Illig Serena, Bachelery M. L., Lubbecke J. F. (2020). Why do Benguela Ninos lead Atlantic Ninos ?. Journal of Geophysical Research : Oceans, 125 (9), e2019JC016003 [27 p.]. ISSN 2169-9275.
Source
Journal of Geophysical Research : Oceans, 2020,
125 (9), e2019JC016003 [27 p.] ISSN 2169-9275
We investigate the lag between warm interannual Sea Surface Temperature (SST) events in the eastern-equatorial Atlantic, the Atlantic Ninos, and the occurrence of Benguela Ninos along the southwestern Angolan coast. While it is commonly agreed that both events are associated with equatorial and subsequent coastal-trapped wave propagations driven remotely by a relaxation of the trade-winds, it is surprising that SST anomalies off Angola tend to precede the ones in the eastern-equatorial sector by similar to 1 month. To explain this counterintuitive behavior, our methodology is based on the experimentation with a Tropical Atlantic Ocean model. Using idealized wind-stress perturbations from a composite analysis, we trigger warm equatorial and coastal events over a stationary and then, seasonally varying ocean mean-state. In agreement with the linear dynamics, our results show that when the interannual wind-stress forcing is restricted to the western-central equatorial Atlantic, the model yields equatorial events leading the coastal ones. This implies that neither the differences in the ocean stratification between the two regions (thermocline depths or modal wave contributions) nor the seasonal phasing of the events explains the observed temporal sequence. Only if wind-stress anomalies are also prescribed in the coastal fringe, the coastal warming precedes the eastern-equatorial SST anomaly peak, emphasizing the role of the local forcing in the phenology of Benguela Ninos. A weaker South-Atlantic Anticyclone initiates the coastal warming before the development of eastern-equatorial SST anomalies. Then, equatorward coastal wind anomalies, driven by a convergent anomalous circulation located on the warm Atlantic Nino, stop the remotely forced coastal warming prematurely. Plain Language Summary We investigate the Sea Surface Temperature (SST) interannual fluctuations in the Tropical Atlantic. We focus on the extreme warm events that occur every few years in the Gulf of Guinea, the Atlantic Ninos, and along the coast of Angola-Namibia, the Benguela Ninos. It is commonly agreed that both events are forced by equatorial and subsequent coastal waves triggered in the western-central basin by a relaxation of the trade-winds. Yet, we observe that the coastal warming tends to precede the one in the Gulf of Guinea by similar to 1 month. We explain this counterintuitive fact, using experimentation with a Tropical Atlantic Ocean model. Using idealized wind-stress perturbations, we simulate warm equatorial and coastal events on top of a stationary and then, seasonally varying ocean mean-state. Results show that when wind-stress perturbations are confined to the western-central equatorial Atlantic, the model yields equatorial events leading the coastal variability, consistent with the propagation path of the waves. This implies that neither the ocean mean-state nor its seasonal variability controls the timing between events. We further show that only when wind-stress anomalies are prescribed within the coastal fringe, warm events off Angola precede the eastern-equatorial SST anomaly. Both warmings originate from a reduction of the strength of the South-Atlantic Anticyclone. Nevertheless, local processes initiate the coastal warming before the remotely forced equatorial waves impact the eastern-equatorial SST. Benguela Ninos then also stop earlier due to the development of coastal wind anomalies associated with a convergent anomalous circulation located on the warm Atlantic Nino event.
