Until 2010, the longest globally-complete estimate of the four-dimensional atmospheric circulation was from a dataset produced jointly by NOAA's National Centers for Environmental Prediction and the National Center for Atmospheric Research: the NCEP-NCAR Reanalysis. This dataset of computer-generated weather map reconstructions or "reanalyses" starts from 1948, leaving many important climate events such as 1930's Dust Bowl drought uncovered. To expand the coverage of global gridded reanalyses, the 20th Century Reanalysis Project is an effort led by NOAA's Earth System Research Laboratory Physical Sciences Division and the University of Colorado CIRES Climate Diagnostics Center to produce a reanalysis dataset spanning the entire twentieth century, assimilating only surface observations of synoptic pressure, monthly sea surface temperature and sea ice distribution. The pressure observations have been assembled through international cooperation under the auspices of the Atmospheric Circulation Reconstructions over the Earth initiative, ACRE, and working groups of the Global Climate Observing System and World Climate Research Program. The Project uses a recently-developed Ensemble Filter data assimilation method which directly yields each six-hourly reanalysis field or weather map as the most likely state of the global atmosphere, and also estimates uncertainty in that map. This dataset will provide the first estimates of global tropospheric variability spanning 1871 to present at six-hourly temporal resolution and 2 degree longitude by 2 degree latitude resolution.
There are 3 sample animations of the data available for Science On a Sphere®. All three animations are showing columnar averaged precipitable water (that is, the total amount of water vapor there is in a column extending from the surface to the top of the atmosphere). Water isn't directly assimilated into the 20th Century dataset but is produced by the atmospheric circulation interacting with water sources such as the oceans. It is a good tracer of atmospheric motions.
This dataset illustrates a La Nina(1917) and El Nino (1919). Note that during the La Nina, most of the atmospheric water in the tropical Pacific tended to be in the west while during El Nino, it shifts eastward to the central Pacific.