An Observational Constraint on CMIP5 Projections of the East African Long Rains and Southern Indian Ocean Warming

Two outlying projections of the East African Long Rains suggest the seasonal rainfall may double by the late 21st century. Previous work has linked these extremes—found in the IPSL-CM5A model—to an exceptional March to May warming of the southern Indian Ocean. The current study shows a strong feedback between sea surface temperature (SST) increases and reduced low-level cloud cover (with similar behavior in other southern subtropical oceans). An observational constraint is developed by demonstrating a correlation across 28 models between the strength of present-day interannual SST-cloud sensitivity and future SST response. Verification of the present-day sensitivity finds that IPSL-CM5A's feedbacks are very likely overestimated. It is therefore suggested its projections should be discounted for the March to May southern Indian Ocean and East African Long Rains. This narrows the CMIP5 plausible range of Long Rains totals by a third.