Some overlooked features of tropical Atlantic climate leading to a new Niño-like phenomenon

The Atlantic Niño, an equatorial zonal mode akin to the Pacific El Niño/Southern Oscillation (ENSO), is phase-locked to boreal summer when the equatorial easterly winds intensify and the thermocline shoals in the Gulf of Guinea. Using a suite of satellite and in-situ observations, we report a new mode of tropical Atlantic variability that displays many characters of the zonal mode but peaks in November-December (ND), statistically independent of the Atlantic Niño in the preceding summer and the Pacific ENSO. The origin of this ND zonal mode lies in an overlooked aspect of the seasonal cycle in the equatorial Atlantic.

In November the equatorial easterly winds intensify for the second time, lifting the thermocline in the Gulf of Guinea. An analysis of high-resolution climatological data shows that this secondary thermocline shoaling induces a noticeable SST cooling in the central equatorial Atlantic. The shoaling thermocline increases the SST sensitivity to surface wind changes, reinvigorating equatorial ocean-atmosphere interaction. The resultant ocean-atmospheric anomalies are organized into patterns that give rise to positive mutual feedback as Bjerknes envisioned for the Pacific ENSO. This ND zonal mode significantly affects interannual rainfall variability in coastal Congo-Angola during its early rainy season. It tends to further evolve into a meridional mode in March-April, affecting precipitation in northeast Brazil. Thus it offers potential predictability for climate over the Atlantic sector in early boreal winter, a season for which local ocean-atmosphere variability was poorly understood.