The NVAP data-set does contain some interannual variability of the order 1-1.5mm on the global average. There is also a strong annual cycle driven by the variability of the Northern Hemisphere. Included in this topic is a plot of global averages of total PWC for the 5 NVAP years 1988 - 1992. The year 1988 tends to be the upper bound of the envelope and 1992 the lower bound. In the next 2 examples we examine the causes of the interannual variability of PWC. The hemispheric NVAP averages show that the Northern Hemisphere's annual cycle and interannual variability is greater than the Southern Hemipshere.
1988 and 1992 were much different years globally. 1988 was the strongest La Niña on record and was the year of the Mid-west USA drought. 1992 was a moderate El Niño year. The difference plot of the total PWC for 1988 - 1992 shows higher PWC in 1988 with the exception of the El Niño region in the Central Pacific.
The NVAP project has produced a stable 5 year data-set of global total PWC ( or water vapor). Anomalies produced show the global and hemispheric total water to be quite variable. We noticed that the years 1988 and 1992 were very different in the global average. To investigate whether this was indeed an accurate difference we examined the lower tropospheric temperature anomalies from Christy and Spencer (1995). These results showed that 1988 (a strong La Niña year) was much warmer globally than average and that 1992 ( a mild El Niño year) was much cooler globally. This relationship is shown as a comparison of the NVAP and lower tropospheric temperature anomalies. If the global temperature anomalies were warmer than normal it would make sense that the water vapor anomalies were also above average. There are some differences between the hemispheres. During 1991 when the temp/PWC anomaly relationship breaks down we can see that the N. Hemisphere anomalies continued to track together but the S. Hemisphere diverged. A plot of just the Global Anomalies without the hemispheres is also available.
