Some Overlooked Features of Tropical Atlantic Climate Leading to a New Niño-Like Phenomenon*

Abstract

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. A suite of satellite and in situ observations reveals a new mode of tropical Atlantic variability that displays many characteristics of the zonal mode but instead peaks in November-December (ND). This new mode is found to be statistically independent from both 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, increasing upwelling and lifting the thermocline in the Gulf of Guinea. An analysis of high-resolution climatological data shows that these dynamical changes induce a noticeable SST cooling in the central equatorial Atlantic. The shoaling thermocline and increased upwelling enhance 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 the following 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 otherwise poorly understood.

On the relationship between Atlantic Niño variability and ocean dynamics

“…While this could hint at the empirical models having difficulties to predict dynamical SST during December, it could also be due to a feedback linking the three variables to each other, in this case the Bjerknes feedback. Okumura and Xie (2006)'s findings support the second possibility. Note that, in contrast to May and June, the December peak of enhanced dynamical SST variance is captured by both the FLX and STD experiments, indicating that the KCM appears to be able to reproduce the variability associated with Okumura and Xie (2006)'s Atlantic Niño II.…”

“…Okumura and Xie (2006)'s findings support the second possibility. Note that, in contrast to May and June, the December peak of enhanced dynamical SST variance is captured by both the FLX and STD experiments, indicating that the KCM appears to be able to reproduce the variability associated with Okumura and Xie (2006)'s Atlantic Niño II.…”

“…This peak could be associated with Okumura and Xie (2006)'s Atlantic Niño II, a secondary Niño-like phenomenon in the tropical Atlantic that is able to organize SST anomaly growth into a Bjerknes feedback. Note that our adjusted models choose SSH, u10, and an interaction term between the two predictors to capture dynamical SST in December.…”

Seasonal variability of the Equatorial Undercurrent at 10°W as inferred from recent in situ observations

“…Such an erosion may also occur in boreal winter as evidenced in December 2002 during the second upwelling season [Okumura and Xie, 2006], though with a weaker amplitude and located in the shallowest upper thermocline (above s q = 25.0).…”

The South Atlantic Anticyclone as a key player for the representation of the tropical Atlantic climate in coupled climate models

“…For this season, two other mechanisms for easterly wind acceleration over the equatorial Atlantic are identified: (1) the pressure gradients induced by monsoon heating over western Africa and the northern tropical Atlantic is limited to the eastern Atlantic south of West Africa; and (2) the equatorward momentum advection is a significant contributor across the basin. The accelerated easterlies contribute to the equatorial cooling by inducing upwelling and by shallowing the thermocline in the east (Okumura and Xie 2006). Richter et al (2012b) showed that JJA SST biases in the eastern Equatorial Atlantic in the CCSM global coupled model can be associated with a weakened east-west tilt of the thermocline due to a deeper thermocline in the eastern half of the equatorial Atlantic, which develops in response to a reduced strengthening of the MAM zonal wind stress.…”