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 longwa ve radiation (OLR) data record are examined. A simple procedure propos ed by Kousky and Kayano (1994) is used to reduce these effects. Large- scale interannual modes of OLR within the tropics are determined by pe rforming empirical orthogonal function analyses on OLR anomalies and o n bias-corrected OLR anomalies. Both analyses result in two physically meaningful leading modes, which describe different large-scale anomal ous features of the OLR related to the southern oscillation. A mode re lated to changes in the satellite observing system is found only in th e analysis for the uncorrected OLR anomalies. Evidence is presented in dicating that estimates of the mean daily OLR are affected by changes in the equator-crossing times due either to the replacement of satelli tes or to long-term drifts from the orbital plane during the lifetime of individual satellites.