Outgoing longwave radiation biases and their impacts on empirical orthogonal function modes of interannual variability in the tropics

The effects of different equator‐crossing times on the outgoing longwave radiation (OLR) data record are examined. A simple procedure proposed by Kousky and Kayano (1994) is used to reduce these effects. Large‐scale interannual modes of OLR within the tropics are determined by performing empirical orthogonal function analyses on OLR anomalies and on bias‐corrected OLR anomalies. Both analyses result in two physically meaningful leading modes, which describe different large‐scale anomalous features of the OLR related to the southern oscillation. A mode related to changes in the satellite observing system is found only in the analysis for the uncorrected OLR anomalies. Evidence is presented indicating that estimates of the mean daily OLR are affected by changes in the equator‐crossing times due either to the replacement of satellites or to long‐term drifts from the orbital plane during the lifetime of individual satellites.