The Met Office Hadley Centre's mean sea level pressure (MSLP) data set, HadSLP2, is a unique combination of monthly globally-complete fields of land and marine pressure observations on a 5 degree latitude-longitude grid from 1850 to 2004. This product is also available in an updated form using NCEP/NCAR reanalysis fields, giving the near real time product, HadSLP2r.
HadSLP2 was created using marine observations taken from ships and buoys and land observations from around the globe. Missing data have been filled in a statistically optimal way. Various indices can be derived from these data. Below is shown the evolution in the Southern Oscillation Index, an indicator of changes in the El Niño phenomenon.
Further information and data about HadSLP2 are available
Daily resolution data for the N. Atlantic and Europe are available from the EMULATE dataset.
References
Allan, R. J. and Ansell, T. J. (2006) 'A new globally complete monthly historical mean sea level pressure data set (HadSLP2): 1850-2004', Journal of Climate, 19 5816-5842.