For the TOPEX/POSEIDON (T/P) Extended Mission (TPEM) we proposed studies in two general areas. First, we are examining tropical Pacific mass and heat balances based on descriptions we are making of tropical geostrophic and Ekman current variations, which are coordinated with the analysis of in situ data and numerical model studies. Second, we are studying the interaction of the tropical Pacific Ocean with the subtropics. Specifically, we will: • examine the extended period of anomalous

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... s in the Pacific during the 1990's, • explore tropical processes that may lead to covariations between the subtropical gyres in the northern and southern hemispheres, • study the interaction of mesoscale eddies with the North Equatorial Current (NEC). One focus of our TPEM proposal was understanding the connection of tropical Pacific mass and heat transports with interannual variability, work which was begun during the primary T/P mission. During the primary mission, we focused on the relationship of the Pacific to Indian Ocean throughflow (ITF) to the heat content of the western Pacific warm pool, and we showed [Lukas et al., 1995; Potemra et al., 1996] that throughflow monitoring by altimetric sea level is feasible. We realized during the primary T/P mission, however, that we would have to be careful in choosing a reference surface for the T/P data before attempting to analyze low frequency anomalies. During the TPEM we have used in situ sea level data to estimate the magnitude of these errors, and we are also attempting to determine a temporal subset of the entire T/P time series that could be used as an improved reference period [Mitchum et al., 1997]. Briefly, we used 17 sea level gauges in the western tropical Pacific that had time series at least 20 years in length. These sea level time series were referenced to the 20-year mean, and used to estimate the errors that would have occurred if only the last N years of data were available to make the average (Figure 1). We also found that using data only from the last two quarters of the calendar year, rather than the entire year, significantly reduced the errors. We are continuing this work with sea surface heights from numerical models to better study the spatial patterns of the errors.