Numerical Modeling of Agulhas Retroflection and Ring Formation with Isopycnal Outcropping

Abstract

Abstract Experiments with a pure-isopycnic coordinate numerical model on a domain representing the oceanic region within a few thousand kilometers of South Africa and in which isopycnal outcropping is allowed around the edges of the Agulhas Current Retroflection and Agulhas rings are described. Previous experiments with a quasi-isopycnic coordinate model have exhibited a vigorous retroflection and active ring formation for certain parameter ranges. But mean layer thickness choices have been such that isopycnal outcropping was confined to the model subpolar gyre interior to the southwest of the retroflection. In order to make more close comparison with observation and with analytical theory describing the behavior of warm lenses, we invoke the unique aspect of the pure-isopycnic coordinate model which allows coordinate surfaces to outcrop as do density surfaces in frontal zones. Four new experiments are described, proceeding from a 3-layer case—in which the first interface density jump is decreased by 50% compared to previous experiments—to a 5-layer case with three layers in the mean upper 250 m. Isopycnal outcropping along the subtropical convergence and around the edge of the Indian Ocean subtropical gyre is a primary feature of three of the new experiments. In addition, some characteristics of ring formation are more realistic than in previously published experiments: e.g., it is more frequent and is typically west or southwest of the Agulhas Dank tip as opposed to on the southeast side. The new experiments are generally less energetic. Energy input by the wind is pumped into the lower layers and dissipated more efficiently. For very thin upper layers, an inequity develops between the mean upper layer thicknesses in the Indian and Atlantic Oceans: the top layer fluid tends to become trapped in the Atlantic Ocean. This, in turn, has an impact on the dynamics of the retroflection and ring formation. These must, therefore, be viewed as preliminary outcropping experiments. It is concluded that, in the future, some account must be made for thermodynamic forcing, by which upper layer Atlantic Ocean water downwells and returns along intermediate isopycnals to the Indian Ocean where it then upwells back into the top layer.