North Atlantic Oscillation (NAO) Index

The NAO index (NAOI) is defined as the difference in the normalized monthly sea level pressure (SLP) regionally zonal-averaged over the North Atlantic sector from 80°W to 30°E between 35°N and 65°N (Li and Wang, 2003). The NAOI captures well large-scale circulation features of the NAO, and is essentially a measure of the intensity of zonal winds across the central North Atlantic between 35°N to 65°N. A systematic comparison of NAO indices shows that the NAOI provides a much more faithful and optimal representation of the spatial-temporal variability associated with the NAO, suggesting the NAOI as a better choice for describing and monitoring variability of the broad-scale NAO and for diagnosing relationships between the NAO and global climate variations (Li and Wang, 2003). The daily NAO index (NAOI) is defined as same as the monthly NAO index (Li and Wang, 2003) except for the daily resolution.

Correlation coefficients between monthly sea level pressure and (a), the NAOI (Li and Wang, 2003), (b), NAOI_R (Rogers, 1984), (c), NAOI_H (Hurrell, 1995), (d), NAOI_J (Jones et al., 1987), (e), NAOI_M (Moses et al., 1987), and (f), NAOI_B (Barnston & Livezey, 1987) in the Northern Hemisphere for the period 1958-2000. The contour interval is 0.1. The pink (blue) shading indicates significant at the 99.9% confidence level.

The North Atlantic Oscillation (NAO) Index

The NAO index. Top. DJF (1872-2010); Middle. JJA (1873-2010); Bottom. Annual (1873-2010)

The yellow lines indicate 7-year Gaussian-type filtered values.

Related References

• Li, J. P., and J. X. L. Wang, 2003: A new North Atlantic Oscillation index and its variability. Adv. Atmos. Sci., 20(5), 661-676. PDF
• Li, J. P., 2005: Coupled air-sea oscillations and climate variations in China, in: Climate and Environmental Evolution in China (First Volume), Edt. D. Qin, Beijing: China Meteorological Press, 324-333. PDF