The amplitude of the pattern defined by the leading empirical orthogonal function of winter monthly mean NH MSLP anomalies poleward of 20 ° N (Thompson and Wallace, 1998, 2000). (ipcc.ch)
The difference in average MSLP between SH middle and high latitudes (usually 45 ° S and 65 ° S), from gridded or station data (Gong and Wang, 1999; Marshall, 2003), or the amplitude of the leading empirical orthogonal function of monthly mean SH 850 hPa height poleward of 20 ° S (Thompson and Wallace, 2000). (ipcc.ch)
The NPI is the average MSLP anomaly in the Aleutian Low over the Gulf of Alaska (30 ° N — 65 ° N, 160 ° E — 140 ° W; Trenberth and Hurrell, 1994) and is an index of the PDO, which is also defined as the pattern and time series of the first empirical orthogonal function of SST over the North Pacific north of 20 ° N (Mantua et al., 1997; Deser et al., 2004). (ipcc.ch)