The Influence of Solar Forcing on Tropical Circulation

The response of the seasonal tropical circulation to an 11-yr solar cycle forcing is studied with the Goddard Institute for Space Studies (GISS) ModelE, which includes fully interactive atmospheric chemistry. To identify characteristic solar signals in the tropical circulation, the model experiments are carried out with certain imposed conditions: a doubly amplified solar forcing and the present-day and preindustrial greenhouse gases and aerosol conditions, with the mixed layer or fully coupled dynamic ocean model. In both the model and the NCEP reanalysis, tropical humidity increases in response to enhanced solar irradiance are found to be statistically significant in both solstice seasons. Changes are also found in the vertical velocities for both the Hadley and Walker circulations in some areas of the Pacific region. With present-day greenhouse gas and aerosol conditions, the ascending branch of the Hadley cell is enhanced near the equator, and the intertropical convergence zone (ITCZ) is shifted northward in response to solar forcing during the boreal winter. Enhancement of the meridionally averaged vertical velocity over the western Pacific indicates strengthening of the Walker circulation in response to solar forcing in both solstice seasons. In present-day conditions, the tropical circulation response to an 11-yr solar forcing is generally consistent with that derived from previous observational works.