GRACE Science Team Meeting :: Session B.4.1 to B.4.9

Seasonal Variability of the Red Sea
(J. Wahr, D. Smeed, E. Leuliette, S. Swenson)

Decadal Ocean Bottom Pressure Increase on the Sahul Shelf and its Contribution to Contemporary Regional Sea Level Rise
(R. M. Ponte; K. J. Quinn; C. Piecuch)

Understanding Oceanographic Contribution to Polar Motion
(S. Kwon, D.P. Chambers)

Tests of Seven Global Ocean Tide Models with GRACE Range-Rate Data
(R. D. Ray, S. B. Luthcke)

Posters

Consolidating the mass-budget in the Arctic Ocean from GRACE, satellite altimetry and other data
(O. Andersen, Y. Cheng, P. Knudsen)

Interannual variability in sea surface temperature and West Antarctic mass change
(Y. Firing; C. Boening; M. M. Watkins; D. Wiese)

Ocean tides in GRACE monthly averaged gravity fields.
(P. Knudsen, O. B. Andersen)

Non-seasonal fluctuations of the Arctic Ocean mass observed by GRACE
(D. Volkov, F. Landerer)

Title: GRACE Release-05 observed ocean variability at sub-annual and interannual periods and its relation to sea level
Presenter: Quinn, Katherine
Co-Authors: C. G. Piecuch; R. M. Ponte

Abstract: Knowing the relationship between ocean bottom pressure (OBP) and sea level (SL) is important for understanding the ocean response to atmospheric forcing and the partition between mass and steric contributions to SL, among other aspects of ocean variability. Previous theoretical and model-based studies of the relationship between OBP and SL suggest primarily barotropic variability at mid to high latitudes for scales greater than a few hundred kilometers and periods less than a few months. At interannual timescales it is generally assumed that SL changes are mostly steric. We use GRACE Release-05 solutions (weekly and monthly) combined with satellite altimetry to investigate the relationship between OBP and SL at large scales (>750 km) and for sub-annual and interannual periods. The observed OBP and SL fields are significantly coherent at mid to high latitudes for sub-monthly to semi-annual periods, providing global observational evidence for the barotropic nature of large-scale ocean variability at these spatial and temporal scales. We show broader regions of correspondence between OBP and SL using Release-05 rather than Release-04 data, demonstrating improvements in the GRACE data at high frequencies. Elevated interannual OBP signals are observed over deep extratropical regions (e.g., Southern Ocean basins) and shallow or semi-enclosed areas (e.g., Indonesian and Nordic seas). In these places, considerable interannual SL variance is explained by OBP variance. Correlation between SL and OBP is significant in many regions, including instances of significant negative correlation suggestive of active baroclinic processes. Results exemplify the good quality of GRACE Release-05 data at interannual timescales.

Title: Assimilation of GRACE data in a global, eddying, ocean and sea ice model
Presenter: Boening, Carmen
Co-Authors: C. Boening; D. Menemenlis

Abstract: We present preliminary results from the assimilation of 2004-2005 Gravity Recovery and Climate Experiment (GRACE) data in a global, eddying, ocean and sea ice model. For GRACE data, we used the Jet Propulsion Laboratory (JPL) mass concentration (mascon) solution. The numerical ocean model is the Massachusetts Institute of Technology general circulation model (MITgcm) as configured for ocean data assimilation by the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project. The adjoint method is used to adjust MITgcm initial and surface boundary conditions in order to reduce a weighted quadratic distance between simulation and observations, while satisfying ocean model equations exactly during the complete 2-year assimilation period.

Three ECCO2 solutions will be compared to the mascon solution: (1)iter00, a simulation that has not been constrained by ocean data; (2) iter30, a simulation that has been constrained by altimetry, sea surface temperature (SST), and vertical temperature and salinity (T/S) profiles; and (3) iter32, a simulation that also includes GRACE data constraints. With small regional exceptions, e.g., some eddy-rich regions in Southern Ocean and North Atlantic, iter00 explains 50% or more of GRACE data variance. The non-GRACE-data-constrained solution iter30 explains substantially more GRACE data variance than iter00, especially in Southern Ocean where iter30 explains up to 60% of the GRACE-iter00 residual variance. A notable exception is the South Atlantic, where, for yet-to-be-determined reasons, iter30 explains less GRACE variance than iter00.

At writing of abstract, two forward-adjoint iterations have been carried out based on iter30 as baseline simulation and including GRACE data constraints in addition to altimetry, SST, and T/S profiles. The resulting simulation, iter32, reduces the GRACE contribution to the cost function without increasing the cost of the preexisting model-data difference terms. These preliminary mode-data comparison and data assimilation results indicate the potential for GRACE mascon solutions to help separate steric from non-steric contributions to sea surface height in time-evolving estimates of the global ocean circulation.

