FY 1995

Open-ocean validation of TOPEX/POSEIDON sea level in the western equatorial Pacific

Picaut, J.L., A.J. Busalacchi, M.J. McPhaden, E.J. Katz, L. Gourdeau, F.I. González, E.C. Hackert, and M. Bushnell

Océanographie Physique No. 5, Centre ORSTOM de Nouméa, Conventions Sciences de la Mer, 76 pp (1995)

During the verification phase of the TOPEX/POSEIDON radar altimeter mission a rigorous open-ocean validation experiment was conducted in the western equatorial Pacific Ocean. From August-September, 1992 to February-March, 1993 two TOGA-TAO moorings at 2°S-156°E (1739 m depth) and 2°S-164.4°E (4400 m depth) were outfitted with additional temperature, salinity, and pressure sensors to measure precisely the dynamic height from the surface to the bottom at 5 min intervals directly beneath two TOPEX/POSEIDON crossovers. Bottom pressure gauges and inverted echo sounders were deployed as well. A predeployment design study using full depth CTD casts, subsequently confirmed by post deployment analyses, indicated this suite of instruments was capable of measuring sea surface height fluctuations to within 1-2 cm. The validation experiment also benefited from the comprehensive set of ocean-atmosphere measurements that were made in the region during the TOGA-COARE Intensive Observation Period of November 1992-February 1993. The surface relative to bottom dynamic height fluctuations observed in situ during the 6-7 month experiment had a standard deviation of 5 cm with excursions of order ±15 cm. Energetic steric sea level variability was found to exist on short time scales of order hours to a few days, most notably the quasi permanence of strong semi-diurnal internal tides. Such internal tides were noted to induce changes in surface dynamic height with a standard deviation of 2 dyn cm. At the shallower of the two sites, 2°S-156°E, a possible nonlinear rectification of the internal tide was observed occasionally to change the dynamic height by as much as 30 cm over less than an hour. On time scales longer than the 10-day repeat of the TOPEX/POSEIDON satellite, the low-frequency fluctuations of dynamic height were related to interannual variations corresponding to the 1991-93 ENSO, to the seasonal cycle and to intraseasonal variations associated with the 40-60 day oscillations of the equatorial zonal wind field. Instantaneous comparisons between the 1-sec TOPEX/POSEIDON altimeter retrievals and the 5-min dynamic height were performed with regard to several tide models, the barotropic tide measured in situ, ECMWF surface air pressure, and the surface air pressure measured in situ. Depending on the choice of altimeter and of the environmental corrections applied to the altimeter data, the rms differences between the satellite and the in situ measurements of sea level were as low as 3.3 cm at 2°S-156°E and 3.7 cm at 2°S-164.4°E. When additional satellite data in the general vicinity of the mooring are included, and after the use of a 30-day low-pass filter, the satellite and in situ data were found to be highly correlated, with correlation coefficients of about 0.95 and rms differences around 1.8 cm.