Solar irradiance anomalies caused by clear-sky transmission variations above Mauna Loa: 1958-99

The clear-sky transmission of the atmosphere contributes to determining the amount of solar irradiance that reaches various levels in the atmosphere, which in turn is fundamental to defining the climate of the earth. As of th e end of 1999, sustained clear-sky solar transmission over the mid-Pacific, as viewed from Mauna Loa, Hawaii, reached its highest level of clarity sin ce before the eruption of Mount Pinatubo in 1991 and appears to be continui ng to increase toward baseline levels established during 1958-62 and not su stained since. This record is used to answer the question as to impact of t ransmission variations, which can be attributed to either upward scattering or absorption above the station, on the net solar irradiance at 3.4 km, th e altitude of the isolated mountain-top observing site. Net solar irradianc e at a given level describes the total solar irradiance absorbed below that level. Monthly mean net solar anomalies caused by transmission variations, relative to the 1958-62 baseline, range from -14 to 2 W m(-2) and averaged -1.45 W m(-2) (-0.7%) between 1963 and 1999. Because of inherent attribute s of this transmission record, the observed fluctuations in the record are of unusually high precision over the entire period of record and are also r epresentative of an extended surrounding region. Irradiance anomalies have a long-term precision of better than 0.1 W m(-2) (similar to0.05%) per deca de. Any possible linear trend for the entire 42 yr is limited by the data t o between about 0.0 and -0.1 W m(-2) decade(-1), or any net shift over the 42 yr must be in the range of about 0.0 to -0.35 W m(-2) (0.0% to -0.15%). The transmission fluctuations are potentially caused by various atmospheric constituents, primarily aerosols, ozone, and water vapor, but the role of a specific constituent cannot be uniquely isolated on the basis of the tran smission record alone. Aerosols have the greatest potential influence on th e record and in general have the ability to cause both scattering and absor ption such that the net radiative heating effect in the entire atmospheric column cannot be determined from the transmission data alone. However, beca use the largest anomalies in the record are known to be due to volcanic eru ptions that produce predominantly conservative scattering aerosols, those l arge anomalies resulted in net radiative cooling tendencies in the entire a ssociated atmospheric column.