The Atlantic Subtropical Cells : mean state and variability from an observational perspective
The Atlantic Subtropical Cells (STCs) are shallow wind-driven overturning circulations that consist of poleward Ekman transport from the tropics to the subtropics in the surface layer, subduction in the subtropics and equatorward geostrophic flow at thermocline level. They are eventually closed by equatorial and eastern boundary upwelling. To date, the Atlantic STCs have mainly been investigated in general circulation or data assimilation models while the only observational study has been conducted before the Argo float era. In this thesis, for the first time, the mean state of the horizontal branches of the Atlantic STCs is characterized along 10°N and 10°S based on 13 years of hydrographic data from Argo floats. The interface depth between the surface layer and the thermocline layer is defined by the seasonally varying depth at which the meridional velocity reverses sign from poleward to equatorward (30 - 70m). The lower boundary of the thermocline layer is characterized by another flow reversal at the 26.0 kg m-3 isopycnal. Within the thermocline layer, a mean equatorward transport of about 9 Sv (5 Sv at the western boundary west of 32°W, 4 Sv in the interior) along 10°S is observed. Here, the western boundary transport at 10°S is derived from ship section data because of a transport underestimation by Argo float data. In contrast, only about 3 Sv of equatorward transport are entering the equatorial region at thermocline level along 10°N of which the majority is concentrated at the western boundary west of 50°W. This asymmetric thermocline layer transport convergence is largely balanced by a rather symmetric net transport divergence in the surface layer, where the Ekman divergence exceeds the geostrophic convergence. Overall, the resulting residual of about 3 Sv is attributed to western boundary transport uncertainty at 10°N and diapycnal transport equatorward of 10°N/S associated with the northward return flow of the Atlantic Meridional Overturning Circulation that partly upwells in the tropics. With this newly available Argo float data set, another part of this thesis focuses on the observed transport variability of the individual horizontal branches of the Atlantic STCs with an emphasis on their connection to tropical sea surface temperature (SST) anomalies. For the first time, surface layer and thermocline layer transport time series associated with the Atlantic STCs are derived from Argo float observations. Both layers are dominated by seasonal variability. The thermocline layer convergence varies from 7-14 Sv whereas the surface layer divergence varies from 0 to more than 20 Sv between boreal summer and winter, respectively. Although interannual transport fluctuations at thermocline level are relatively weak, they are suggested to modulate equatorial SST anomalies at time scales of ~5 years and longer. Remarkably, at these time scales, only positive anomalies of the interior part of the thermocline layer transport convergence are leading negative equatorial SST anomalies. At shorter time scales, both SST and interior thermocline layer transport convergence anomalies are rather forced in parallel by local changes of zonal wind stress. Moreover, at thermocline level, the western boundary transport component is anti-correlated with interior transport anomalies on all time scales and in both hemispheres. Due to the small time lags between both transport components, it is suggested that locally forced westward propagating Rossby waves cause the anti-correlation. However, Rossby waves that are forced poleward of the zonal sections at 10°N/S and reach the western boundary to propagate equatorward are expected to distort the initial time lag due to the locally forced Rossby waves. Overall, the results of this thesis provide a new detailed description of the different characteristics of the Atlantic STCs in terms of both their mean state and temporal variability while shedding light on processes that so far have only been observed in the Pacific Ocean or in model studies. It further proves the relevance and capability of Argo float observations within the scope of the Atlantic STC circulation while also addressing their limitations. Eventually, the results aim to provide a benchmark against which general circulation models should be validated.
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