AO Index (SV NAM index)

24 November, 2023
Yoshihiro Tachibana (Mie University)

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The definition of AO index (SV NAM index)
1) EOF was applied to a temporal covariance matrix of geopotential height fields for individual calendar months using a zonally averaged monthly geopotential height field from 1000 to 200 hPa for the area poleward of 40N.
2) The daily time series of the AO index (SV NAM index) is obtained by projecting daily zonal mean geopotential height anomalies onto the EOF of each month.

Details are shown in Ogi et al. (2004), Tachibana et al. (2010), and Otomi et al. (2013).

Notice and background of AO index (SV NAM index)
The AO was first defined by Thompson and Wallace (2000) which is based on an invariant EOF spatial pattern throughout the year,and Ogi et al. (2004) identified seasonal variations of the Northern Hemisphere annular mode (SV NAM) from 1958 to 2002 by performing an empirical orthogonal function (EOF) analysis. Ogi et al. (2004) and Tachibana et al. (2010) demonstrated that in winter, but not in summer, the SV NAM accords well with the AO defined by Thompson and Wallace (2000) and used by the Climate Prediction Center of the U.S. National Oceanic and Atmospheric Administration (NOAA/CPC). Ogi et al. (2005), Tachibana et al. (2010), and Otomi et al. (2013) also demonstrated that the SV NAM successfully captures anomalous summertime weather conditions associated with blocking anticyclones, such as the hot summers in Europe in 2003 and 2010, whereas the original AO of Thompson and Wallace (2000), mainly reflects atmospheric variabilities in winter and cannot capture such a hot summer. Therefore, Ogi et al. (2005) redefined the summertime SV NAM as the summer AO. Therefore, we further renamed the SV NAM index defined by Ogi et al. (2004) as the AO index.

Ogi, M., Y. Tachibana and K. Yamazaki (2003) Impact of the wintertime North Atlantic Oscillation (NAO) on the summertime atmospheric circulation. Geophysical Research Letters, 30, 1704. doi: 10.1029/2003GL017280. (Link)

Ogi, M., K. Yamazaki and Y. Tachibana (2003) Solar cycle modelation of the seasonal linkage of the North Atlantic Oscillation (NAO). Geophysical Research Letters, 30, 2170. doi: 10.1029/2003GL018545. (Link)

Ogi, M., K. Yamazaki and Y. Tachibana (2004) The summertime annular mode in the Northern Hemisphere and its linkage to the winter mode. Journal of Geophysical Research, 109:D20114. doi: 10.1029/2004JD004514. (Link)

Ogi, M., Y. Tachibana and K. Yamazaki (2004) The connectivity of the winter North Atlantic Oscillation (NAO) and the summer Okhotsk high. Journal of the Meteorological Society of Japan, 82, 905-913. doi: 10.2151/jmsj.2004.905. (Link)

Ogi, M., K. Yamazaki and Y. Tachibana (2005) The summer northern annular mode and abnormal summer weather in 2003, Geophysical Research Letters, 32, L04706. doi: 10.1029/2004GL021528. (Link)

Otomi, Y., Y. Tachibana and T. Nakamura (2013) A possible cause of the AO polarity reversal from winter to summer in 2010 and its relation to hemispheric extreme summer weather. Climate Dynamics, 40, 1939-1947, doi: 10.1007/s00382-012-1386-0. (Link)

Ando, Y., M. Ogi and Y. Tachibana (2015) Abnormal Winter Weather in Japan during 2012 Controlled by Large-Scale Atmospheric and Small-Scale Oceanic Phenomena. Monthly Weather Review, 143, 54-63, doi: 10.1175/MWR-D-14-00032.1. (Link)

Tachibana, Y., T. Nakamura, H. Komiya and M. Takahashi (2010) Abrupt evolution of the summer Northern Hemisphere annular mode and its association with blocking. Journal of Geophysical Research, 115:D12125. doi: 10.1029/2009JD012894. (Link)

Thompson, D. W. J. and J. M. Wallace (2000) Annular Modes in the Extratropical Circulation. Part I: Month-to-Month Variability. Journal of Climate, 13:1000-1016. doi: 10.1175/1520-0442(2000)013<1000:AMITEC>2.0.CO;2. (Link)