The effect of greenhouse-gas-induced changes in SST on the annual cycle of zonal mean tropical precipitation

Forced by an increase in greenhouse gasses, CMIP5 models project changes to the seasonality of both tropical SST and precipitation. Nearly all models project an amplification and a phase delay of the annual cycle for both quantities, indicating a greater annual range and extrema reached later in the year. For precipitation this means enhanced summer ITCZ and monsoonal precipitation and delays to the meridional movement of the ITCZ and a later start and end to the monsoons. Analyzing the moisture budget of the CMIP5 models, we corroborate previous studies that found that the increase in the amplitude of tropical precipitation is mainly due to an increased annual mean moisture gradient with some negative compensation from a reduction in the seasonal amplitude of the tropical circulation. We also find that the delays in the phase of precipitation are linked to delays in the phase of the circulation. We detail these changes and investigate their nature and relationship in coupled simulations with idealized CO2 forcing and in an AGCM forced with SST perturbations which represent idealizations of the changes in the annual mean, amplitude, and phase as simulated by the CMIP5 models. We find that a uniform SST warming is sufficient to force both an amplification and a delay of the annual cycle of precipitation of similar magnitude as in the CMIP5 models. AGCM simulations solely forced with seasonal SST changes do not reproduce the seasonal changes in precipitation found in the CMIP5 models.