Wavelet Transform

[nickbsb]

In "Wavelet Transform",TC98 says explicitly that "this red-noise was estimated from( α1+√α2) /2 where α1and α2 are the lag-1 and lag-2 autocorrelations of the Niño3 SST". So this is NOT a constant value, valid for each and every time series, but one which must be estimated. when α2<0,α=?, how to compute it? Thank you!