1. Any one of the semi-permanent highs and lows that appear on mean charts of sea level pressure. As originally used by L. Teissenenc de Bort in 1881, this term was applied to maxima and minima of pressure on daily charts. The main centers of action in the Northern Hemisphere are the Icelandic low, the Aleutian low, the Azores high and/or Bermuda high, the Pacific high, the Siberian high (in winter), and the Asiatic low (in summer). Other less intense or less consistent mean systems may be considered. Fluctuations in the nature of these centers are intimately associated with relatively widespread and long-term weather changes. 2. As used by Sir Gilbert Walker, any region in which the variation of any meteorological element is related to weather of the following season in other regions.