 1.9 cm as an upper bound (one standard deviation). If we consider uncertainty in the steric-corrected altimeter maps, the uncertainty is closer to 1.3 cm.

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