Tracks generated using a bespoke tracking algorithm, identifying and linking upper-tropospheric vortices (described fully in Hunt et al, 2018, QJRMS - see linked documentation to this record), using data derived from ERA-Interim reanalysis data and selected CMIP5 model runs (with some modifications such as the vorticity level used).

In essence the algorithm works by:

1. locating all mid-tropospheric relative vorticity maxima;

2. group multiple peaks by using a neighbourhood filter, then integrate to find the parent vortex centre;

3. link potential candidates together across time steps to form tracks using a nearest-neighbour approach incorporating local wind speed;

4. surviving tracks are filtered by duration (> 2 days) and location (must pass through [20-36.5N, 60-80E]).