Figure 3.29. (Top) Time series of the NPI (sea level pressure during December through March averaged over the North Pacific, 30°N to 65°N, 160°E to 140°W) from 1900 to 2005 expressed as normalised departures from the long-term mean (each tick mark on the ordinate represents two standard deviations, or 5.5 hPa). This record reflects the strength of the winter Aleutian Low pressure system, with positive (negative) values indicative of a weak (strong) Aleutian Low. The bars give the winter series and the smooth black curves show decadal variations (see Appendix 3.A ). Values were updated and extended to earlier decades from Trenberth and Hurrell (1994). (Bottom) As above but for SSTs averaged over the tropical Indian Ocean (10°S–20°N, 50°E –125°E; each tick mark represents two standard deviations, or 0.36°C). This record has been inverted to facilitate comparison with the top panel. The dashed vertical lines mark years of transition in the Aleutian Low record (1925, 1947, 1977). Updated from Deser et al. (2004).