Response of an Atmospheric GCM to Tropical Atlantic SST Dipole

Unlike the Pacific, the tropical Atlantic Ocean does not have a dominant mode of climate variability, but instead sees several modes co-existing. Among them is a sea surfa ce temperature (SST) dipole pattern that, through its meridional SST gradient, is known to affect rainfall over the Sahel and northeast Brazil. Our understanding of this dipole is inadequate at this time and it remains an open question whether it represents a mode of air-sea interaction or is just a fortuitous coincidence between independent SST variability on the two sides of the equator. Simple/intermediate coupled ocean-atmosphere models consistently produce the dipole as an intrinsic mode of the system, but more sophisticated coupled general circulation models (GCMs) give a somewhat murky picture.

An atmospheric GCM is forced with a prescribed SST dipole in the tropical Atlantic to investigate whether the response allows any positive feedback onto the ocean. The added SST anomalies are constant in time and anti-symmetric about the equator with maxima at 15S and 15N. The 40-year integration indicates that the GCM's response in the Tropics is robust and characterized by weakened (enhanced) trade winds in the warmer (colder) hemisphere, a wind anomaly pattern favoring the wind-evaporation-SST feedback. An additional positive feedback is identified between the low-level clouds and SST, much as in observations. The paper will also discuss the extratropical response--particularly the projection onto the NAO--and its seasonality.