Zonal wave 3 amplitude (v250)

Amplitude of zonal wavenumber 3 in the 40–65°S mean 250-hPa meridional-wind anomaly. In the spirit of Goyal et al. (2022), who use EOFs of 500-hPa v; the store has v only at 925/850/250/200 hPa, so 250 hPa (equivalent-barotropic) and a Fourier fit replace them. — Goyal et al. 2022, J. Climate 35, 5137–5149, doi:10.1175/JCLI-D-21-0927.1