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

Abstract

<p><strong>Abstract.</strong> Ocean color observations show semiannual variations of chlorophyll in the Atlantic cold tongue with a main bloom in boreal summer and a secondary bloom in December. In this study, ocean color and in situ measurements, and a coupled physical-biogeochemical model are used to investigate the processes that drive this variability. Results show that the main phytoplankton bloom in July-August is driven by a strong vertical supply of nitrate in May-July and the secondary bloom in December is driven by a shorter and moderate supply in November. The upper ocean nitrate balance is analyzed and shows that vertical advection controls the nitrate input in the equatorial euphotic layer and that vertical diffusion and meridional advection are key in extending and shaping the bloom off equator. Horizontal advection mostly acts to bring nitrate low water below the mixed layer. Our results also give insights on the influence of intraseasonal processes in these exchanges. Observations and model show that the Equatorial Undercurrent brings low-nitrate water (relatively to off-equatorial surrounding waters) but still rich enough to enhance the cold tongue productivity.</p>

A NEMO-based model of <i>Sargassum</i> distribution in the Tropical Atlantic: description of the model and sensitivity analysis (NEMO-Sarg1.0)

“…For the physical component of the simulation, we use the regional NEMO-based configuration described in Hernandez et al (2016Hernandez et al ( , 2017 and Radenac et al (2020) that covers the tropical Atlantic between 35°S and 35°N and from 100°W to 15°E. The resolution of the horizontal grid is ¼° and there are 75 vertical levels, 24 of which are in the upper 100 m of the ocean.…”

Multidecadal Intensification of Atlantic Tropical Instability Waves

“…TIWs redistribute heat and freshwater through advection and mixing (Foltz et al., 2020; Hummels et al., 2013; Inoue et al., 2019; Jochum & Murtugudde, 2006; Jochum et al., 2007; Kennan & Flament, 2000; Moum et al., 2009; Wenegrat & McPhaden, 2015) and affect the biogeochemistry of the equatorial upper ocean by influencing chlorophyll concentration (Grodsky et al., 2008; Menkes et al., 2002; Sherman et al., 2022; Shi & Wang, 2021), dissolved oxygen variability (Eddebbar et al., 2021), and nitrate distributions (Radenac et al., 2020). Through their impact on SST, TIWs also feed back onto the large‐scale atmospheric circulation (Caltabiano et al., 2005; Seo & Xie, 2011; Seo et al., 2007; Wu & Bowman, 2007b).…”

How do Climate Modes Shape the Chlorophyll‐a Interannual Variability in the Tropical Atlantic?

“…Concomitant to this cold SSTA, a strong negative SSHA and a positive Chl‐a anomaly are depicted (Figure 4d). The latter can be attributed to the enhanced upwelling of cold nutrient‐rich deep water into the surface layer (Radenac et al., 2020), driven by remotely forced equatorial wave triggered by the reinforcement of easterly trades in the western‐central basin. This analysis also reveals a localized moderate positive Chl‐a MJJ peak at 25°W which seems to detach itself from the core eastern Chl‐a anomaly and differs from the SSTA pattern.…”

“…2008; Radenac et al. 2020). The study of the winter bloom is beyond the scope of this paper, but additional attention could be given to its interannual variability in future research projects.…”