Modulation of Tropical Instability Waves and chlorophyll concentration by equatorial waves during the 2021 Atlantic Niño

During the boreal summer of 2021, the central and eastern equatorial Atlantic experienced persistent sea surface temperature (SST) anomalies of more than +1°C for several months. These episodic extreme events are referred to as Atlantic Niños, with 2021 marking the strongest event since 1984. Atlantic Niños are known to have far-reaching impacts on, for instance, rainfall over the surrounding continents, and on ocean dynamics through changes in thermohaline gradients and circulation. A pronounced event like the 2021 Atlantic Niño in combination with the steadily expanding coverage by satellite and in-situ observations provides the rare opportunity to study an Atlantic Niño event in unprecedented detail. Here we focus on the influence of the 2021 Atlantic Niño on tropical instability wave (TIW) activity and surface chlorophyll concentration.

The 2021 Atlantic Niño was initiated by a strong downwelling Kelvin wave excited by westerly wind bursts in the western and central equatorial Atlantic. The eastward propagating Kelvin wave induced strong eastward flow anomalies on the equator causing a reduction of the meridional shear of the near-surface zonal flow in the central equatorial Atlantic. At the same time, the Kelvin wave-induced deepening of the thermocline weakened the seasonal development of the equatorial Atlantic cold tongue. The reduction of both the meridional SST gradient and the meridional shear of zonal velocity largely suppressed barotropic and baroclinic instability, which is required for the generation of TIWs. Consistent with these changes, we find that 2021 was one of the least active years in terms of TIW-related temperature, salinity, sea level, and current variability. The overall reduction in TIW activity is characterized by weak TIW activity before and enhanced TIW activity after the climatological TIW peak resulting in a delay of the TIW season. This delayed onset of TIW activity is expected to have considerable consequences for the local heat and freshwater budgets. Low TIW activity and positive SST anomalies also impacted the concentration and distribution of surface chlorophyll as observed by daily gap-free satellite observations. After the initial Kelvin wave, surface chlorophyll concentration dropped to extraordinarily low values and was anti-correlated with the evolution of SST anomalies. The absence of TIWs is also apparent in weaker than normal surface chlorophyll concentration variability on intraseasonal time scales, highlighting the interplay of TIWs and chlorophyll. Our results demonstrate how the 2021 Atlantic Niño impacted oceanic variability, but further analysis is needed to better understand the consequences of such events for regional heat and freshwater budgets as well as for nutrients, productivity, and marine ecosystems.