A Reexamination of the Mechanism of the Semiannual Oscillation in the Southern Hemisphere

Abstract

The cause of the semiannual oscillation (SAO) at middle and high southern latitudes, proposed by van Loon, is reexamined using observations and general circulation model (GCM) simulations. The model results and the more recent observed data [sea surface temperature (SST), ocean heat storage, temperature profiles in the upper ocean, and atmosphere transient eddy momentum and heat fluxes] support van Loon's original hypothesis that the mechanism involves the different annual cycles of temperature between the Antarctic polar continent and the surrounding midlatitude southern oceans. A strong semiannual oscillation is noted in the observed atmospheric transient eddy momentum flux in southern midlatitudes corresponding with the two times of year when the circumpolar trough around Antarctica is most intense. The products of the dynamical coupling of ocean and atmosphere?the annual cycle of SST near 50°S and associated ocean beat storage?are important to the amplitude and phase of the SAO in the atmosphere. GCM simulations are analyzed to provide insights into the consequences of changing elements of the ocean forcing near 50°S. The GCM simulation with the specified annual cycle of SSTs has the correct phase of the SAO but reduced amplitude. The model with a simple mixed-layer ocean (shallow fixed depth with no dynamics) produces an altered annual cycle of SSTs and ocean heat storage at 50°S and a similarly altered SAO as a consequence. These model results, along with the observed upper-ocean temperature profiles and heat storage values, suggest that changes in the annual cycle of SST and ocean heat storage near 50°S could lead to a modulation of the observed SAO and affect its role in the El Niño?Southern Oscillation and the Indian monsoon.