B.4 - Oceanography
(Convener: Don Chambers)

Understanding Sea Level Rise: The cross-spectral analysis of the sea level budget
(Leuliette, Smith)

How well can GRACE Measure Transport Variations of the Antarctic Circumpolar Current?
(Chambers, Makowski, Bonin)

Evaluation of High-Frequency Oceanographic Signal in GRACE Data: Implications for De-aliasing
(Bonin, Chambers)

Using Ocean Bottom Pressure from GRACE to Understand Transport Variations of the Antarctic Circumpolar Current
(Makowski, Chambers, Bonin)

The 2010 decline in global mean sea level and its relation to ENSO
(Boening, Landerer, Nerem, Willis)

Evaluation and interpretation of inter-annual variations in Arctic Ocean mass from GRACE using in situ observations and ocean models
(Peralta-Ferriz, Morison, Wahr, Kwok, Bonin, Chambers)

Session: B.4 - Oceanography
Title: Understanding Sea Level Rise: The cross-spectral analysis of the sea level budget
First Author: Eric Leuliette
Presenter: Eric Leuliette
Co-Authors: Walter H.F. Smith

Abstract: We investigate the spectrum of recent changes in total sea level measured by the Jason series of satellite altimeters and changes in the components of sea level -- steric sea level derived from Argo profiles and ocean mass from GRACE maps of gravity. The sea level budget is closed when the sum of these independently measured components agrees with measurements of total sea level, indicating that the observations can be used to meaningfully explain the causes of sea level change. With cross-spectral analysis between total sea level and its components during 2005-2011, the relative contributions of each component can be assessed at different frequencies. Using an admittance analysis on Jason, GRACE, and Argo data, we demonstrate that the budget closes at all significant frequencies, including those in the intra-annual, annual, and interannual.

Increased warming of the Earth in the 21st century will produce accelerating rates of sea level rise both from increased thermal expansion and mass input to the ocean from continental ice sheets and glaciers. In this scenario, the relative contribution of additional ocean mass to sea level rise is expected to increase from the proportion observed during the late 20th century. In order to isolate the climate signal, a better understanding of the interannual variability of the sea level budget components is necessary for interpreting changes in the relative contributions. For example, the patterns of heat storage anomalies associated with ENSO variability likely alter the relative contributions of steric sea level and ocean mass at the seen at 4-8 year periods of ENSO phases compared to other long-period variability. Using sea level and steric sea level from 18 years of output from ECCO-JPL assimilation ocean model, we investigate the effect of ENSO variability on the sea level budget.

Finally, we demonstrate how admittance maps can be used to understand the geographical patterns of the relative contributions of sea level rise at different frequencies. However, application of these methods using observations will only be practical with a long gravity data record from a successful GRACE Follow-on mission.

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Session: B.4 - Oceanography
Title: How well can GRACE Measure Transport Variations of the Antarctic Circumpolar Current?
First Author: Don Chambers
Presenter: Don Chambers
Co-Authors: Jessica K Makowski, Jennifer A Bonin

Abstract: Because of the inherent resolution of the data and smoothing required to reduce noise, GRACE will never be able to measure the transport variability along individual fronts of the Antarctic Circumpolar Current. However, it may be possible to measure the average transport variation between fronts over wide areas, such as a basin. A previous estimate by Zlotnicki et al. (2007) suggested that basin-scale average transport variations could be computed to 5 - 7 Sverdrups (Sv, where 1 Sv = 106 m3/s), based on differencing averages of bottom pressure (BP) across the current and computing a very approximate scaling parameter.

We have been testing whether computing transport variability using the full integral of bottom pressure derivatives between front positions for 15-30° longitude boxes give better results. In order to quantify the uncertainty in the GRACE measurements due to smoothing and noise, we have used a simulated data set based on BP from JPL_ECCO, hydrology from GLDAS, and ice mass loss trends from Antarctica and Greenland, with random and correlated noise consistent with GRACE added. We find that we can recover the "true" average transport variability to within 3 to 5 Sv for boxes as narrow as 30° in longitude over much of the Indian and Pacific Oceans. Errors increase as one approaches West Antarctica due to leakage of ice mass loss. These differences are of the same order as the difference between models, which suggests GRACE may contribute to understanding the ACC dynamics.

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Session: B.4 - Oceanography
Title: Evaluation of High-Frequency Oceanographic Signal in GRACE Data: Implications for De-aliasing
First Author: Jennifer Bonin
Presenter: Jennifer Bonin
Co-Authors: Don P Chambers

Abstract: The sub-monthly portion of a recent daily-resolution GRACE data set (ITG-Grace2010) is evaluated over the ocean by comparing with the high-frequency component of sea level variability measured by satellite altimetry. The current ocean model (OMCT) used to remove the high-frequency non-tidal ocean mass variations in GRACE data processing is also assessed. We demonstrate that the OMCT model does not adequately represent the true sub-monthly variability in non-tidal ocean mass variability, but that ITG-Grace2010 does. The differences are not small, and indicate that a new model, or a data set derived from sub-monthly GRACE information, should be considered for de-aliasing in future GRACE processing.

