GRACE Science Team Meeting

Session: B.4 - Oceanography
(Convener: )

15:15-15:30 Global mean ocean mass changes from GRACE and its uncertainties
First Author: Liangjing Zhang
Co-Authors: H. Dobslaw, V. Klemann, M. Thomas
15:30-15:45 Improving global tide models using GRACE data
First Author: Gary Egbert
Co-Authors: R. D. Ray
15:45-16:00 Using ECCO2 to understand bottom pressure and AMOC variability
First Author: Jordan Meyer
Co-Authors: D. Chambers
16:00-16:15 Observing bottom currents associated with the Atlantic Meridional Overturning Current (AMOC) using GRACE
First Author: Don Chambers
Co-Authors: J. Meyer
16:15-16:30 Deep-ocean warming in the subtropical South Pacific over the last decade
First Author: Felix Landerer
Co-Authors: D Volkov, S-K Lee, R Lumpkin
16:30-16:45 Linking Oceanic Tsunamis and Geodetic Gravity Changes of Large Earthquakes
First Author: Yuning Fu
Co-Authors: Y.T. Song, R.S. Gross
POSTER Direct Calculation of Relative Sea Level Fingerprints from GRACE Mascon Solutions
First Author: Benjamin Krichman
POSTER Accounting for gravitational and loading effects of land ice on absolute sea level?
First Author: Rui Ponte
Co-Authors: K.J. Quinn

Title: Global mean ocean mass changes from GRACE and its uncertainties
Presenter: Zhang, Liangjing
Co-Authors: H. Dobslaw, V. Klemann, M. Thomas

Abstract: Global mean sea level is an important indicator of climate change, and its medium- to long-term prediction is important for many economic and societal decisions. However, large differences are found between the ocean mass component of the sea level budget from previous analysis due to the different data, post-processing methods, and time spans applied. In our recent efforts at GFZ to post-process GRACE Stokes coefficients into gridded Level-3 surface mass anomaly estimates, the barystatic sea level change is estimated together with geocenter variations through an iterative method (Bergmann et. al. 2014) for the period from April 2002 to September 2016, resulting in an average mass-induced sea-level rise of 1.93 mm/y.

We analyze uncertainties arising from geocenter variations (0.16 mm/y), C20 replacement (0.18 mm/y), glacial isostatic adjustment (GIA: 1.24 mm/y) and different spatial smoothing and leakage corrections (0.13 mm/y), thereby revealing the importance of an accurate GIA model for the sea-level estimate. Further, the global mass budget is closed by considering GRACE-based mass changes over the continents, that need to correspond additive-inversely to the changes in ocean mass. Results are further compared with estimates from other GRACE releases including the latest JPL RL05M data. Good agreement is found between the land contributions to sea level change, while the directly calculated barystatic sea level change is found to be larger than that from JPL RL05M, especially after the year 2012.

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Title: Improving global tide models using GRACE data
Presenter: Egbert, Gary
Co-Authors: R. D. Ray

Abstract: We will discuss our ongoing efforts to use GRACE data to improve global tide models, primarily at high latitudes. Estimates of errors in the GOT4.7 model, parameterized in spherical harmonics up to degree and order 90, have been estimated from GRACE Level-1B data. The resulting estimates can already be used to correct tide models used for orbit calculations, and have been made available to the mission. However, to be useful for most all other applications, these corrections need to be incorporated into a modern high resolution tidal model. We are using the OSU Tidal Inversion Software (OTIS) to assimilate the GRACE results, initially to derive a correction to our current global tide model, TPXO.8. In future we plan to experiment with simultaneous assimilation of multi-mission altimetry and GRACE tide solutions. The principal modifications of OTIS required for this work include implementation of representer calculations for observation functionals corresponding to spherical harmonic coefficients, and characterization of data errors. We will outline our approach, and discuss progress towards initial inversion results.

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Title: Using ECCO2 to understand bottom pressure and AMOC variability
Presenter: Meyer, Jordan
Co-Authors: D. Chambers

Abstract: Inter-ocean transports are of fundamental interest to physical oceanography and ocean climate considerations, but are poorly understood and difficult to measure because long-term direct measurements of strait circulations are an expensive alternative and their implementation remains logistically challenging. We hypothesize that the magnitude and variability of inter-ocean transports vary with sea-surface height (SSH) and ocean bottom pressure (OBP) gradients between two inter-connected oceans.

The Atlantic Meridional Overturning Circulation (AMOC) is an important component of the Earth’s climate system and variability in its magnitude can have large effects on the transport of heat to the northern hemisphere. A few efforts are in place to monitor this circulation, the longest running being the RAPID array of moorings at 26.5°N. Remotely sensed ocean bottom pressure data from the Gravity Recovery and Climate Experiment (GRACE) and the GRACE-Followon mission may provide a way to measure the deep currents associated with the AMOC along the entire western side of the Atlantic. This could allow us to better understand how representative AMOC variability measured at one latitude is for the entire basin. To better understand the relationship between bottom pressure and AMOC transport, we use ocean bottom pressure and velocity data from the second Estimating the Circulation and Climate of the Ocean (ECCO2) state estimate run at Jet Propulsion Laboratory. We have calculated transport of the current above and below 1000m using monthly averaged meridional velocity estimates from ECCO2 at 26.5°N and compared those to the transports calculated by the RAPID array, finding reasonable agreement. We have furthermore examined the transport in ECCO2 at all latitudes in the Atlantic and performed a principal component analysis to quantify the leading mode of variability and test if it is coherent at all latitudes. Finally, we have examined the cross-correlation between the transport calculated with velocity estimates and ocean bottom pressure anomalies to understand the pattern and spatial extent of pressure changes, to see if they are recoverable with new GRACE mascon solutions.

