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aer 



Atmospheric and 
Environmental Research, Inc. 



Establishing an lERS Sub-Center for Ocean Angular Momentum 

Principal Investigator: RUI M. PONTE 



FINAL REPORT CONTRACT NO. NAS5-98182 



Submitted to: 

NASA Goddard Space Flight Center 

Mail Code 219 

Greenbelt, MD 20771 



Submitted by: 

AtmoHpheric and Environmental Research, Inc, 

131 Hartwell Avenue 

Lexington, MA 02421-3126 USA 

Phone: (781)761-2288 



April 2, 2001 



FINAL REPORT 

The project entitled "Establishing an lERS Sub-Center for Ocean Angular Momen- 
tum" came to existence at AER on September 21, 1998, under the direction of the Principal 
Investigator R.M. Ponte, and lasted for two and a half years with a total funding of $97,800. 
The focus of the project was to establish a new archival center for ocean angular momen- 
tum (0AM) products — the Special Bureau for the Oceans (SBO) — as part of a broader 
effort by the International Earth Rotation Service (lERS) to create a Global Geophysical 
Fluids Center (GGFC) dealing with all fluids affecting the rotation of the Earth. This 
report summarizes activities carried out under the project. 

To get the SBO off the ground, major efforts were directed at calculating and analyz- 
ing 0AM quantities from several ocean models. Particular attention was given to 0AM 
time series computed from 11-|- years of output from the MIT primitive equation model 
described in Ponte et al. (1998, Nature, 391, p. 476). Detailed analyses of 0AM variability 
in relation to regional changes in oceanic currents and mass field and to Earth rotation 
parameters were performed. Results on the axial 0AM component were written up as a 
paper entitled "Global and regional axial ocean angular momentum signals and length-of- 
day variations (1985-1996)", co-authored with D. Stammer. The paper appeared in the 
July 15, 2000 issue of the Journal of Geophysical Research-Oceans. A reprint is appended 
to this report. As part of the project, we completed the process of preparing, documenting, 
and archiving the MIT model 0AM series (all its three components about the polar and 
equatorial axes). The dataset is now publicly available at SBO. 

Another major effort was dedicated to the analysis of an 0AM dataset obtained from 
an MIT model run that was constrained by a number of available global ocean observations 
(sea level from altimetry, monthly mean hydrography, and sea surface temperatures). The 
potential improvements on OAM values provided by the data constraints were addressed in 
detail. Results were written up in a paper entitled "Improving ocean angular momentum 
estimates using a model constrained by data" co-authored with D. Stammer and C. Wun- 
sch. The paper is now in press at Geophysical Research Letters. A reprint is appended to 
this report. An invited talk focusing on these results was also prepared and delivered by 
the PI at the EGS General Assembly held in Nice in April 2000. These OAM series are 
under consideration for potential archival at SBO. 

To be able to provide OAM series spanning at least several decades, feasibility checks 
on a number of available ocean products were carried out. A 50-year ocean analysis product 
recently developed at the University of Maryland by James Carton and collaborators was 
chosen based on those checks. Contact with the relevant investigators was established and 
appropriate data was obtained. Software to calculate OAM quantities from the ocean fields 
was developed and tested. Extensive analyses of the monthly time series in comparison with 



other 0AM products and in the context of the Earth's rotation budget were performed. 
The 50-year long series reveals the importance of 0AM for polar motion excitation at 
interannual periods. Work towards a paper documenting the new data set and analysis 
was initiated. Release of the new dataset to SBO archives is envisioned in the near future. 

In addition to long 0AM time series, the need for high temporal resolution products, 
important in studies of rapid rotation signals, was also considered. Calculation of 6- 
hourly 0AM values was pursued using the barotropic model of Ponte (1993, Dynamics of 
Atmosphere and Oceans, 18, p. 209), which accounts for both wind and pressure forcing 
efiFects. Preliminary results were obtained from a model run with optimized parameters, 
as judged from comparisons with altimeter sea level measurements. Six-hourly time series 
beginning in 1992 have been created and plans were laid out to analyze and test their 
overall quality. 

