Boebel, Olaf

Flechtner, F.

Kanzow, T.

Karstensen, J.

Macrander, Annecke

Schmidt, Roland

Schröter, Jens

English

Macrander, A.

Schmidt, R.

Boebel, O.

Schröter, J.

Karstensen, Johannes

OceanRep

Oceanic mass variability observed by bottom pressure sensors and GRACE satellites

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a)b)3528211470-7-14-21-28Geostrophic transport equivalent / SvOceanic Mass Variability Observed by Bottom Pressure Sensors and GRACE SatellitesA. Macrander (1,*), T. Kanzow (2), F. Flechtner (3), R. Schmidt (3), O. Boebel (1), J. Schröter (1) and J. Karstensen (4)(1) Alfred-Wegener Institut für Polar- und Meeresforschung, Bremerhaven, Germany, (2) National Oceanography Centre, Southampton, Great Britain,(3) GeoForschungsZentrum (GFZ) Potsdam, Germany, (4) Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR), Kiel, Germany.* amacrander@awi-bremerhaven.de, fax: +49 471 4831 17976. Future Aims• Extended measurements of OBP in the South Atlantic(ACC array) and North Atlantic (MOVE at 16°N, FramStrait at 79°N) by AWI• Cooperation with projects observing OBP at otherlocations (MOVE, RAPID and others)• Establishing a global data base of OBP measurements• Validation of GRACE gravimetry data using all availableground truth sites of OBP measurements to improveGRACE estimates of oceanic mass flux variability5. ConclusionsResultsOcean bottom pressure (OBP) timeseries at 26°N (RAPID),16°N (MOVE) and 44-50°S (ACC array) have been evaluated:• In the tropical North Atlantic, the observed monthly OBPvariability is about <= 0.01 dbar. In the Antarctic CircumpolarCurrent, OBP variability is larger (0.04 dbar).• The performance of GRACE to capture oceanic massvariability varies greatly between different ground truth sites.In the tropical North Atlantic, GRACE shows unrealisticallylarge signal amplitudes, whereas in the ACC region, GRACEhas a much better skill to observe OBP variability. Further research• Validation of GRACE data at various OBP ground-truth sites• Determine the reasons for the GRACE deficiencies in thetropical North Atlantic and better skills in the ACC region• Investigation of ACC array PIES data, altimetry and GRACEwith respect to mass transport and heat content variability ofthe ACC• Validation/ Comparison with Ocean Models4. Antarctic Circumpolar Current (ACC)In mid and high latitudes, OBP variability islarger than in the tropics due to the morebarotropic structure of ocean currents, anda larger Coriolis parameter. In the ACC,GRACE may hence serve better to detectoceanic mass flux variability.Here, data from two PIES deployed from2002 to 2005 are shown, covering thenorthern part of the ACC (Fig. 4).In 2006, the array is to be extended to6 - 9 PIES, improving integratedgeostrophic estimates of the ACC andensuring a 2-dimensional coverage oflarge-scale coherent OBP signalsobserved by GRACE.Stiftung Alfred-Wegener-Institutfür Polar- und Meeresforschungin der Helmholtz-Gemeinschaft1. IntroductionGravity field products of the Gravity Recovery and ClimateExperiment (GRACE) satellite mission are assessed to estimate thecapability of space-borne gravity measurements to detect thetemporal variability of the oceanic mass distribution and its currents.Here, GRACE data is validated against in-situ measurements ofocean bottom pressure (OBP) at three different sites:• RAPID (RAPID Climate Change) array at 26°N in the tropical NorthAtlantic• MOVE (Meridional Overturning Variability Experiment) array at 16°Nin the tropical North Atlantic• ACC array at 44°S - 50°S in the Antarctic Circumpolar CurrentFig. 5 a) Monthly averages of OBP in ACC array. Red: Northern site at 44°S 8°E, blue:Southern site at 50°S 1°E. Solid lines: In-situ observations, dashed lines: GRACE.b) OBP differences between both sites. Solid blue line: In-situ, dashed red line: GRACE.• OBP variability 0.04 dbar on timescales of 3 months to1 year (Fig. 5 a)• GRACE variability amplitude similar to in-situobservations. Significant correlation of 0.45 at northernsite, 0.68 at southern site (Fig. 5 a)• GRACE shows some capability to determine OBPdifferences between both sites (correlation to in-situobservations 0.33, Fig. 5 b)• Independend validation of numerical models: Both in-situand GRACE OBP differences yield a barotropic transportRMS variability of 11 Sv (Fig. 5b). This is consistent withthe FESOM model transport variability range. The modelmean transport across the array is 40 +/- 4 Sv, total ACCtransport 135 Sv (Fig. 4).Fig. 4: ACC in FESOM ocean model [see Böning, 2006]. Transportstreamfunction (colour shading) and u*h  transport vectors (monthlymean Jan 2005). In the northern part of the ACC,  PIES deployed2002-2005 (see Fig. 5).  