Title: Modes of Variability and Trends in Arctic Ocean Bottom Pressure
Presenter: Morison, James
Co-Authors: C. Peralta-Ferriz; J.H. Morison

Abstract: We present an empirical orthogonal function (EOF) analysis of GRACE Arctic Ocean bottom pressure (OBP) and a study of interannual GRACE trends in OBP and dynamic ocean topography (DOT) and OBP trends in ice-ocean models. The EOF analysis of GRACE monthly OBP with seasonal cycle removed yields two modes that are particularly significant [Peralta-Ferriz, 2013]. The first is essentially a basin wide increase in mass forced by southerly winds in Fram and Bering straits. It is most important in the winter and is similar in form and causality to the sub-monthly variation of Peralta-Ferriz et al. [2011]. The second mode is characterized by mass increases on the Russian shelves and is related to forcing by the Arctic Oscillation (AO). The model-based interannual trend analysis confirms the results of Morison et al. [2012] that from 2005 to 2008, while the Canadian Basin became increasingly anticyclonic, nearly the whole rest of the Arctic Ocean became more cyclonic. Decreasing freshwater content in the cyclonic cell very nearly balanced freshening in the Canadian Basin. Model versus GRACE comparisons show that the models (PIOMAS and ECCO2) underestimate the OBP trends as observed by GRACE.

Title: Baroclinic Rossby Waves observed by GRACE in the northwestern Tropical Pacific
Presenter: Piecuch, Christopher
Co-Authors: C. Piecuch

Abstract: Ocean bottom pressure (OBP) behavior derived from Release-05 Gravity Recovery and Climate Experiment (GRACE) spherical harmonic coefficients is investigated along the northwestern Tropical Pacific for the case of interannual timescales. Variations in OBP are significantly anti-correlated to collocated fluctuations in sea surface height (SSH) observed by satellite radar altimeters. To interpret the OBP observations, a linear model of the OBP response to interior wind stress curl is used, comprising contributions from barotropic Sverdrup dynamics as well as baroclinic Rossby waves; model solutions are evaluated numerically using time-mean stratification from an ocean atlas and time-varying winds from an atmospheric reanalysis. Model and data compare favorably: simulated and observed time series are significantly correlated, and the model generally explains more than half of the data variance. The good correspondence between model and data speaks to the good quality of the GRACE-derived fields. Results constitute the first satellite-gravimetric documentation of baroclinic Rossby waves and confirm linear theoretical suggestions that baroclinic processes contribute importantly to OBP changes at low latitudes and low frequencies.

Title: Understanding Transport Variability of the Antarctic Circumpolar Current Using Ocean Bottom Pressure
Presenter: Makowski, Jessica
Co-Authors: J. Makowski; D.P. Chambers; J.A. Bonin

Abstract: Previous studies have suggested that ocean bottom pressure (OBP) can be used to measure the transport variability of the Antarctic Circumpolar Current (ACC). The OBP observations from the Gravity Recovery and Climate Experiment (GRACE) are used to calculate transport along the choke point between Antarctica and Australia. There has been some evidence to suggest that Southern Hemisphere winds and the Southern Annular Mode (SAM) or the Antarctic Oscillation (AAO) play a significant role in accelerating/decelerating ACC transport, along with some contribution from buoyancy forcing. We will examine whether average zonal wind stress, wind stress curl, local zonal winds, or the SAM are representative of the low frequency zonal mass transport variability. Preliminary studies suggest that seasonal variation in transport across the Australia-Antarctica choke point is driven by winds along and north of the northern front of the ACC, the Sub Tropical front (STF). It also appears that interannual variations in transport are related to wind variations centered south of the Sub Antarctic Front (SAF). We have observed a strong negative correlation/positive correlation across the STF of the ACC in the Indian Ocean, which suggests wind stress curl may also be responsible for transport variations.

Title: Seasonal Variability of the Red Sea
Presenter: Wahr, John
Co-Authors: J. Wahr; D. Smeed; E. Leuliette; S. Swenson

