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

Rodrigue Anicet Imbol Koungue,Julien Jouanno,Mathieu Rouault,Serena Illig,Peter Brandt

doi:10.1029/2019jc015013

Abstract

AbstractA systematic study of Benguela Niño and Benguela Niña events during 1958 to 2015 including those that developed before the satellite era (1982) is carried out using an ocean general circulation model in combination with a linear equatorial model. Altogether, 21 strong warm and cold anomalous coastal events are identified among which 6 undocumented extreme coastal events are reported. Results suggest that most of these extreme coastal events including the newly identified ones are linked to remote equatorial forcing via mode 2 equatorial Kelvin waves. The latter propagates after approaching the African coast poleward as coastally trapped waves leading surface temperature anomalies along the Angola‐Benguela current system by one month. One to two months before the peak of Benguela Niños or Niñas usually occurring in March–April, a large‐scale wind stress forcing is observed with both local (variations of alongshore coastal wind stress) and remote forcing developing simultaneously. Results further suggest that surface temperature anomalies off Southern Angola and in the Angola‐Benguela Front are associated with equatorial dynamics and meridional wind stress fluctuations off the southwestern African coast north of 15°S. Similar mechanisms are observed for Northern Namibia in combination with forcing by local meridional wind stress variations.

Highlights

The Benguela Upwelling Systems (BUS) is among the most productive marine ecosystems in the world ocean supporting important fisheries (Chavez & Messié, 2009) due to upwelling dynamics
Description of the Coastal Domains of Interest The interannual sea surface temperature (SST) coastal variability along the western coast of Africa is studied within three key domains as defined in Rouault et al (2018) which are illustrated in Figure 2d: (1) the Southern Angola domain (10°S– 15°S; 1°‐width coastal fringe), a low wind speed tropical warm water region affected by the Angola Current (Kopte et al, 2017; Tchipalanga et al, 2018) and coastal trapped waves (CTWs); (2) the Angola‐Benguela Front (ABF) domain (15°S–19°S; 1°‐width coastal fringe), which is a transition region between the warm tropical water and cold upwelled water separated by the ABF in its middle; and (3) the Northern Namibia domain (19°S–24°S; 1°‐width coastal fringe), a wind‐driven upwelling region with cold upwelled waters best characterized as Northern Benguela Upwelling System (NBUS). 2.3.5
Within the upwelling‐driven Northern Namibia domain, weak cyclonic wind stress curl generating negative Ekman pumping that could decrease the coastal upwelling resulting in positive surface temperature anomalies

Summary

Introduction

The Benguela Upwelling Systems (BUS) is among the most productive marine ecosystems in the world ocean supporting important fisheries (Chavez & Messié, 2009) due to upwelling dynamics. These extreme warm events are called Benguela Niños (Shannon et al, 1986; Florenchie et al, 2004; Rouault et al, 2007; Ostrowski et al, 2009; Imbol Koungue et al, 2017; Rouault et al, 2018). Extreme coastal events (Benguela Niño or Niña) can last from few months to half a year or more (Florenchie et al, 2004; Rouault, 2012; Rouault et al, 2003) They impact on the regional climate and rainfall (Hansingo & Reason, 2009; Rouault et al, 2003, 2009), responsible for drought or flooding in the neighboring countries. They affect the ecosystem and the productivity through a change in the upwelling intensity that modulates the biogeochemical properties and fish habitat (Blamey et al, 2015; Boyer et al, 2001; Ostrowski et al, 2009)

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Results