GFI/BCCR Seminar: Thermodynamic origins and dipole structure of the Atlantic Niño

Hyacinth Nnamchi (Department of Geography, University of Nigeria):

Thermodynamic origins and dipole structure of the Atlantic Niño

Abstract
The Equatorial Atlantic Ocean is characterized by a zonal SST mode often considered the Atlantic equivalent of the Pacific El Niño and is hence christened “Atlantic Niño”. The Atlantic Niño peaks in austral winter. The current understanding is that the Atlantic Niño is caused by the Bjerknes feedback mechanisms; that is, the growth of the SST anomaly is achieved through the dynamical interactions among equatorial wind perturbations, SST and thermocline. But compared to the Pacific, the Bjerknes coupling is of the order of 50% weaker and the equatorial wind perturbations account for only ~15-35% of the observed equatorial Atlantic variability.

In this talk, I will show that (1) the Atlantic Niño is largely driven by atmospheric changes in extra-tropical South Atlantic Ocean (SAO) and is therefore linked to opposite sign in SST over the south-western part of the SAO—together, these form the SAO dipole. I will further show that (2) the equatorial Atlantic variability simulated by conventional OGCMs and by coupling the atmosphere to a 50m slab of motionless ocean—and then, I will argue that thermodynamic processes alone are capable of generating the  Atlantic Niño.