Abstract: Seasonal variability in mass and sea surface height within the Red Sea, occurs mostly through the exchange of heat with the atmosphere, evaporation from the ocean surface, and the inflow and outflow of water through the strait of Bab el Mandab that opens into the Gulf of Aden to the south. The effects of precipitation, of water exchange through the Suez Canal to the north, and of runoff from the adjacent land, are all small. Flow through the Bab el Mandab occurs through a multi-layer pattern, that includes a summer influx of cool water at intermediate (~100 m) depths. Thus, summer water in the southern Red Sea is warm near the surface (due to higher air temperatures) but cool at intermediate depths. Summer water in the northern Red Sea is warm at all depths. The temperature profile affects the water density, which impacts the sea surface height but has no effect on vertically integrated mass. This “steric effect” can be inferred by differencing altimeter and GRACE measurements. Here, we subtract GRACE estimates of mass variability, from altimeter (Jason-1, Jason-2, and Envisat) estimates of sea surface height variations, to obtain monthly estimates of the steric signal averaged over the northern Red Sea and southern Red Sea, separately. We compare those results with the steric signals estimated using sea surface temperature (SST) data from the Modis instrument aboard NASA’s Aqua satellite. We focus on the seasonal cycle. We obtain good agreement for the Northern Red Sea by assuming a mixed-layer depth of 73 meters when converting the SST results to a steric signal. For the Southern Red Sea the agreement is fair, but not as good. We find, though, that if we assume the seasonal steric signal leads the seasonal SST signal by one month, and we use a mixed-layer depth of 62 meters, we obtain agreement with the altimeter-minus-GRACE results for the Southern Red Sea that is as good as the agreement we obtain for the Northern Red Sea without assuming a time lag. At the moment, the reasons for this are unclear.

Title: Decadal Ocean Bottom Pressure Increase on the Sahul Shelf and its Contribution to Contemporary Regional Sea Level Rise
Presenter: Piecuch, Christopher
Co-Authors: R. M. Ponte; K. J. Quinn; C. Piecuch

Abstract: Satellite altimeter data indicate that sea level (SL) rose on the Sahul Shelf north of Australia at a rate of about 1.5+/-0.4 cm/yr over the 2003-2012 decade. To interpret the SL rise, we use contemporaneous time series of ocean bottom pressure (OBP) derived from Release-05 Gravity Recovery and Climate Experiment (GRACE) gravity coefficients processed by the Jet Propulsion Laboratory. Analysis reveals an important increase in OBP at a rate of 0.7+/-0.3 cm/yr, contributing about half of the observed rise in SL, but also hints at the importance of steric change. Assuming salinity change is not important, the residual steric change of 0.8+/-0.2 cm/yr corresponds to a depth-averaged temperature change of 0.4+/-0.1 degrees C/yr. Although paucity of hydrographic time series on the Sahul Shelf precludes direct comparison, comparable subsurface temperature changes were observed by the Argo profiling float array in the northwestern tropical Pacific Ocean over the 2004-2012 period. Further analysis suggests that the Sahul Shelf OBP increase is mostly remotely forced by a contemporaneous intensification of equatorial Pacific easterly winds. Results demonstrate that GRACE data can be useful for interpreting decadal change in some shallow coastal seas, where the relation between SL, OBP and steric height has been difficult to observe.

Title: Understanding Oceanographic Contribution to Polar Motion
Presenter: Kwon, Sarah
Co-Authors: S. Kwon; D.P.Chambers

Abstract: Many studies have shown that mass redistribution within the oceans and ocean currents are significant contributors to polar motion. Chambers and Willis (2009) have previously identified a significant low-frequency mass exchange between the Pacific and Indo-Atlantic Oceans. Here, we examine how much this large-scale mass exchange contributes to polar motion by using ocean bottom pressure data from a model for 1993 to 2011. We find that the interbasin exchange of mass explains nearly 53% of the y-component of polar motion driven by ocean mass variations, but less than 3% the x-component. On the other hand, redistribution of mass within the Pacific alone explains nearly 60% of the variance in the x-component driven by ocean mass variations. The remainder of the variance is explained by redistribution within the Indo-Atlantic Ocean. The motion component of polar motion based on currents from the same model was also calculated using only data from areas poleward of of 30°S to the contribution of the Antarctic Circumpolar Current on polar motion.

Title: Tests of Seven Global Ocean Tide Models with GRACE Range-Rate Data
Presenter: Ray, Richard
Co-Authors: R. D. Ray; S. B. Luthcke

Abstract: An international project, being led by Detlef Stammer (University of Hamburg), is currently performing a comprehensive assessment of state-of-the-art global ocean tide models. A wide variety of tests are being performed, including even tests of tidal currents with moored current meters and with acoustic tomographic arrays. The elevation tests include our long-wavelength tests presented in this talk based on GRACE range-rate data. For each of seven global ocean tide models, we have processed a multi-year set of GRACE range-rate data, using non-tidal forward models as compete as possible to remove all non-tidal energy. We then convert the range-rate residuals to range or range-acceleration residuals, bin the data globally, and perform tidal analyses on these binned data. Regions of significant tidal amplitudes indicate regions where the tide model is inadequate, although the results can be confused owing to (a) GRACE's limited spatial resolution and (b) the side lobes inherent in ranging residuals. Some models show surprising large residuals in M2 over the North Atlantic Ocean, caused (we suspect) by relatively small errors in elevation spread over basin scales. All models display large residuals in polar regions, although some are worse than others and some constituents are worse than others. These tests will be compared with some of the other model tests, such as standard tide-gauge assessments. No one model is best in every test.

