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We have studied the spatial and temporal variation of the surface longwaveradiation (downwelling and net) over a 21-year period in the tropical andsubtropical Pacific Ocean (40 S–40 N, 90 E–75 W). The fluxes were computed usinga deterministic model for atmospheric radiation transfer, along withsatellite data from the ISCCP-D2 database and reanalysis data from NCEP/NCAR(acronyms explained in main text), for the key atmospheric and surface inputparameters. An excellent correlation was found between the downwellinglongwave radiation (DLR) anomaly and the Niño-3.4 index time-series,over the Niño-3.4 region located in the central Pacific. A highanti-correlation was also found over the western Pacific (15–0 S, 105–130 E).There is convincing evidence that the time series of the mean downwellinglongwave radiation anomaly in the western Pacific precedes that in theNiño-3.4 region by 3–4 months. Thus, the downwelling longwave radiationanomaly is a complementary index to the SST anomaly for the study of ENSOevents and can be used to asses whether or not El Niño or La Niñaconditions prevail. Over the Niño-3.4 region, the mean DLR anomalyvalues range from +20 Wm−2 during El Niño episodes to −20 Wm−2during La Niña events, while over the western Pacific (15–0 S, 105–130 E)these values range from −15 Wm−2 to +10 Wm−2, respectively. Thelong- term average (1984–2004) distribution of the net downwelling longwaveradiation at the surface over the tropical and subtropical Pacific for thethree month period November-December-January shows a net thermal cooling ofthe ocean surface. When El Niño conditions prevail, the thermalradiative cooling in the central and south-eastern tropical Pacific becomesweaker by 10 Wm−2 south of the equator in the central Pacific (7–0 S,160–120 W) for the three-month period of NDJ, because the DLR increase islarger than the increase in surface thermal emission. In contrast, thethermal radiative cooling over Indonesia is enhanced by 10 Wm−2 duringthe early (August–September–October) El Niño phase

ENSO surface longwave radiation forcing over the tropical Pacific