Description: El Niño-Southern Oscillation (ENSO) in the Pacific is asymmetric for warm and cold events with respect to amplitude, spatial patterns and temporal evolution. Here the symmetry of the Atlantic Niño mode, which many previous studies have argued is governed by atmosphere–ocean dynamics similar to those of ENSO, is investigated using two different ocean reanalysis products. Calculation of Bjerknes feedback terms for the Pacific reveals a pronounced asymmetry between warm and cold events, though unlike most previous studies, the largest asymmetry is found in the relationship between eastern Pacific thermocline depth and SST anomalies. For the Atlantic, cold events are effectively mirror images of warm events with Bjerknes feedbacks of similar strength. The analysis supports not only the conclusion that Atlantic Niños are more symmetric than ENSO, but the hypothesis itself that the Bjerknes feedback is operative in the Atlantic given the strength of the relationship between the key variables involved.

Description: Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) is a multinational program initiated in 1997 in the tropical Atlantic to improve our understanding and ability to predict ocean-atmosphere variability. PIRATA consists of a network of moored buoys providing meteorological and oceanographic data transmitted in real time to address fundamental scientific questions as well as societal needs. The network is maintained through dedicated yearly cruises, which allow for extensive complementary shipboard measurements and provide platforms for deployment of other components of the Tropical Atlantic Observing System. This paper describes network enhancements, scientific accomplishments and successes obtained from the last 10 years of observations, and additional results enabled by cooperation with other national and international programs. Capacity building activities and the role of PIRATA in a future Tropical Atlantic Observing System that is presently being optimized are also described.

Description: Based on velocity data from a long-term moored observatory located at 0°N, 23°W we present evidence of a vertical asymmetry during the intraseasonal maxima of northward and southward upper-ocean flow in the equatorial Atlantic Ocean. Periods of northward flow are characterized by a meridional velocity maximum close to the surface, while southward phases show a subsurface velocity maximum at about 40 m. We show that the observed asymmetry is caused by the local winds. Southerly wind stress at the equator drives northward flow near the surface and southward flow below that is superimposed on the Tropical Instability Wave (TIW) velocity field. This wind-driven overturning cell, known as the Equatorial Roll, shows a distinct seasonal cycle linked to the seasonality of the meridional component of the south-easterly trade winds. The superposition of vertical shear of the Equatorial Roll and TIWs causes asymmetric mixing during northward and southward TIW phases. Key Points: - Composites of Tropical Instability Waves at 0°N, 23°W show a surface (subsurface) velocity maximum during northward (southward) phases - Meridional wind stress forces a seasonally-varying, shallow cross-equatorial overturning cell-the Equatorial Roll - The superposition of Tropical Instability Waves and Equatorial Roll causes asymmetric mixing during north- and southward phases

Description: Regional climate variability in the tropical Atlantic, from interannual to decadal time scales, is inevitably connected to changes in the strength and position of the individual components of the tropical current system with impacts on societally relevant climate hazards such as anomalous rainfall or droughts over the surrounding continents (Bourlès et al., 2019; Foltz et al., 2019). Furthermore, the lateral supply of dissolved oxygen in the tropical Atlantic upper-ocean is closely linked to the zonal current bands (Brandt et al., 2008; Brandt et al., 2012; Burmeister et al., 2020) and especially to the Equatorial Undercurrent (EUC) and its long-term variations with potential implications for regional marine ecosystems (Brandt et al., 2021). The eastward flowing EUC is located between 70 to 200 m depth and forms one of the strongest tropical currents with maximum velocities of up to 1 m s-1 and maximum variability on seasonal time scales (Brandt et al., 2014; Johns et al., 2014). In the intermediate to deep equatorial Atlantic, variability on longer time scales is mainly governed by alternating, vertically-stacked, zonal currents (equatorial deep jets (EDJs); Johnson and Zhang, 2003). At a fixed location, the phases of these jets are propagating downward with time, implying that parts of their energy must propagate upward towards the surface (Brandt et al., 2011). In fact, a pronounced interannual cycle of about 4.5 years, that is associated with EDJs, is projected onto surface parameters such as sea surface temperature or precipitation (Brandt et al., 2011) further demonstrating the importance of understanding equatorial circulation variability and its role in tropical climate variability. While variability in the zonal velocity component on the equator is focused on seasonal to interannual time scales (Brandt et al., 2016; Claus et al., 2016; Kopte et al., 2018), meridional velocity fluctuations dominate the intraseasonal period range (20 to 50 days) due to the presence and passage of westward propagating Tropical Instability Waves (TIWs; Grodsky et al., 2005; Bunge et al., 2007; Wenegrat and McPhaden, 2015; Tuchen et al., 2018; Specht et al., 2021). In general, intraseasonal variability in the central equatorial Atlantic is mainly attributed to TIWs in the upper ocean (Athie and Marin, 2008), while intraseasonal variability in the deep ocean is associated with the signature of equatorial Yanai waves (Ascani et al., 2015; Tuchen et al., 2018, Körner et al., 2022). The observed and modelled interaction between intraseasonal equatorial waves and the aforementioned EDJs was found to maintain the deep equatorial circulation against dissipation (Greatbatch et al., 2018; Bastin et al., 2020) pointing toward the importance of intraseasonal variability for equatorial ocean dynamics. These findings are largely based on, or underpinned by a unique and steadily expanding data set of current velocity observations in the central equatorial Atlantic Ocean. Since 2001, current velocities have been measured almost continuously as part of a multilateral collaboration, the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA), that regularly services a moored observatory located at 0°N/23°W (Bourlès et al., 2019). The significance of this data set is characterized by the length of the time series and by the full-depth coverage of current velocity observations which allow for a detailed analysis of both upper-ocean and deep-ocean dynamics on a wide range of time scales and frequencies. For instance, it enables the decomposition of the current velocity time series into vertical modes pointing toward the existence of resonant basin modes and identifying different sources of deep intraseasonal variability (Brandt et al., 2016; Claus et al., 2016; Greatbatch et al., 2018; Tuchen et al., 2018, Körner et al. under review). Here, we present 20 years of full-depth current velocity observations at 0°N/23°W. The aim of this study is to provide the scientific community with a publicly available reference data set that could be used in manifold ways, including, for instance, the validation of ocean models or reanalysis products.