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2. Methods

We use contour and particle advection on isentropic surfaces with winds derived from ECMWF analyses. The contours or particles are either initialized with the isentropic potential vorticity also derived from the analyses (Sections 3-5) or are defined in such a way as to highlight the mixing geometry of the 2-dimensional (isentropic) flow (Section 6). The horizontal resolution of the analyses is spectral T106 (or T213 in section 3), corresponding to a grid-point resolution of approximately 1.3¡ (or 0.65¡ in section 3), and the temporal resolution of the analyses is 6 hourly. The importance of the temporal resolution of the advecting winds for the accurate generation of fine scales is discussed in Waugh & Plumb (1994).

The advection model is the contour advection with surgery model of Dritschel (1989) adapted to advect both contours and particles with the imposed wind field, both forward and backward in time. The advection time-step is half an hour and the advecting winds are interpolated from the six hourly fields using cubic splines as described in Mariotti et al. (2000). Depending on the application, either a 2nd order or a 4th order Runga Kutta scheme is used for the avection.

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