Observed transport variability of the Atlantic Subtropical Cells and their impact on tropical sea-surface temperature variability

The Atlantic Subtropical Cells (STCs) are shallow wind-driven overturning circulations connecting the tropical upwelling areas with the subtropical subduction regions. In both hemispheres they are characterized by equatorward transport at thermocline level, upwelling at the equator and poleward Ekman transport in the surface layer. STCs are suggested to impact sea surface temperature variability in tropical upwelling regions on interannual to decadal time scales through the variability either in STC transport and/or hydrographic properties. Here we present a 21st century mean state of the horizontal branches of the Atlantic STCs. Argo float data and repeated ship sections show that the equatorward part of the STCs can be observed between the 26.0 kg m-3 isopycnal and a seasonally varying upper boundary (30-70 m). Transport estimates within this layer reveal that the southern hemisphere contributes about 3 times more to the transport convergence between 10°N and 10°S than the northern hemisphere. In contrast, poleward transports in the surface layer driven by the Ekman divergence are rather symmetric. Overall, a residual transport of about 3 Sv remains. This missing transport could either be linked to diapycnal transport across the 26.0 kg m-3 isopycnal, as part of the Atlantic Meridional Overturning Circulation which partly upwells in the tropics, or to uncertainties of the transport estimates, particularly at the western boundary at 10°N. From 2010 to 2017, both Ekman divergence and thermocline layer convergence between 10°N and 10°S suggest an increase in STC transport with a dominating contribution from the northern hemisphere. The observations further show opposing thermocline layer transports at the western boundary and in the interior basin that are partly compensating each other. Implications of the increase in STC transport and variability of the STC hydrographic variability in the tropical Atlantic will be discussed

Deep Intraseasonal Variability in the Central Equatorial Atlantic

Besides the zonal flow that dominates the seasonal and long-term variability in the equatorial Atlantic, energetic intraseasonal meridional velocity fluctuations are observed in large parts of the water column. We use 15 years of partly full-depth velocity data from an equatorial mooring at 23°W to investigate intraseasonal variability and specifically the downward propagation of intraseasonal energy from the near-surface into the deep ocean. Between 20 and 50 m, intraseasonal variability at 23°W peaks at periods between 30 and 40 days. It is associated with westward-propagating tropical instability waves, which undergo an annual intensification in August. At deeper levels down to about 2000 m considerable intraseasonal energy is still observed. A frequency–vertical mode decomposition reveals that meridional velocity fluctuations are more energetic than the zonal ones for periods < 50 days. The energy peak at 30–40 days and at vertical modes 2–5 excludes equatorial Rossby waves and suggests Yanai waves to be associated with the observed intraseasonal energy. Yanai waves that are considered to be generated by tropical instability waves propagate their energy from the near-surface west of 23°W downward and eastward to eventually reach the mooring location. The distribution of intraseasonal energy at the mooring position depends largely on the dominant frequency and the time, depth, and longitude of excitation, while the dominant vertical mode of the Yanai waves plays only a minor role. Observations also show the presence of weaker intraseasonal variability at 23°W below 2000 m that cannot be associated with tropical instability waves

The Atlantic Subtropical Cells inferred from observations

The Atlantic Subtropical Cells (STCs) are shallow wind‐driven overturning circulations connecting the tropical upwelling areas to the subtropical subduction regions. In both hemispheres they are characterized by equatorward transport at thermocline level, upwelling at the equator and poleward Ekman transport in the surface layer. This study uses recent data from Argo oats complemented by ship sections at the western boundary as well as reanalysis products to estimate the meridional water mass transports and to investigate the vertical and horizontal structure of the STCs from an observational perspective. The seasonally varying depth of meridional velocity reversal is used as the interface between the surface poleward ow and the thermocline equatorward ow. The latter is bounded by the 26.0 kg m‐3 isopycnal at depth. We find that the thermocline layer convergence is dominated by the southern hemisphere water mass transport (9.0 ±1.1 Sv from the southern hemisphere compared to 2.9 ±1.3 Sv from the northern hemisphere) and that this transport is mostly confined to the western boundary. Compared to the asymmetric convergence at thermocline level, the wind‐driven Ekman divergence in the surface layer is more symmetric, being 20.4 ±3.1 Sv between 10°N and 10°S. The net poleward transports (Ekman minus geostrophy) in the surface layer concur with values derived from reanalysis data (5.5 ±0.8 Sv at 10°S and 6.4 ±1.4 Sv at 10°N). A diapycnal transport of about 4 Sv across the 26.0 kg m‐3 isopycnal is required in order to maintain the mass balance in the STC circulation

Deep Intraseasonal Variability in the Central Equatorial Atlantic

Besides the zonal flow that dominates the seasonal and long-term variability in the equatorial Atlantic, energetic intraseasonal meridional velocity fluctuations are observed in large parts of the water column. 15 years of full-depth velocity data from an equatorial mooring at 23°W are used to investigate intraseasonal variability and specifically the downward propagation of intraseasonal energy from the surface into the deep ocean. Near the surface (20 to 50 m), intraseasonal variability at 23°W peaks at periods between 30 to 40 days. It is associated with westward propagating Tropical Instability Waves, which undergo an annual intensification in August. Enhanced energy levels of equatorial intraseasonal variability are observed down to about 2000 m. A frequency-vertical mode decomposition shows that meridional velocity fluctuations are more energetic than the zonal ones for periods < 50 days. The energy peak at 30 to 40 days and vertical modes 2 to 5 excludes equatorial Rossby or gravity waves and suggests Yanai waves to be associated with the observed intraseasonal energy. Yanai waves that are considered to be generated by Tropical Instability Waves propagate their energy from near the surface west of 23°W down- and eastward to eventually reach the mooring location. The distribution of intraseasonal energy depends largely on the dominant frequency and the time, depth, and longitude of excitation with the dominant vertical mode of the Yanai waves playing only a minor role. Observations also indicate the presence of weaker intraseasonal variability at 23°W below 2000 m that is not associated with Tropical Instability Waves

Maintenance of the seasonal cycle and the interannual variability by intra-seasonal stochastic variability in the equatorial Atlantic

The variability of the zonal circulation along the equator in the Atlantic Ocean is dominated by the seasonal cycle and the presence of the equatorial deep jets (EDJs). The seasonal cycle is externally driven by surface wind variability, however the mechanism which generates and maintains the EDJs against dissipation is not fully understood yet. Additionally, intra-seasonal stochastic variability, the tropical instability waves (TIWs), is generated in the upper ocean by both baroclinic and barotropic instability. The intra-seasonal energy at the equator reaches to depths of about 2000 m. We argue that the intra-seasonal variability gets distorted by the presence of the lower frequency zonal velocity variability. This causes a systematic convergence of intra-seasonal momentum flux such that the seasonal cycle and the EDJs are maintained against dissipation. The presence of this mechanism is demonstrated from two OGCM simulations and moored observations at 23W in the equatorial Atlantic