IMPACT OF ALTIMETER, THERMISTOR, AND EXPENDABLE BATHYTHERMOGRAPH DATAON RETROSPECTIVE ANALYSES OF THE TROPICAL PACIFIC-OCEAN

This study explores the relative impact of three major components of t he tropical Pacific Ocean observing system on data assimilation analys es. The extensive observing components that are available during the 2 -year period from October 1992 through September 1994 include sea leve l derived from TOPEX/POSEIDON altimetry, thermistor data from the Trop ical Ocean/Global Atmosphere Tropical Atmosphere-Ocean (TOGA TAO) moor ing array, and expendable bathythermographs (XBTs) from the Volunteer Observing Ship program. In the first part of this study, methods are i ntroduced to assimilate these data into a primitive equation model usi ng optimal interpolation. The resulting velocity and thermal analyses are then compared with a numerical simulation that has no data assimil ation. The most innovative aspect of the assimilation procedure is the development of a statistical model that uses satellite altimetry to u pdate the subsurface thermal field. To determine the impact of individ ual components of the observing system, three additional experiments a re conducted. In each experiment, one component is withheld from the a ssimilation. The resulting analysis is then compared with the missing data set. Our results show (1) To resolve major features of the season al cycle, it is necessary to have either the altimeter or the XBT data . In the absence of XBTs the analysis appears to develop a significant temporal drift in its thermal structure. (2) Intraseasonal variabilit y such as tropical instability waves is best resolved using altimetry because of that data set's superior spatial resolution. (3) Thermistor data from the TOGA TAO mooring array are helpful, but not as crucial, in resolving the seasonal cycle within +/-8 degrees latitude as the X BTs. The mooring array is too coarsely spaced to allow this analysis s ystem to resolve tropical instability waves.