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ENSO surface shortwave radiation forcing over the tropical Pacific

by Kg Pavlakis
Atmospheric Chem Phys ()
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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.

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