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Title: Observing bottom currents associated with the Atlantic Meridional Overturning Current (AMOC) using GRACE
Presenter: Chambers, Don
Co-Authors:J. Meyer

Abstract: Utilizing yearly-averaged bottom pressure anomaly mascon maps from both the Center for Space Research and the Jet Propulsion Laboratory, we investigate meridional bottom currents along the continental slope in the western Atlantic. This is different than the method used by Landerer et al (2015), which examined bottom pressure gradients across the Atlantic basin. We show that method is corrupted by long-wavelength anti-correlated trends in GRACE-observed bottom pressure in the western and eastern half of the basins, leading to a trend in deep transport in the opposite direction than is observed by an in situ array. Computing local bottom pressure gradients (i.e., geostrophic bottom currents) minimizes the effect of this long-wavelength signal. We find variability in the local bottom currents along the continental shelf north of the Gulf Stream separation that is consistent with the observed upper geostrophic transport at 26.5°N, including a similar trend -- indicating a slow down of the upper limb of the AMOC. We also find anti-correlated bottom currents east of the shelf in water deeper than 4000 meters that is consistent with the inferred slow-down of the North Atlantic Deep Water.

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Title: Deep-ocean warming in the subtropical South Pacific over the last decade
Presenter: Landerer, Felix
Co-Authors: D Volkov, S-K Lee, R Lumpkin

Abstract: The persistent energy imbalance at the top of the atmosphere, inferred from satellite measurements, indicates that the Earth’s climate system continues to accumulate excess heat. As only sparse and irregular measurements of ocean heat below 2000m depth exist, one of the most challenging questions in global climate change studies is whether the excess heat has already penetrated into the deep ocean. Here we report on a comprehensive analysis of satellite (altimetry, gravimetry) and in situ measurements, and find that a significant deep-ocean warming occurred in the subtropical South Pacific Ocean over the past decade (2005–2014). The local accumulation of heat accounted for up to a quarter of the global ocean heat increase for that time period, with an inferred deep ocean (below 2000 m) contribution of up to 10%. We further demonstrate that this heat accumulation is consistent with a decade-long intensification of the subtropical convergence, possibly linked to the persistent La Niña-like state.

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Title: Linking Oceanic Tsunamis and Geodetic Gravity Changes of Large Earthquakes
Presenter: Fu, Yuning
Co-Authors: Y.T. Song, R.S. Gross

Abstract: Large earthquakes at subduction zones usually generate tsunamis and coseismic gravity changes. These two independent oceanic and geodetic signatures of earthquakes can be observed individually by modern geophysical observational networks. The Gravity Recovery and Climate Experiment (GRACE) twin-satellites can detect gravity changes induced by large earthquakes, while altimetry satellites and Deep-Ocean Assessment and Reporting of Tsunamis (DART) buoys can observe resultant tsunamis. In this study, we introduce a method to connect the oceanic tsunami measurements with the geodetic gravity observations, and apply it to the 2004 Sumatra Mw 9.2 earthquake, the 2010 Maule Mw 8.8 earthquake and the 2011 Tohoku Mw 9.0 Earthquake. Our results indicate consistent agreement between these two independent measurements. Since seafloor displacement is still the largest puzzle in assessing tsunami hazards and its formation mechanism, our study demonstrates a new approach to utilizing these two kinds of measurements for better understanding of large earthquakes and tsunamis.

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Poster Title: Direct Calculation of Relative Sea Level Fingerprints from GRACE Mascon Solutions
Presenter: Krichman, Benjamin

Abstract: Relative sea level (RSL) refers to the sea level as measured by a tide gauge or ocean bottom pressure recorder, rather than the absolute sea level measured from the geocenter. The RSL change resulting from mass exchange between the oceans and continental sources is not evenly distributed across the world ocean due to self-attraction and loading effects. These spatial variations in RSL changes are referred to as the RSL fingerprints. The fingerprints can be thought of as a weighing factor that relates the local RSL to the mean ocean RSL. RSL fingerprints are generally calculated by solving the sea level equation with model or land data inputs. Here, an attempt is made to calculate RSL fingerprints directly as a ratio of local behavior to the mean ocean, using GRACE mascon solutions. Specifically, the fingerprints corresponding to the annual mass cycle of the ocean are calculated directly and compared to previous work.

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Poster Title: Accounting for gravitational and loading effects of land ice on absolute sea level?
Presenter: Ponte, Rui
Co-Authors: K.J. Quinn

Abstract: Gravitational attraction and loading (GAL) effects associated with ongoing long-term changes in land ice are expected to cause spatially varying trends in absolute sea level (ASL), as measured by satellite altimeters. The largest spatial gradients in ASL trends, predicted from solving the Sea Level Equation using GRACE retrievals of mass distribution over land for the period 2005-15, occur near Greenland and West Antarctica, consistent with a strong local land ice loss. Misinterpreting the estimated static GAL trends in ASL as dynamic pressure gradients can lead to errors in large-scale geostrophic transports of order 10 Sv (1 Sv=1,000,000 cubic meters per second) across the Southern Ocean and the subpolar North Atlantic over the analyzed decade. South of Greenland, where altimeter sea level and hydrography (Argo) data coverage is good, the residual ASL minus steric height trends are similar in magnitude and sign to the gravitationally based predictions. In addition, estimated GAL-related trends are as large, if not larger than other factors such as deep steric height, dynamic bottom pressure, and glacial isostatic rebound. Thus, using GRACE to estimate and account for static GAL effects on ASL records, commonly neglected in oceanographic studies, seems important for quantitative interpretation of the observed ASL trends.

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