The Effect of Greenhouse-Gas-Induced Changes in SST on the Seasonality of 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 date to the monsoons. We detail these changes and investigate their nature and relationship in an AGCM forced with idealized changes in the annual mean and annual cycle of SST. By 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 increase in the annual mean moisture gradient with some negative compensation from a reduction in the seasonal amplitude of the tropical circulation. We also find that the phase delay of precipitation is related to a phase delay of tropical circulation.

We reproduce the sign and relative magnitude of these seasonality changes in an AGCM with a prescribed SST uniformly warmer than in a control simulation. The seasonal responses of the terms in the moisture budget for this simulation are very similar to those for CMIP5, despite the AGCM not having a coupled ocean or the many complicated forcings of the CMIP5 models.

In the second experiment, we change only the seasonal characteristics of SST. For an amplified annual cycle of SST we find an amplified annual cycle of precipitation and tropical circulation. Likewise for a delayed SST we find a delayed annual cycle of precipitation and tropical circulation. There are also cross-effects: the phase of SST affects the amplitude of precipitation and the amplitude of SST affects the phase of precipitation. While a delayed and amplified seasonal cycle of SST can reproduce the sign of the seasonal changes of precipitation in the CMIP5 models, the magnitude is too small and the budget indicates that different processes are operating than in the coupled models. These results inform our understanding of the hydrological cycle and help explain future projection for tropical precipitation.