Equatorial deep jets and their influence on the equatorial mean circulation in an idealised model forced by intraseasonal momentum flux convergence

Equatorial deep jets (EDJ) are vertically stacked, downward propagating zonal jets that alternate in direction with depth. In the tropical Atlantic, they have been shown to influence both surface conditions and tracer variability. Despite their importance, the EDJ are absent in most ocean models, most likely due to a lack of vertical resolution. However, when the vertical resolution is sufficiently high, EDJ oscillating at a period of about 4.5 years are present in idealised model configurations driven by steady wind forcing. We have used such a model for a basin-wide reconstruction of the intraseasonal eddy momentum flux convergence, which has recently been shown to be a large contributor to the EDJ maintenance (Greatbatch et al., 2018). When we apply the diagnosed momentum flux convergence that is oscillating at the EDJ period as forcing in a model without wind forcing, EDJ develop, allowing us to verify the mechanism proposed in Greatbatch et al. (2018). Additionally, we can isolate the nonlinear effect that the EDJ have on the time mean zonal flow at intermediate depths. We can show that the EDJ drive time mean zonal flow that is similar in structure to the mean flow measured by Argo floats at 1000 m depth, contributing (in our idealised setup) about one fifth of the total magnitude of the observed flow, the rest likely resulting from direct forcing by downward propagating intraseasonal waves as shown before in other studies.

 

References:

Greatbatch, R. J., M. Claus, P. Brandt, J.-D. Matthießen, F. P. Tuchen, F. Ascani, M. Dengler, J. Toole, C. Roth, J. T. Farrar, 2018: Evidence for the Maintenance of Slowly Varying Equatorial Currents by Intraseasonal Variability. Geophysical Research Letters, 45, 1923-1929.