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Triskels and Symmetries of Mean Global Sea-Level Pressure

The evolution of mean sea-level atmospheric pressure since 1850 is analyzed using singular spectrum analysis. Maps of the main components (the trends) reveal striking symmetries of order 3 and 4. The northern hemisphere (NH) displays a set of three positive features, forming an almost perfect equilateral triangle. The southern hemisphere (SH) displays a set of three positive features arranged as an isosceles triangle, with a possible fourth (weaker) feature. This geometry can be modeled as Taylor-Couette flow of mode 3 (NH) or 4 (SH). The remarkable regularity and order three symmetry of the NH triskel occurs despite the lack of cylindrical symmetry of the northern continents. The stronger intensity and larger size of features in the SH is linked to the presence of the annular AAO. In addition to the dominant trends, quasi-periodic components of 130, 90, 50, 22, 15, 4, 1.8, 1, 0.5, 0.33, and 0.25 years, i.e. the Jose, Gleissberg, Hale and Schwabe cycles, the annual cycle and its first three harmonics are identified.