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Session: B.4 - Oceanography
Title: Using Ocean Bottom Pressure from GRACE to Understand Transport Variations of the Antarctic Circumpolar Current
First Author: Jessica Makowski
Presenter: Jessica Makowski
Co-Authors: Don P Chambers, Jennifer A Bonin

Abstract: Previous tests using GRACE to measure the variability of barotropic transport of the Antarctic Circumpolar Current (ACC) have been based on differencing basin-scale averages of bottom pressure (BP) across the current and computing a very approximate scaling parameter. Here, we will compute transport variability using the full integral of bottom pressure derivatives between front positions for 15-30° longitude boxes. Results are first computed using a simulated data set that includes BP from JPL_ECCO, hydrology from GLDAS, and Antarctica and Greenland ice mass loss trends in order to study the effect of smoothing required to reduce noise in GRACE measurements. Preliminary results from the study are presented, along with some initial results using GRACE data.

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Session: B.4 - Oceanography
Title: The 2010 decline in global mean sea level and its relation to ENSO
First Author: Carmen Boening
Presenter: Carmen Boening
Co-Authors: Carmen Boening, Felix W. Landerer, R. Steven Nerem, Joshua Willis

Abstract: Besides a long-term trend identified in sea level records observed by satellite altimetry since the early 1990s, global mean sea level (GMSL) variability is dominated by interannual fluctuations of up to 5-7 mm in a single year. These variations are connected to net precipitation changes over land and oceans, related to the El Nino Southern Oscillation (ENSO). In late 2009, GMSL increased significantly, followed by a drop of about 6 mm from mid 2010 to mid 2011. GMSL change can be caused by steric contributions, which are induced by changes in density, and also by ocean mass changes, which are induced by the exchange of freshwater between the ocean and the continents. In this study, we examine the mechanisms that lead to these large interannual fluctuations by comparing the different contributions to GMSL and their relationship to ENSO as observed with GRACE, satellite altimetry and hydrographic profilers. Our study indicates that both mass and steric contributions play a role in causing the 2009-2011 anomaly, but the mass contributions dominate (~4 mm of the decrease in 2010/11). By studying past events, we find that the maximum in ocean mass lags the El Nino events by a few months indicating a delayed response in the global water cycle due to ENSO. Steric contributions are in phase with ENSO suggesting an immediate response in ocean warming/cooling.

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Session: B.4 - Oceanography
Title: Evaluation and interpretation of inter-annual variations in Arctic Ocean mass from GRACE using in situ observations and ocean models
First Author: Cecilia Peralta-Ferriz
Presenter: Cecilia Peralta-Ferriz
Co-Authors: J. Morison, R. Kwok, J. Wahr, J. Bonin, D. Chambers

Abstract: In the last few decades, the Arctic Ocean has observed drastic changes in climate variables, e.g., reduced sea-ice extent and thickness, heat and freshwater content and distribution, etc. These changes coupled to the circulation of the Arctic Ocean. Thus an accurate monitoring and interpretation of the changes in the Arctic Ocean circulation is essential to the understanding role of the Arctic in climate. Due to its global coverage and time resolution in the Arctic, GRACE represents a fundamental on-going dataset for Arctic Ocean mass variations and mass distribution. In order to improve our interpretation of the recent Arctic Ocean mass changes and their effects on the ocean circulation patterns, we evaluate the efficiency of GRACE and ocean models (OMCT, PIOMAS, and ECCO2) to accurately observe ocean mass changes and their spatial distribution. Preliminary inter-comparisons are shown, as well as comparisons with in situ observations from pressure and tide gauges. Trends in ocean bottom pressure (OBP), dynamic ocean topography (DOT) and steric pressure (StP = OBP-DOT) using GRACE OBP, DOT from ICESat, and OBP and DOT from the ocean models, reveal that at inter-annual timescales, the OBP is only a small fraction of DOT. This indicates that at inter-annual timescales, the deep Arctic Ocean tends to adjust baroclinically in agreement with Vinogradova et al. [2007] and Bingham and Hughes [2008]. Consequently, longer term changes in OBP are a fraction of corresponding DOT changes.

Bingham, R. J., and C. W. Hughes (2008), The relationship between sea-level and bottom pressure variability in an eddy permitting ocean model, Geophys. Res. Lett., 35, L03602.
Vinogradova, N., R. M. Ponte, and D. Stammer (2007), Relation between sea level and bottom pressure and the vertical dependence of oceanic variability, Geophys. Res. Lett., 113, L03608.

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