Title: Consolidating the mass-budget in the Arctic Ocean from GRACE, satellite altimetry and other data
Presenter: Andersen, Ole
Co-Authors: O. Andersen; Y. Cheng; P. Knudsen

Abstract: The GRACE satellites have greatly improved our knowledge on mass variations in the Arctic Ocean. However, it is still challenging to validate GRACE findings and to study sea level changes and ocean circulation in the Arctic Ocean due to lack of other remote sensing observations in large parts of the Arctic Ocean, such as satellite altimetric data. Sea level variation in the Arctic Ocean plays an important role in the global climate system. Based the GRACE data (R05) and reprocessed Envisat altimetry data, we estimates the contributions of mass transport on sea level changes in the Arctic Ocean. Our studies demonstrate that the mass variations contributes 56% to the sea level changes of -1.6 mm/year in the regions 66°N-82°N over period 2003-2011. The ice sheet lost in Greenland causes the significant water-thickness reduction around Greenland. That is because that the horizontal gravitational pull from that region is weakened and water levels around the sources sink. On the other hand, the water mass accumulated along the Norwegian Coast and in the Arctic Ocean interior, such as the East Siberian and Beaufort Seas. Moreover, the other contributors, such as steric sea level variations responsible to the remained sea level changes of -0.7 mm/year in the Arctic Ocean.

Title: Interannual variability in sea surface temperature and West Antarctic mass change
Presenter: Firing, Yvonne
Co-Authors: Y. Firing; C. Boening; M. M. Watkins; D. Wiese

Abstract: Accelerating ice mass loss in West Antarctica has been measured since 2003 by the Gravity Recovery and Climate Experiment (GRACE), and may be contributed to by both atmospheric and oceanic forcing. The connections between West Antarctic melting and changes in local ocean temperature are examined using satellite observations of sea surface temperature and height. Interannual signals in sea surface temperature in the Amundsen and Bellingshausen Seas and mass loss from the adjacent glaciers and ice sheets are correlated; however, much of this signal is due to atmospheric circulation patterns and associated coastal precipitation anomalies.

Title: Ocean tides in GRACE monthly averaged gravity fields.
Presenter: Knudsen, Per
Co-Authors: P. Knudsen; O. B. Andersen

Abstract: The GRACE gravity field satellite mission maps the Earth's gravity fields and its variations with unprecedented accuracy during its 11-year lifetime. Unless ocean tide signals and their load upon the solid earth are removed from the GRACE data, their long period aliases obscure more subtle climate signals which GRACE aims at. In this analysis the results of Knudsen and Andersen [2002, 2007] have been revised using actual post-launch orbit parameter of the GRACE mission. Tidal errors may affect the GRACE data up to harmonic degree around 40 and they will not cancel in the GRACE monthly averaged temporal gravity fields. The S2 and the K2 terms have alias frequencies much longer than 30 days, so they remain almost unreduced in the monthly averages. In this analysis the tidal residuals are extracted using the CSR-RL05 monthly geoid variations from GRACE data over the period from January 2003 to September 2012 made available by UTCRS.

Title: Non-seasonal fluctuations of the Arctic Ocean mass observed by GRACE
Presenter: Landerer, Felix
Co-Authors: D. Volkov; F. Landerer

Abstract: Time variable gravity observations from the GRACE satellites reveal strong non-seasonal bottom pressure variability in the Arctic Ocean on 2 to 6 months time scales and a record-high bottom pressure anomaly in February of 2011. Here, we examine the nature and driving forces behind those variations. Our findings indicate that the non-seasonal variability of the Arctic Ocean mass is strongly coupled to wind forcing. The zonal wind pattern is correlated with a di-pole pattern of Arctic Ocean mass changes. Westerly wind intensification over the North Atlantic at about 60°N and over the Russian continental shelf break causes the ocean mass to decrease in the Nordic seas and in the central Arctic, and to increase over the Russian Arctic shelf. The time evolution of this pattern is significantly correlated with the Arctic Oscillation index. Basin-wide Arctic Ocean mass fluctuations are related to northward wind anomalies over the northeastern North Atlantic and Nordic seas, and over the Bering Sea. We show that positive (negative) Arctic Ocean mass anomalies are associated with anticyclonic (cyclonic) anomalies of the large-scale ocean circulation pattern. Based on an ocean model output, we conclude that the observed non-seasonal Arctic Ocean mass variability is mostly explained by the net horizontal wind driven transports, and the contribution of fresh water fluxes is negligible. We demonstrate that the net transport anomalies across the North Atlantic (Bering Strait) contributed about 3 cm (1 cm) to the record-high mass-related sea level anomaly in February 2011.

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