Impact of intraseasonal wind bursts on sea surface temperature variability in the far eastern tropical Atlantic Ocean during boreal spring 2005 and 2006: focus on the mid-May 2005 event

Abstract

Abstract. The impact of boreal spring intraseasonal wind bursts on sea surface temperature variability in the eastern tropical Atlantic Ocean in 2005 and 2006 is investigated using numerical simulation and observations. We especially focus on the coastal region east of 5° E and between the Equator and 7° S that has not been studied in detail so far. For both years, the southerly wind anomalies induced cooling episodes through (i) upwelling processes, (ii) vertical mixing due to the vertical shear of the current, and for some particular events (iii) a decrease in incoming surface shortwave radiation. The strength of the cooling episodes was modulated by subsurface conditions affected by the arrival of Kelvin waves from the west influencing the depth of the thermocline. Once impinging the eastern boundary, the Kelvin waves excited westward-propagating Rossby waves, which combined with the effect of enhanced westward surface currents contributed to the westward extension of the cold water. A particularly strong wind event occurred in mid-May 2005 and caused an anomalous strong cooling off Cape Lopez and in the whole eastern tropical Atlantic Ocean. From the analysis of oceanic and atmospheric conditions during this particular event, it appears that anomalously strong boreal spring wind strengthening associated with anomalously strong Hadley cell activity prematurely triggered the onset of coastal rainfall in the northern Gulf of Guinea, making it the earliest over the 1998–2008 period. No similar atmospheric conditions were observed in May over the 1998–2008 period. It is also found that the anomalous oceanic and atmospheric conditions associated with the event exerted a strong influence on rainfall off northeast Brazil. This study highlights the different processes through which the wind power from the South Atlantic is brought to the ocean in the Gulf of Guinea and emphasizes the need to further document and monitor the South Atlantic region.

Physical drivers of the nitrate seasonal variability in the Atlantic cold tongue

“…On average in the 1.5 • S-0.5 • N, 20-5 • W region, our model results show that horizontal eddy advection is responsible for nitrate decrease, especially in August-September, and that vertical eddy advection supplies the euphotic layer with nitrate. The intraseasonal nitrate variations may have the same origins as temperature modulations observed at periods between 10 and 50 d in the cold tongue de Coëtlogon et al, 2010;Jouanno et al, 2013;Herbert and Bourlès, 2018). TIWs are observed west of 10 • W at periods between 20 and 50 d (Jochum et al, 2004;Athié and Marin, 2008;Jouanno et al, 2013).…”

“…The physical component of the simulation is based on the NEMO (Nucleus for European Modeling of the Ocean; Madec and the NEMO team, 2016) numerical code. We use the regional configuration described in Hernandez et al (2016Hernandez et al ( , 2017 (Dussin et al, 2016) using bulk formulae from Large and Yeager (2009). Temperature, salinity, current, and sea level from the MERCATOR global reanalysis GLORYS2V4 (Storto et al, 2018) are used to force the model at the lateral boundaries.…”

“…They are active in boreal summer, decrease in fall, emerge again at the end of the year with lesser intensity than in summer, and disappear in spring (Jochum et al, 2004;Caltabiano et al, 2005;Perez et al, 2019). East of 10 • E, the impacts of wind-forced equatorial waves superimpose at different frequencies (Houghton and Colin, 1987;Athié et al, 2008;de Coëtlogon et al, 2010;Jouanno et al, 2013;Herbert and Bourlès, 2018): Kelvin waves at periods between 25 and 40 d, mixed Rossby-gravity waves between 15 and 20 d, and inertia-gravity waves between 5 and 11 d.…”

The Tropical Atlantic Observing System

“…The Benguela Niño and SAOD have been linked to the AZM (Lübbecke et al, 2010;Richter et al, 2010;Nnamchi et al, 2016Nnamchi et al, , 2017Rouault et al, 2018), but it is unclear whether the Benguela Niño and AZM are part of the same climate mode or closely related but distinct modes (Polo et al, 2008;Goubanova et al, 2013;Bachèlery et al, 2016a,b;Illig et al, 2018a,b;Illig and Bachèlery, 2019). Intraseasonal wind bursts seem to play an important role in AZM evolution during some years (Marin et al, 2009;Herbert and Bourlès, 2018). The AZM has been shown to impact surface chlorophyll-a concentration in the eastern equatorial Atlantic (Grodsky et al, 2008), but its broader effect on primary productivity is still unknown.…”

Physical drivers of the nitrate seasonal variability in the Atlantic cold tongue

“…On average in the 20° W-5° W, 1.5° S-0.5° N region, our model results show that horizontal eddy advection is responsible for nitrate decrease, especially in August-September, and that vertical eddy advection supplies the euphotic layer with nitrate. The intraseasonal nitrate variations may have the same origins as temperature modulations observed at periods between 10 and 50 days in the cold tongue (Marin et al, 2009;de Coëtlogon et al, 2010;Jouanno et al, 2013;Herbert and Bourlès, 2018). TIW are observed west of 10° W at periods between 20 and 50 days (Jochum et al, 2004;Athié and Marin, 2008;Jouanno et al, 2013).…”