The Atlantic Subtropical Cells Inferred from Observations Tuchen, Franz Philip; Lübbecke, Joke F.; Schmidtko, Sunke ...
Journal of geophysical research. Oceans,
November 2019, 2019-11-00, 20191101, Letnik:
124, Številka:
11
Journal Article
Recenzirano
Odprti dostop
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 floats 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 flow and the thermocline equatorward flow. 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 3 Sv across the 26.0 kg m−3 isopycnal is required in order to maintain the mass balance of the STC circulation.
Plain Language Summary
The Atlantic Subtropical Cells (STCs) are shallow wind‐driven overturning circulations connecting the tropics to the subtropical regions within the upper 300 m. In both hemispheres, they are characterized by equatorward transport at subsurface level and poleward transport in the surface layers. They are closed by upwelling at the equator and subduction in the subtropics. STCs are suggested to impact sea‐surface temperature variability in tropical upwelling regions thereby influencing, for example, precipitation patterns. The boundary between the two branches is approximated by the depth at which the meridional velocity reverses. The lower boundary of the deep equatorward branch is defined by an isoline of potential density. We find that at subsurface level, the equatorward branches converge in the tropics with more transport coming from the southern hemisphere. At the surface, a more symmetric divergence of water mass is observed in the tropics. The surface layers are also influenced by geostrophic transport generally counteracting the wind‐driven divergence. In total, the net surface divergence and the subsurface convergence yield a residual. It is suggested that this water mass volume deficit originates from below the STCs and enters the subsurface layers in the tropics where it is lifted to the surface.
Key Points
Equatorward and poleward transports associated with the Atlantic Subtropical Cells are estimated from observations and reanalysis data
Estimates show asymmetry in thermocline transports (three times more transport from the south) and symmetric flow divergence at the surface
Transport budget reveals a residual of 3 Sv likely linked to the upper‐layer flow of the Atlantic meridional overturning circulation
Abstract
In the equatorial Atlantic Ocean, meridional velocity variability exhibits a pronounced peak on intraseasonal time scales whereas zonal velocity dominantly varies on seasonal to interannual ...time scales. We focus on the intraseasonal meridional velocity variability away from the near-surface layer, its source regions, and its pathways into the deep ocean. This deep intraseasonal velocity variability plays a key role in equatorial dynamics as it is an important energy source for the deep equatorial circulation. The results are based on the output of a high-resolution ocean model revealing intraseasonal energy levels along the equator at all depths that are in good agreement with shipboard and moored velocity data. Spectral analyses reveal a pronounced signal of intraseasonal Yanai waves with westward phase velocities and zonal wavelengths longer than 450 km. Different sources and characteristics of these Yanai waves are identified: near the surface between 40° and 10°W, low-baroclinic-mode Yanai waves with periods of around 30 days are excited. These waves have a strong seasonal cycle with a maximum in August. High-frequency Yanai waves (10–20-day period) are excited at the surface east of 10°W. In the region between the North Brazil Current and the Equatorial Undercurrent, high-baroclinic-mode Yanai waves with periods between 30 and 40 days are generated. Yanai waves with longer periods (40–80 days) are shed from the deep western boundary current. The Yanai wave energy is carried along beams eastward and downward, thus explaining differences in strength, structure, and periodicity of the meridional intraseasonal variability in the equatorial Atlantic Ocean.
Significance Statement
Past studies show that intraseasonal meridional kinetic energy is important for the deep equatorial circulation (DEC). However, numerical studies use intraseasonal variability with varying characteristics to investigate the formation and maintenance of the DEC. This is partly because of sparse observations at depth that are limited to single locations. This study investigates intraseasonal meridional kinetic energy in the equatorial Atlantic in a high-resolution ocean model that is tested against available shipboard and moored observations. We analyze the spatial and temporal distribution and the baroclinic structure of intraseasonal variability. Using the model, we identify different sources and pathways of intraseasonal energy in the deep equatorial Atlantic. We offer groundwork for further studies on the formation and maintenance of the DEC.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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