easyclimate.field.teleconnection.index_ao_nam

easyclimate.field.teleconnection.index_ao_nam#

The Arctic Oscillation (AO)/ Monthly Northern Hemisphere Annular Mode (NAM) Index

The Arctic Oscillation (AO) Index (or Monthly Northern Hemisphere Annular Mode (NAM) Index) is a key metric used to describe large-scale atmospheric variability in the Northern Hemisphere, particularly influencing mid-to-high latitude weather patterns. It is defined by the leading mode of Empirical Orthogonal Function (EOF) analysis of sea-level pressure (SLP) anomalies north of 20°N. The AO Index quantifies fluctuations in atmospheric pressure between the Arctic and mid-latitudes, with positive and negative phases reflecting distinct circulation patterns. In the positive phase, lower Arctic pressure and higher mid-latitude pressure strengthen westerly winds, confining cold air to polar regions, often leading to milder winters in North America and Europe. The negative phase, with higher Arctic pressure and weaker winds, allows cold air to move southward, causing colder, stormier weather in these regions.

The AO’s role in climate variability is significant, as it modulates temperature and precipitation, especially in winter. The AO Index, typically derived from monthly or seasonal SLP data, reflects the strength of the polar vortex, with positive values indicating a stronger vortex and negative values a weaker one. It is closely linked to the North Atlantic Oscillation (NAO) due to shared variability patterns.

The AO’s fluctuations are driven by internal atmospheric dynamics, stratospheric processes, and external forcings like sea surface temperatures. Its teleconnections make it a critical factor in seasonal weather predictions and long-term climate modeling. In a warming climate, Arctic amplification may alter AO dynamics, making its study essential for understanding future climate trends.

See also

  • Thompson, D. W. J., & Wallace, J. M. (1998). The Arctic oscillation signature in the wintertime geopotential height and temperature fields. Geophysical Research Letters, 25(9), 1297–1300. https://doi.org/10.1029/98gl00950

  • Fang, Z., Sun, X., Yang, X.-Q., & Zhu, Z. (2024). Interdecadal variations in the spatial pattern of the Arctic Oscillation Arctic center in wintertime. Geophysical Research Letters, 51, e2024GL111380. https://doi.org/10.1029/2024GL111380

  • Li, J., and J. X. L. Wang (2003), A modified zonal index and its physical sense, Geophys. Res. Lett., 30, 1632, doi: https://doi.org/10.1029/2003GL017441, 12.

  • Thompson, D. W. J. , & Wallace, J. M. . (1944). The Arctic oscillation signature in the wintertime geopotential height and temperature fields. Geophys. Res. Lett., doi: https://10.1029/98GL00950

Functions

calc_index_AO_EOF_Thompson_Wallace_1998(...)

The calculation of monthly mean Arctic Oscillation (AO) index using empirical orthogonal functions (EOFs) method over the entire Northern Hemisphere:

calc_index_NAH_zonal_lat_Li_Wang_2003(...[, ...])

The calculation of Monthly Northern Hemisphere Annular Mode (NAM) Index using normalized monthly zonal-mean sea level pressure (SLP) between 35°N and 65°N.