Forcing of the Atlantic Equatorial Deep Jets Derived from Observations

Abstract

The equatorial deep jets (EDJs) are a 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 238W, the vertically propagating EDJs are best described as superimposed oscillations of the 13th to the 23rd 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 238W yields a basinwide 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 to 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, requires the phase of the forcing of the EDJs to propagate downward.

The emergence of equatorial deep jets in an idealised primitive equation model: an interpretation in terms of basin modes

“…This shows the strong zonal coherence of the jets and also the slight tendency to tilt down towards the east, consistent with Fig. 6 in the reconstruction of Claus et al (2016) and the analysis of Youngs and Johnson (2015) based on data from Argo floats.…”

“…The experiments show the development of vertically alternating zonal jets along the equator that are analogous to the equatorial deep jets (EDJs) that have been observed, for example in the equatorial Atlantic (Brandt et al 2011;Claus et al 2016). We noted that the energy in the model runs associated with the zonal velocity along the equator is mostly confined to frequencies associated with resonant basin modes (Cane and Moore 1981;Yang and Liu 2003), a feature of the solution we have been able to exploit in order to describe the emergence of the EDJs in the model solutions.…”

“…The analogy with basin modes was first noted by Ascani et al (2006) in an idealised model experiment but has since become generally accepted as a model for the observed EDJs (D'Orgeville et al 2007;Greatbatch et al 2012;Claus et al 2014;Youngs and Johnson 2015;Claus et al 2016). Indeed, basin modes appear to be important for describing not only the EDJs in the equatorial Atlantic Ocean but also the semi-annual and annual cycles .…”

“…The EDJs are a major feature of the equatorial current system and consist of a series of vertically stacked zonal jets that alternate in direction with depth. The Atlantic EDJs have a vertical length scale of around 300-700 m and their period is estimated to be 5 ± 1 years by Johnson and Zhang (2003), later refined to 4.5 years by Brandt et al (2011) with an uncertainty of about 150 days estimated by Claus et al (2016) for the, then available, 10 year time series. The observed downward phase propagation of the EDJs implies, according to linear theory, an upward energy propagation, a feature confirmed in a nonlinear model study by Matthießen et al (2015) (see their Fig.…”

“…This mechanism implies that deep jets are supported by the intraseasonal waves over a considerable depth range. In fact Claus et al (2016) have diagnosed the forcing for the deep jets from the mooring data at 23 • W on the equator and find that the implied power input that maintains the deep jets takes place over the whole depth range occupied by the jets. Concerning the vertical phase propagation of the jets, we note that in the particular case studied by D' Orgeville et al (2007), the vertical phase propagation is not in agreement with observations, although it should be noted that their model runs are shorter than those considered by Ascani et al (2015) and Matthießen et al (2015) and possibly do not show fully developed EDJ cycles.…”