further present deployments,  plannedarray extension.  hydrographic moorings in sea-ice covered regions.3. MOVE and RAPIDThe MOVE and RAPID mooring arrays(Fig. 1) in the tropical North Atlanticmonitor the Meridional OverturningCirculation (MOC) by means ofintegrating geostrophic measurements.A number of PIES and pressure gaugesprovides timeseries of ocean bottompressure (OBP) allowing ground-truthvalidation of GRACE OBP.Fig. 2 a) MOVE OBP timeseries (blue)and GRACE OBP RL01 product (red).b) OBP differences and correspondinggeostrophic current velocity anomalies.2. DataGRACE satellite data:Here, de-tided and de-aliased monthly averaged GRACE GSM+GAC RL03 productsprovided by GFZ Potsdam [see Flechtner, 2006] are used as GRACE OBP estimate.A spatial Gaussian filter of 1000 km (MOVE, RAPID) or 500 km (ACC) has been appliedfor smoothing of smaller scale variability in the GRACE gravity field solutions. Spatialdistribution of short-term variability is shown in Fig. 1.In-situ data:MOVE and ACC arrays: Pressure sensors / Inverted Echo Sounder (PIES, manufacturedby University of Rhode Island). RAPID array: SeaBird Seagauges SBE26. All instrumentsare deployed at the sea floor and use the same type of pressure sensor. For comparisonwith GRACE, de-trended timeseries of monthly averages are used.Fig. 1: Standard deviation in mbar of OBP inferred from GFZ monthly gravity field solutions. Note, that the largest variability of gravityis observed over land (hydrological cycle). The smallest amplitudes are found over the ocean in low latitudes. OBP ground truth sites: OBP sensor timeseries shown here;  Other OBP sensors currently deployed;  planned OBP sensors/ PIES deployments.Fig. 3 a) RAPID in-situ OBP (blue)and GRACE OBP RL03 product (red)at mooring site MAR2. b) OBPdifferences MAR2-EB1.• Monthly OBP variability of 0.01 dbar observed inMOVE and RAPID moorings (Figs. 2a and 3a)• GRACE data show a seasonal cycle of 0.04 dbar(Figs. 2a and 3a)• OBP differences also greatly overestimated byGRACE (Figs. 2b and 3b)• Reasons for low performance of GRACE to observeoceanic variability still unclearAcknowledgements“Improved GRACE Level-1 and Level-2 Products and their Validation by Ocean Bottom Pressure” is funded by Bundesministerium fürBildung und Forschung (BMBF) and Deutsche Forschungsgemeinschaft (DFG) in the framework of the the joint Geotechnologien project.Participating institutions are GFZ Potsdam, TU München, Uni. Bonn, AWI Bremerhaven, TU Dresden.Workpackage WP 400 “Evaluation of GRACE Gravity Fields by Ocean Bottom Pressure” is carried out at Alfred-Wegener-Institut fürPolar- und Meeresforschung (AWI), Bremerhaven, Germany.OBP data provided by IFM-GEOMAR Kiel (MOVE), NOC Southampton (RAPID), AWI Bremerhaven (Fram Strait and ACC arrays).Related contributions to EGU 2006:Böning, C., Timmermann, R., Danilov, S., Schröter, J., Boebel, O. (2006): A global finite element ocean model: Circulation andbottom pressure anomalies in the South Atlantic, EGU General Assembly 2006, Abstract EGU06-EGU06-A-04139, Vienna,Austria, 02-07 April 2006. Poster XY0717 on Thursday, 06 April 2006. Author in attendance: 13:30 - 15:00.Flechtner,F., Koenig, R., Meyer, Ul., Neumayer, K.H., Rothacher, M., Schmidt, R. (2006): Determination of the Earth's GravityField from CHAMP, GRACE and LAGEOS at GFZ Potsdam, EGU General Assembly 2006, Abstract EGU06-A-05902,Vienna, Austria, 02-07 April 2006. Oral presentation on Monday, 02 April 2006 11:30, Lecture Room 6 (K).References:Kanzow, T., F. Flechtner, A. Chave, R. Schmidt, P. Schwintzer, andU. Send (2005), Seasonal variation of ocean bottom pressurederived from Gravity Recovery and Climate Experiment (GRACE):Local validation and global patterns, J. Geophys. Res.,110, C09001,doi:10.1029/2004JC002772.EGU General Assembly, Wien, Austria, 5 April 2006, 17:30 - 19:00, XY0753Weblinks:GRACE: http://www.gfz-potsdam.de/grace/MOVE: http://www.ifm.uni-kiel.de/allgemein/research/projects/clivar/send/clivar.htmlRAPID: http://www.noc.soton.ac.uk/rapid/rapid.php“Improved GRACE Level-1 and Level-2 Products and their Validation by Ocean BottomPressure”:  http://www.geotechnologien.de/forschung/forsch2.2l.html (German only).a)b)Locations M3, M1, MAR2 and EB1 indicated on Fig. 1.a)b)RAPIDMOVEACCMOVERAPIDFram StraitM1M3MAR2OBP standard deviation / mbarEB1

Oceanic Mass Variability Observed by Bottom Pressure Sensors and GRACE Satellites

Oceanic Mass Variability Observed by Bottom Pressure Sensors and GRACE Satellites

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