On the Nature and Origin of Atmospheric Annual and Semi-annual Oscillations

Abstract: This paper proposes a joint analysis of variations of global sea-level pressure and of Earth's rotation (RP), expressed as the coordinates of the rotation pole and length of day. Sea-Level-Pressure (SLP) extracted components are a weak trend, eleven quasi-periodic or periodic components ($\sim$130, 90, 50, 22, 15, 4, 1.8 yr), an annual cycle and its first three harmonics. These periods are characteristic of the space-time evolution of the Earth's rotation axis and are present in many characteristic features of solar and terrestrial physics. We focus mainly on the annual and to a lesser extent semi-annual components. Maps of the first three components of SLP (that together comprise more than 85% of the data variance) reveal interesting symmetries. The trend is very stable and forms a triskel structure that can be modeled as Taylor-Couette flow of mode 3. The annual component is characterized by a large negative anomaly extending over Eurasia in the NH Summer (and the opposite in the NH Winter) and three large positive anomalies over Australia and the southern tips of South America and South Africa in the SH Spring (and the opposite in the SH Autumn) forming a triskel. The semi-annual component is characterized by three positive anomalies (an irregular triskel) in the NH Spring and Autumn (and the opposite in the NH Summer and Winter), and in the SH Spring and Autumn by a strong stable pattern consisting of three large negative anomalies forming a clear triskel within the 40$^{\circ}$-60$^{\circ}$ annulus formed by the southern oceans. A large positive anomaly centered over Antarctica, with its maximum displaced toward Australia, and a smaller one centered over Southern Africa complement the pattern.