Earth-emitted radiation varies in space and time due to variations of atmospheric and surface temperature, clouds, and water vapor. The method of empirical orthogonal functions (EOFs) has been applied to a 10-year data set of outgoing longwave radiation to study this variability. Spherical harmonic functions are used as a basis set for producing equal area map results. The following findings are noted. The first EOF accounts for 66% of the variance. After that, each EOF accounts for only a small variance, forming a slowly converging series. The first two EOFs describe mainly the annual cycle. The third EOF is primarily the semiannual cycle, although many other EOFs also contain significant semiannual parts. These results reaffirm earlier studies based on a shorter data set. In addition, a much stronger spring/fall mode was found in the central equatorial Pacific Ocean for the second EOF than had been found earlier. This difference is attributed to the use of broadband radiometer data which were available for the present study. The earlier study used data from a window-channel instrument which is not as sensitive to water vapor variations. The fourth EOF describes much of the 1976-1977 and 1982-1983 ENSO phenomena. There is typically a gap in the spectrum between a semiannual peak and the annual cycle for all but the first EOF. A semiannual dipole straddles the Asian-Australian monsoon track.