On other miscellaneous activities, the PI began examining the possible calculation of 
torque quantities representing exchanges of 0AM with the solid Earth and atmosphere. A 
number of ocean models were explored with regard to their formulation of the momentum 
equations, and conservation equations for the three 0AM components consistent with the 
model approximations were derived. Work towards developing the best procedures and 
software to compute the torques for future MIT model runs was carried out. 

The PI participated in a number of meetings directly related to SBO activities. These 
meetings included the lERS Workshop that took place in October 1998, in Potsdam, Ger- 
many, where the PI presented an invited talk entitled "Importance of ocean signals in the 
excitation of polar motion" on the session dedicated to GGFC. The PI also participated in 
informal SBO meetings that took place in October 1998 at Keystone, Colorado, and Oc- 
tober 1999 at St. Raphael, France, on the occasion of TOPEX/Poseidon-Jasonl meetings, 
and in April 2000 in Nice, France, on the occasion of the EGS General Assembly, where 
SBO activities were discussed and planned. 

Throughout the project, the PI was involved in exchanges with other investigators 
to define the scope of SBO activities and products, which led to the preparation of a 
document entitled "The lERS Special Bureau for the Oceans", by Gross et al. This note 
will be published as part of an lERS Technical Report and serves as an introduction of 
SBO to the geophysical community. The PI also helped in setting up the SBO web page, 
hosted at the Jet Propulsion Laboratory and available for use by all interested investigators 
at http://euler.jpl.nasa.gov/sbo/. 



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1, AGENCY USE ONLY (Leave blank) 



2. REPORT DATE 
April 2, 2001 



REPORT TYPE AND DATES COVERED 
Final, 9/21/98-3/20/01 



TITLE AND SUBTITLE 

Establishing an lERS Sub-Center for Ocean Angular Momentum 



6, AUTHORS 



Rul M, Ponte 



5, FUNDING NUMBERS 



C NAS5-98182 



7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

Atmosptieric and Environmental Research, Inc, 
131 Hartwell Avenue 
Lexington, MA 02421-3126 



8, PERFORMING ORGANIZATION 
REPORT NUMBER 



9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 

NASA Goddard Space Flight Center 
Mail Code 219 
Greenbelt, MD 02771 



10. SPONSORING/MONITORING AGENCY 
REPORT NUMBER 



1 1 . SUPPLEMENTARY NOTES 



Preprints appended 



12a, DISTRIBUTION/AVAILABILITY STATEMENT 



12b. DISTRIBUTION CODE 



13. ABSTRACT (Maximum 200 words) 

With the objective of establishing the Special Bureau for the Oceans (SBO), a new archival center for ocean angular momentum (OAM) 
products, we have computed and analyzed a number of OAM products from several ocean models, with and without data assimilation. 
All three components of OAM (axial term related to length of day variations and equatorial terns related to polar motion) have been 
examined in detail, in comparison to the respective Earth rotation parameters. An 1 1+ year time series of OAM given at 5-day intervals 
has been made publicly available. Other OAM products spanning longer periods and with higher temporal resolution, as well as products 
calculated from ocean model/data assimilation systems, have been prepared and should become part of the SBO archives In the near 
future. 



14. SUBJECT TERMS 



Ocean angular momentum, ocean mass and currents. Earth rotation 



15. NUMBER OF PAGES 
3 



16. PRICE CODE 



17. SECURITY CLASSIFICATION 
OF REPORT 

Unclassified 



18. SECURITY CLASSIFICATION 
OF THIS PAGE 

Unclassified 



19. SECURITY CLASSIFICATION 
OF ABSTRACT 

Unclassified 



20. LIMITATION OF ABSTRACT 



NSN 7540-01-280-5500 



Computer Generated 



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