Coastal trapped waves and tidal mixing control primary production in the tropical Angolan upwelling system

Abstract

Eastern boundary upwelling systems are hotspots of marine life and primary production. The strength and seasonality of upwelling in these systems are usually related to local wind forcing. However, in some tropical upwelling systems, seasonal maxima of productivity occur when upwelling favorable winds are weak. Here, we show that in the tropical Angolan upwelling system (tAUS), the seasonal productivity maximum is due to the combined effect of coastal trapped waves (CTWs) and elevated tidal mixing on the shelf. During austral winter, the passage of an upwelling CTW displaces the nitracline upward by more than 50 m. Thereby, nitrate-rich waters spread onto the shelf, where elevated vertical mixing causes a nitrate flux into the surface mixed layer. Interannual variability of the productivity maximum is strongly correlated to the amplitude of the upwelling CTW as seen in sea level data. Given that CTWs are connected to equatorial forcing, a predictability of the strength of the productivity maximum is suggested.

Long-term climatological trends driving the recent warming along the Angolan and Namibian coasts

“…9b). It's worth noting that Bachèlery et al (2020) and (Körner et al 2024) also emphasized this connection with the equatorial variability, suggesting that changes in the magnitude of equatorially-forced CTW activity could lead to decadal modulation of the Angolan and Namibian coastal variability. This insight implies decadal changes in the equatorial winds triggering the equatorial forcing.…”

“…Consequently, the relatively lower temperatures in the NBUS can be attributed to the quasipermanent wind-driven upwelling in the region (Fig. 4b; Veitch et al 2010), while in the TAUS the seasonal variability of the MLT is not driven by the modulation of the surface winds but rather by the passage of equatorially forced annual and semi-annual CTWs (Körner et al 2023(Körner et al , 2024. The analysis of the ML heat budget reveals that the surface forcing contribution (SFORC, see Eq.…”

“…River runoffs are not included in the simulation, instead the sea surface salinity is restored to the CARS2009 climatological values. This configuration has been successfully used to study the coastal dynamics off southwest Africa on seasonal (Körner et al 2024) and interannual (Illig et al 2020) timescales. Comparisons with observations can be found in Illig et al (2020).…”