Blessing, S., Fraedrich, K., Junge, M., Kunz, T., & Lunkeit, F. (2005). Daily North-Atlantic
Oscillation (NAO) index: Statistics and its stratospheric polar vortex dependence. Meteorologische
Zeitschrift, 14(6), 763-769. doi:10.1127/0941-2948/2005/0085.