Modulation of Equatorial Currents and Tropical Instability Waves During the 2021 Atlantic Niño

Abstract

In the boreal summer of 2021, the equatorial Atlantic experienced the strongest warm event, that is, Atlantic Niño, 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 Niño 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 Niño 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°W show that during the peak of the 2021 Atlantic Niño 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 Niño. Despite extremely weak TIW velocities, modest intraseasonal variability of chlorophyll‐a (Chl‐a) was observed during the Atlantic Niño, due to pronounced meridional Chl‐a gradients that partly compensated for the weak TIWs.

Turbulence From Tropical Instability Waves in the Equatorial Cold Tongues: Quantification From Multiyear Moored Records

“…In fact, the 13 TIWs with the largest TIW KE and the 28 most energetic TIWs in terms of ϵ were all observed when Niño 3.4 < 0. Correlations between ϵ and ATL3 (i.e., the sea surface temperature index commonly used to represent the Atlantic Niño; Zebiak, 1993) are also strong and negative at both 23°W and 10°W, consistent with findings of a recent study which showed that TIW occurrence was suppressed during a large Atlantic Niño event (Tuchen et al, 2024). However, a recent study demonstrated that multiyear averaged vertical profiles of ϵ in the ACT and PCT resemble average profiles collected during weekly monthly short-term experiments in the absence of strong TIWs (Moum et al, 2022), implying that TIWs may have a minimal impact on long-term averages.…”

“…TIW occurrence also exhibits interannual variability: In the Pacific, TIWs are generated more frequently during La Niña and less frequently during El Niño (An, 2008;Imada & Kimoto, 2012;Wu & Bowman, 2007a;Yu & Liu, 2003). In the Atlantic, variability has been attributed to the Atlantic Niña and Atlantic Niño (Olivier et al, 2020;Perez et al, 2012;Tuchen et al, 2024;Tuchen, Perez, et al, 2022;Wu & Bowman, 2007a). To mostly account for seasonal variability, discussion in Sections 4.1 and 4.2 focuses on observations exclusively from the TIW active seasons shown in Figure 3, when properties varied by a smaller amount.…”

“…This has not been done to date on timescales of more than a few weeks-months from ship-based observations and mooring arrays because of difficulties in maintaining long-term records of turbulence. Understanding these processes on long timescales is crucial because TIWs and equatorial ocean dynamics exhibit interannual variability as a result of phenomena including ENSO (An, 2008;He et al, 2024;Imada & Kimoto, 2012;Xue et al, 2020;Yu & Liu, 2003), the Atlantic equatorial mode (i.e., the Atlantic Niño; Wu & Bowman, 2007a;Perez et al, 2012;Olivier et al, 2020;Tuchen, Perez, et al, 2022;Tuchen et al, 2024), and West African monsoon (Brandt et al, 2011). Evaluating multiple years of observations allows for a more complete understanding of TIW-related turbulence and reduces bias due to interannual variations and small sample sizes of existing observations.…”

Impact of the North Atlantic Oscillation on the Decadal Variability of the Upper Subtropical‐Tropical Atlantic Ocean

“…Additionally, near-surface geostrophic velocity data, estimated at 15 m depth from a synthesis product of surface drifting buoy velocities and satellite-derived velocities from sea level anomalies and wind stress, are utilized to determine annual subduction rates (Lumpkin & Garzoli, 2011;Tuchen et al, 2022c). The latest version of this product is available annually from 1993 to 2021 and at 0.25°× 0.25°spatial resolution (Tuchen et al, 2024). For consistency with the other products, we use the data of the time period of 2007-2021.…”