TEC data from nearly 10 years of the TOPEX/Poseidon mission were analyzed to study the TEC climatology. First, the TEC data were binned by season, geomagnetic activity, and solar activity to create longitudinally averaged TEC maps in magnetic latitude and local time. The equatorial anomaly is the most prominent feature in these maps and they show strong TEC variations with solar activity but relatively weak variations with geomagnetic activity in our three Kp bins. These maps also show the annual and semiannual anomalies, but lack the seasonal anomaly. As a final binning, three longitudinal bins (Indian, Pacific, and Atlantic) were added. The TEC measurements display strong longitudinal variations that closely follow the longitudinal variation of the magnetic declination. As a continuation of the study on the TEC measurements, a comprehensive comparison of the TOPEX TEC measurements with the recent version of the International Reference Ionosphere (IRI-2001) was performed. First, it was found that both the IRI and TOPEX TEC show a negligibly small geomagnetic dependency, regardless of the solar activity and seasonal conditions. For solar activity, however, not only the TECs from the IRI and TOPEX measurements, but also the difference between them, strongly depend on the solar activity. The comparison also shows that the daytime low-latitude ionosphere from the IRI always develops earlier than the corresponding TOPEX measurements. With respect to the longitudinal variations of TEC, the IRI TEC show good agreement with the TOPEX measurements for low solar activity, but for high solar activity, large discrepancies occur in the Pacific sector. To better understand the observed TEC variations and to better guide its modeling, we have studied the sensitivity of quiet-time TEC to key atmospheric and ionospheric parameters, such as the neutral densities, neutral wind, neutral and plasma temperatures, plasmaspheric flux, and the O+-O collision frequency. For this study, we have developed a numerical midlatitude ionospheric model, which calculates the O+, H+ densities, and TEC. Our study shows that during the day the neutral wind and the neutral composition have the most important effect on TEC. During the night, most of the parameters can play a significant role in the TEC morphology, except for the plasma temperature. Finally, the TEC varies roughly linearly with respect to all of the parameters except for the neutral wind.