Description: Seasonal variability of the tropical Atlantic circulation is dominated by the annual cycle, but semi-annual variability is also pronounced, despite weak forcing at that period. Here we use multi-year, full-depth velocity measurements from the central equatorial Atlantic to analyze the vertical structure of annual and semi-annual variations of zonal velocity. A baroclinic modal decomposition finds that the annual cycle is dominated by the 4th mode and the semi-annual cycle by the 2nd mode. Similar local behavior is found in a high-resolution general circulation model. This simulation reveals that the annual and semi-annual cycles of the respective dominant baroclinic modes are associated with characteristic basin-wide structures. Using an idealized linear reduced-gravity model to simulate the dynamics of individual baroclinic modes, it is shown that the observed circulation variability can be explained by resonant equatorial basin modes. Corollary simulations of the reduced-gravity model with varying basin geometry (i.e. square basin versus realistic coastlines) or forcing (i.e. spatially uniform versus spatially variable wind) show a structural robustness of the simulated basin modes. A main focus of this study is the seasonal variability of the Equatorial Undercurrent (EUC) as identified in recent observational studies. Main characteristics of the observed EUC including seasonal variability of transport, core depth, and maximum core velocity can be explained by the linear superposition of the dominant equatorial basin modes as obtained from the reduced-gravity model.

Description: The Equatorial Deep Jets (EDJs) are an ubiquitous feature of the equatorial oceans; in the Atlantic Ocean, they are the dominant mode of interannual variability of the zonal flow at intermediate depth. On the basis of more than 10 years of moored observations of zonal velocity at 23°W, the vertically propagating EDJs are best described as superimposed oscillations of the 13th to the 23th baroclinic modes with a dominant oscillation period for all modes of 1650 days. This period is close to the resonance period of the respective gravest equatorial basin mode for the dominant vertical modes 16 and 17. It is argued that since the equatorial basin mode is composed of linear equatorial waves, a linear reduced gravity model can be employed for each baroclinic mode, driven by spatially homogeneous zonal forcing oscillating with the EDJ period. The fit of the model solutions to observations at 23°W yields a basin wide reconstruction of the EDJs and the associated vertical structure of their forcing. From the resulting vertical profile of mean power input and vertical energy flux on the equator, it follows that the EDJs are locally maintained over a considerable depth range, from 500-2500 m, with the maximum power input and vertical energy flux at 1300 m. The strong dissipation closely ties the apparent vertical propagation of energy to the vertical distribution of power input and, together with the EDJs’ prevailing downward phase propagation, require the phase of the forcing of the EDJs to propagate downward.

Description: An intensive three-dimensional survey of the Antarctic Polar Front was made in the Drake Passage in March 1976. The front, which was imbedded within one of the high-velocity cores of the circumpolar current, is viewed as a water mass boundary demarking the northern extent of near-surface antarctic waters. Within the front, water masses are observed to intrude, one above the other, with characteristic vertical scales of 50–100 m. The intrusions are horizontally anisotropic, being elongated in the alongstream direction and constrained primarily to the upper 800 m of the front. The spatial and temporal persistence of the variability is examined through the analysis of continuous vertical profiles of horizontal velocity, temperature, salinity, and oxygen with discrete sampling of nutrients. Analysis of the velocity data showed the mean current flowing to the NNE with speeds of the order of 30–40 cm s−1 in the upper 600 m, with temporal variability over a 28-hour ‘yo-yo’ due primarily to internal gravity waves. The thermohaline variability was not internal wave induced but rather was associated with nearly isentropic advection of different water masses across the front. Cold fresh and warm salty intrusions did not conserve potential density, however, and double-diffusive transfers are strongly suggested as being crucial to an understanding of the dynamics of the intrusions. Applying a model (Joyce, 1977) for lateral mixing we estimate poleward temperature and salinity fluxes due to interleaving of 0.086°C cm s−1 and 0.069‰ cm s−1, respectively. If these values are typical, interleaving could play a significant role in large-scale balance of salt and, to a lesser extent, heat for the Southern Ocean.

Description: Climate variability in the tropical Atlantic Ocean is determined by large-scale ocean–atmosphere interactions, which particularly affect deep atmospheric convection over the ocean and surrounding continents1. Apart from influences from the Pacific El Niño/Southern Oscillation2 and the North Atlantic Oscillation3, the tropical Atlantic variability is thought to be dominated by two distinct ocean–atmosphere coupled modes of variability that are characterized by meridional4, 5 and zonal6, 7 sea-surface-temperature gradients and are mainly active on decadal and interannual timescales, respectively8, 9. Here we report evidence that the intrinsic ocean dynamics of the deep equatorial Atlantic can also affect sea surface temperature, wind and rainfall in the tropical Atlantic region and constitutes a 4.5-yr climate cycle. Specifically, vertically alternating deep zonal jets of short vertical wavelength with a period of about 4.5 yr and amplitudes of more than 10 cm s−1 are observed, in the deep Atlantic, to propagate their energy upwards, towards the surface10, 11. They are linked, at the sea surface, to equatorial zonal current anomalies and eastern Atlantic temperature anomalies that have amplitudes of about 6 cm s−1 and 0.4 °C, respectively, and are associated with distinct wind and rainfall patterns. Although deep jets are also observed in the Pacific12 and Indian13 oceans, only the Atlantic deep jets seem to oscillate on interannual timescales. Our knowledge of the persistence and regularity of these jets is limited by the availability of high-quality data. Despite this caveat, the oscillatory behaviour can still be used to improve predictions of sea surface temperature in the tropical Atlantic. Deep-jet generation and upward energy transmission through the Equatorial Undercurrent warrant further theoretical study.

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.