We present results of an EOF analyses of yearly (1956--2003), global ocean heat content fields (0--700 m layer) on a one-degree, latitude-longitude grid. The observed global ocean heat content integral is well-reproduced by reconstructing this quantity using the first four EOFs. Results of an independent Fourier analysis of these same heat content fields show that the global heat content integral can be accounted for by phenomena with decadal and longer time-scales. The temporal variability of the global integral of 0--700 m ocean heat content is due to gyre and basin-scale spatial variability on decadal time-scales. The strong interdecadal variability observed in the ocean heat content record is associated with the reversal of polarity of the PDO.