Importance of positive cloud feedback for tropical Atlantic interhemispheric climate variability

Abstract

Over the tropical Atlantic during boreal spring, average interhemispheric differences in sea-surface temperature (SST) coincide with a coherent pattern of interannual climate variability often referred to as the Atlantic Meridional Mode. This includes anomalous SST and sea-level pressure roughly anti-symmetric about the equator, as well as cross-equatorial near-surface winds directed toward the warmer hemisphere. Within subtropical marine boundary layer cloud regions in both hemispheres, enhanced cloudiness associated with this variability is co-located with cool SST, a strong temperature inversion, and cold horizontal surface temperature advection, while reduced cloudiness is associated with the opposite meteorological conditions. This is indicative a positive cloud feedback that reinforces the underlying SST anomalies. The simulation of this feedback varies widely among models participating in phase 5 of the Coupled Model Intercomparison Project. Models that fail to simulate this feedback substantially underestimate the amplitudes of typical tropical Atlantic interhemispheric variability in cloudiness off of the equator, SST, and atmospheric circulation. Models that correctly reproduce a positive cloud feedback generally produce higher and more realistic amplitudes of variability, but with substantial scatter. Marine boundary layer clouds therefore appear to be a key element of springtime coupled atmosphere–ocean variability over the tropical Atlantic. A markedly more successful simulation of this variability in climate models may be obtained by better representing boundary layer cloud processes.