ENSO surface shortwave radiation forcing over the tropical Pacific
We have studied the spatial and temporal variation of the downward shortwave radiation (DSR) at the surface of the Earth during ENSO events for a 21-year period over the tropical and subtropical Pacific Ocean (40 degrees S-40 degrees N, 90 degrees E-75 degrees W). The fluxes were computed using a deterministic model for atmospheric radiation transfer, along with satellite data from the ISCCP-D2 database, reanalysis data from NCEP/ NCAR for the key atmospheric and surface input parameters, and aerosol parameters from GADS (acronyms explained in main text). A clear anti-correlation was found between the downward shortwave radiation anomaly (DSR-A) time-series, in the region 7 degrees S-5 degrees N 160 degrees E-160 degrees W located west of the Nino-3.4 region, and the Nino-3.4 index timeseries. In this region where the highest in absolute value DSR anomalies are observed, the mean DSR anomaly values range from-45 Wm(-2) during El Nino episodes to + 40Wm(-2) during La Nina events. Within the Nina 3.4 region no significant DSR anomalies are observed during the cold ENSO phase in contrast to the warm ENSO phase. A high correlation was also found over the western Pacific (10 degrees S-5 degrees N, 120-140 degrees E), where the mean DSR anomaly values range from + 20Wm(-2) to-20Wm(-2) during El Nino and La Nina episodes, respectively. There is also convincing evidence that the time series of the mean downward shortwave radiation anomaly in the off-equatorial western Pacific region 7-15 degrees N 150-170 degrees E, precedes the Nino-3.4 index time-series by about 7 months and the pattern of this anomaly is indicative of ENSO operating through the mechanism of the western Pacific oscillator. Thus, the downward shortwave radiation anomaly is a complementary index to the SST anomaly for the study of ENSO events and can be used to assess whether or not El Nino or La Nina conditions prevail.