Modulation of Equatorial Currents and Tropical Instability Waves During the 2021 Atlantic Ni?o

In the boreal summer of 2021, the equatorial Atlantic experienced the strongest warm event, that is, Atlantic Nino, since the beginning of satellite observations in the 1970s. Such events have far-reaching impacts on large-scale wind patterns and rainfall over the surrounding continents. Yet, developing a paradigm of how Atlantic Nino interacts with the upper-ocean currents and intraseasonal waves remains elusive. Here we show that the equatorial Kelvin wave associated with the onset of the 2021 Atlantic Nino modulated both the background flow and the eddy flux of the equatorial upper-ocean circulation, causing an extremely weak and delayed tropical instability wave (TIW) season. TIW-induced variations of sea surface temperature (SST), sea surface salinity, sea surface height, and eddy temperature advection were exceptionally weak during May to July, the climatological peak of TIW activity, but rebounded in August when higher than normal variability was observed. Moored velocity data at 23 degrees W show that during the peak of the 2021 Atlantic Nino from June to August, the Equatorial Undercurrent was deeper and stronger than usual. An anomalously weak eddy momentum flux strongly suppressed barotropic energy conversion north of the equator from May to July, likely contributing to low TIW activity. Reduced baroclinic energy conversion also might have played a role, as the meridional gradient of SST was sharply reduced during the Atlantic Nino. Despite extremely weak TIW velocities, modest intraseasonal variability of chlorophyll-a (Chl-a) was observed during the Atlantic Nino, due to pronounced meridional Chl-a gradients that partly compensated for the weak TIWs. Every few years the eastern equatorial Atlantic Ocean is significantly warmer than usual during boreal summer. Such warm events are referred to as Atlantic Nino events, and share similarities with El Nino events in the Pacific. In 2021, the strongest Atlantic Nino in at least four decades was observed in the equatorial Atlantic. This study is the first that investigates the complex interaction between Atlantic Nino, tropical Atlantic upper-ocean currents, and equatorial waves based on various observational data sets. We show that the developing 2021 Atlantic Nino weakened both the background flow and the variability of near-surface currents in May, which in turn largely reduced the strength of intraseasonal (20-50 days) waves that are usually generated by instability of the upper-ocean zonal currents. As a consequence, the cooling effect that these waves usually have north of the equator and the warming effect along the equator vanished from May to July 2021. Interestingly, variability of chlorophyll concentration was enhanced, suggesting that enhanced meridional chlorophyll gradients compensated for reduced wave activity. The developing 2021 Atlantic Nino led to weaker equatorial surface currents and reduced vertical shear of upper-ocean horizontal velocityStrong reduction of the surface flow, eddy flux, and meridional temperature gradient in May caused extremely weak and delayed tropical instability wave (TIW) seasonReduced meridional TIW advection contributed to sharpen the north equatorial Chl-a front resulting in modest intraseasonal Chl-a variability