Data have been supplied by Phil Jones.

The North Atlantic Oscillation (NAO) is one of the major modes of variability of the Northern Hemisphere atmosphere. It is particularly important in winter, when it exerts a strong control on the climate of the Northern Hemisphere. It is also the season that exhibits the strongest interdecadal variability. For winter, the difference between the normalised sea level pressure over Gibraltar and the normalised sea level pressure over Southwest Iceland is a useful index of the strength of the NAO. Jones et al. (1997) used early instrumental data to extend this index back to 1823.

Below is a timeseries of the winter (December to March average) of the Jones et al. NAO index, updated to the winter of 2011/12. Note the upward trend from the 1960s to the early 1990s, but also that the trend has not been sustained and has significant year-to-year variability superimposed on it. Note also that the winter 2009/10 had the most negative NAO index measured during the almost 190-year record.

Monthly values of the NAO index up to December 1999 are available on our website. Below is a repeat of the 1999 values and also values through to January 2013. (Note that this Gibraltar minus Iceland version of the NAO index is really most applicable to the winter half of the year.)

Monthly NAO index
Year	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
1999	+0.90	+1.80	-0.72	+0.43	+1.03	+1.39	-1.85	-3.67	-0.51	-0.69	+0.30	+2.13
2000	+0.35	+4.37	+0.54	-3.34	+0.31	+0.89	-2.99	+0.78	-1.10	+2.26	-0.24	-1.41
2001	+0.02	+0.07	-0.68	+1.24	-0.09	-1.33	-1.12	+1.64	-3.83	+0.88	+0.01	-2.25
2002	+2.31	+3.01	+0.09	+0.91	-0.05	+0.90	-0.71	-0.61	-3.58	-1.50	-0.27	-0.98
2003	+0.15	+1.34	+1.08	-1.74	+1.17	-0.86	+0.09	-0.99	+0.35	-3.68	+0.31	-0.85
2004	+0.20	-1.23	+1.07	+1.08	-0.67	-0.38	-0.30	-0.76	+2.51	-2.18	-0.55	+1.27
2005	+1.82	-2.25	-1.29	+0.71	-0.13	-1.00	-0.08	+0.94	+0.50	-0.45	-1.01	-0.81
2006	-0.10	-1.24	-1.12	+0.57	-0.22	-0.41	+0.83	-2.47	-1.02	-1.97	+1.70	+3.08
2007	+1.77	+0.42	+2.03	-0.10	+0.62	-3.34	-1.05	-3.41	-1.18	-0.02	-1.67	+1.42
2008	+1.87	+1.81	+0.37	-2.02	-3.26	-2.05	-1.38	-0.21	-2.07	+0.01	-1.30	-0.58
2009	+0.61	-1.43	+0.15	+1.74	+1.52	-3.05	-0.92	+1.07	-0.63	-2.00	+1.68	-3.72
2010	-2.38	-3.25	-0.80	-1.03	-1.66	-2.40	+0.06	-2.01	-2.38	-2.41	-3.34	-4.62
2011	-1.38	+2.79	-0.44	+2.39	+1.08	-1.58	-3.39	-0.18	+2.97	+1.45	+0.74	+3.20
2012	+2.05	+1.28	+1.78	-2.36	-0.83	-2.58	-1.31	-0.44	-1.44	-3.21	-1.11	+0.60
2013	+1.08

A useful winter season is the December to March average of these values (see Osborn et al., 1999), which is shown in the timeseries above. Recent values of the DJFM NAO index are shown below.

Winter (DJFM) NAO index
Year	Index
1994/5	+2.44
1995/6	-2.32
1996/7	+0.18
1997/8	+0.80
1998/9	+0.98
1999/2000	+1.85
2000/1	-0.50
2001/2	+0.79
2002/3	+0.40
2003/4	-0.20
2004/5	-0.11
2005/6	-0.82
2006/7	+1.83
2007/8	+1.37
2008/9	-0.31
2009/10	-2.54
2010/1	-0.91
2011/2	+2.08
2012/3	+0.84 (Dec-Jan only)

References

• Jones PD, Jonsson T and Wheeler D (1997) Extension to the North Atlantic Oscillation using early instrumental pressure observations from Gibraltar and South-West Iceland. Int. J. Climatol. 17, 1433-1450.
• Osborn TJ (2004) Simulating the winter North Atlantic Oscillation: the roles of internal variability and greenhouse gas forcing. Clim. Dyn. 22, 605-623.
• Osborn TJ (2006) Recent variations in the winter North Atlantic Oscillation. Weather 61, 353-355.
• Osborn TJ (2011) Winter 2009/2010 temperatures and a record-breaking North Atlantic Oscillation index. Weather 66, 19-21.