Global upper ocean heat storage response to radiative forcing from changing solar irradiance and increasing greenhouse gas/aerosol concentrations

Abstract

We constructed gridded fields of diabatic heat storage changes in the upper ocean from 20°S to 60°N from historical temperature profiles collected from 1955 to 1996. We filtered these 42 year records for periods of 8 to 15 years and 15 to 30 years, producing depth-weighted vertical average temperature (DVT) changes from the sea surface to the top of the main pycnocline. Basin and global averages of these DVT changes reveal decadal and interdecadal variability in phase across the Indian, Pacific, Atlantic, and Global Oceans, each significantly correlated with changing surface solar radiative forcing at a lag of 0±2 years. Decadal and interdecadal changes in global average DVT are 0.06°±0.01°K and 0.04°K±0.01°K, respectively, the same as those expected from consideration of the Stefan-Boltzmann radiation balance (i.e., 0.3°K per W m-2) in response to 0.1% changes in surface solar radiative forcing of 0.2 W m-2 and 0.15 W m-2, respectively. Global spatial patterns of DVT changes are similar to temperature changes simulated in coupled ocean-atmosphere models, suggesting that natural modes of Earth's variability are phase-locked to the solar irradiance cycle. A trend in global average DVT of 0.15°K over this 42 year record cannot be explained by changing surface solar radiative forcing. But when we consider the 0.5 W m-2 increase in surface radiative forcing estimated from the increase in atmospheric greenhouse gas and aerosol (GGA) concentrations over this period [Intergovernmental Panel on Climate Change, 1995], the Stefan-Boltzmann radiation balance yields this observed change. Moreover, the sum of solar and GGA surface radiative forcing can explain the relatively sharp increase in global and basin average DVT in the late 1970's.