  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.
