SHORTWAVE RADIATIVE FORCING AND ENSO

In an interesting paper K. G. Pavlakis et al have found that changes in downward shortwave radiation (by changes in cloud cover) are a significant forcing in the enso oscillation.

Abstract. 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_ S–40_ N, 90_ E–75_ 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_ S–5_ N 160_ E–160_W located west of the Ni˜no-3.4 region, and the Ni˜no-3.4 index timeseries. In this region where the highest in absolute value DSR anomalies are observed, the mean DSR anomaly values range from −45Wm−2 during El Ni˜no episodes to +40Wm−2 during La Ni˜na events. Within the Ni˜no-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_ S–5_ N, 120–140_ E), where the mean DSR anomaly values range from +20Wm−2 to −20Wm−2 during El Ni˜no and La Ni˜na 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_ N 150–170_ E, precedes the Ni˜no-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 Ni˜no or La Ni˜na conditions prevail.

Whilst this is an additional indicator,the enso oscillation is a highly coupled mechanism that is both internally driven(self organized far from equilibrium) and externally forced,both the fluctuations and inversions are a result of highly coupled
feedbacks both positive(amplifying) and negative(dissipative),this is a well understood open problem for long term predictive modelling.