Role of interannual Kelvin wave propagations in the equatorial Atlantic on the Angola Benguela Current system

Abstract

The link between equatorial Atlantic Ocean variability and the coastal region of Angola‐Namibia is investigated at interannual time scales from 1998 to 2012. An index of equatorial Kelvin wave activity is defined based on Prediction and Research Moored Array in the Tropical Atlantic (PIRATA). Along the equator, results show a significant correlation between interannual PIRATA monthly dynamic height anomalies, altimetric monthly Sea Surface Height Anomalies (SSHA), and SSHA calculated with an Ocean Linear Model. This allows us to interpret PIRATA records in terms of equatorial Kelvin waves. Estimated phase speed of eastward propagations from PIRATA equatorial mooring remains in agreement with the linear theory, emphasizing the dominance of the second baroclinic mode. Systematic analysis of all strong interannual equatorial SSHA shows that they precede by 1–2 months extreme interannual Sea Surface Temperature Anomalies along the African coast, which confirms the hypothesis that major warm and cold events in the Angola‐Benguela current system are remotely forced by ocean atmosphere interactions in the equatorial Atlantic. Equatorial wave dynamics is at the origin of their developments. Wind anomalies in the Western Equatorial Atlantic force equatorial downwelling and upwelling Kelvin waves that propagate eastward along the equator and then poleward along the African coast triggering extreme warm and cold events, respectively. A proxy index based on linear ocean dynamics appears to be significantly more correlated with coastal variability than an index based on wind variability. Results show a seasonal phasing, with significantly higher correlations between our equatorial index and coastal SSTA in October–April season.

Benguela Niños and Benguela Niñas in Forced Ocean Simulation From 1958 to 2015

“…These extreme warm events are called Benguela Niños (Shannon et al, ; Florenchie et al, ; Rouault et al, ; Ostrowski et al, ; Imbol Koungue et al, ; Rouault et al, ). The inverse of a Benguela Niño is called Benguela Niña (Florenchie et al, ; Imbol Koungue et al, ). These events typically manifest along the coast of Angola and Namibia in the southeastern Atlantic Ocean and tend to reach their maximum in late austral summer mainly during March–April (Florenchie et al, , ; Lübbecke et al, ; Rouault et al, ).…”

“…Recently, Imbol Koungue et al () combined real‐time buoy measurements from the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA), altimetric sea surface height (SSH), and an equatorial Ocean Linear Model (OLM) in order to identify propagation of SSH anomalies (SSHA) along the equator and define an equatorial oceanic index of interannual equatorial Kelvin wave (IEKW) amplitude using the OLM. From 1998 to 2012, they succeeded to link this IEKW proxy to the coastal variability off Angola and Namibia in order to forecast Benguela Niños or Niñas which are of great socioeconomic importance for the countries of Southern Africa.…”

“…Due to the good agreement between PIRATA dynamic height and the OLM SSHA in the monitoring of the IEKW from 1998 to 2012 (cf. Imbol Koungue et al, , Table 1), OLM outputs from 1958 to 2015 were derived. Our methodology is then based on the combination of analysis of an Ocean General Circulation Model (OGCM) output of the tropical Atlantic and of the OLM output from 1958 to 2015 in order to study the anomalous coastal events developing before 1982.…”

Eastern Boundary Circulation and Hydrography Off Angola: Building Angolan Oceanographic Capacities

“…On the contrary, cold and fresh water mass anomalies, likely associated with enhanced contributions of ESACW, were displaced farther to the north between 1996 and 1998, from 2002 to 2006, and since 2015. The interannual subsurface temperature anomalies of up to 0.4°C might represent an additional forcing of upper-ocean temperature and SST variability otherwise largely explained by the impact of upwelling and downwelling CTWs (Florenchie et al 2004;Lübbecke et al 2010;Bachèlery et al 2016;Imbol Koungue et al 2017).…”

“…In addition, altimetry and regional ocean simulations revealed equatorial forced CTWs having interannual time scales (13-20 months; Bachèlery et al 2016). Along the Angolan and Namibian continental margin, CTWs contribute to triggering sea surface temperature (SST) anomalies, which impact atmospheric circulation and rainfall (Shannon et al 1986;Rouault et al 2003;Imbol Koungue et al 2017;Rouault et al 2018). CTWs may also modulate biogeochemical conditions in the coastal upwelling ecosystems and directly impact fisheries by displacing fish eggs and larvae latitudinally along the continental margin (e.g., Ostrowski 2007).…”