An apparent multi-decadal trend in shortwave cloud forcing over the tropical Pacific

The NCEP/NCAR reanalysis of atmospheric data beginning in 1948 has provided an opportunity to study a consistent 50-year record of assimilated weather observations. Through the examination of several fields, we find an apparent long-term decrease in relative humidity and cloud amount in the tropical Pacific. As a result, the shortwave cloud radiative forcing decreased nearly 15 Wm^-2 over the entire period. Two major questions arise: the first is the extent to which the trend over the 50-year period is a real phenomenon rather than the result of observing system evolution. The second is, if the phenomenon is at least partially real, what is its cause?