Southern Annular Mode

Southern Annular Mode. It is a dominant pattern of climate variability in the Southern Hemisphere, describing the north-south movement of the westerly wind belt that circles Antarctica. This oscillation involves a large-scale exchange of atmospheric mass between the mid-latitudes, centered around 40–50°S, and the polar regions, influencing weather and ocean circulation across the hemisphere. Its phase is characterized by changes in the surface pressure difference between these zones, driving significant shifts in storm tracks, temperature, and rainfall.

Definition and characteristics

The primary characteristic is a seesaw in atmospheric pressure between the mid-latitude and polar regions of the Southern Hemisphere. During its positive phase, pressures are below average over Antarctica and above average over the Southern Ocean mid-latitudes, strengthening and contracting the westerly winds toward the pole. Conversely, the negative phase features higher-than-average pressures over the Antarctic continent and lower pressures in the mid-latitudes, weakening and expanding the westerly wind belt equatorward. This meridional shift of the wind and pressure systems is annular, or ring-like, encircling the pole. The associated changes in wind stress have profound effects on the Antarctic Circumpolar Current and sea ice extent.

Climate impacts

Its phases directly affect surface climate across southern continents and oceans. A positive phase typically brings drier conditions to southern Australia, particularly Tasmania and parts of Victoria, while increasing rainfall to southwestern New Zealand and the western Andes of Patagonia. It also influences temperatures, often causing warming over the Antarctic Peninsula and cooling over much of mainland Australia. The negative phase generally reverses these precipitation and temperature patterns, bringing increased rainfall to southern South Africa and southeastern Australia. These shifts alter storm tracks and the intensity of weather systems like those affecting Cape Horn.

Mechanisms and drivers

The fundamental mechanism involves internal atmospheric dynamics and interactions with the ocean and sea ice. It is driven by eddy-mean flow interactions, where transient eddies in the atmosphere reinforce the mean wind and pressure patterns. External drivers can modulate its behavior, including variability in tropical sea surface temperatures from phenomena like El Niño–Southern Oscillation, which can project forcing into the Southern Hemisphere extratropics. Changes in stratospheric circulation, such as sudden stratospheric warming events or the seasonal breakdown of the polar vortex, can also propagate downward to influence its phase. Furthermore, ocean-atmosphere coupling in the Southern Ocean plays a key feedback role.

Trends and climate change

A significant positive trend has been observed since the mid-20th century, largely attributed to anthropogenic forcing including ozone depletion and increased greenhouse gas concentrations. The depletion of the Antarctic ozone hole has been a major driver, strengthening the polar vortex and intensifying the westerly winds, leading to a more frequent positive state. This trend has contributed to observed changes such as the warming of the Antarctic Peninsula, drying trends in southwestern Australia, and alterations in Southern Ocean carbon uptake. Climate models from the Intergovernmental Panel on Climate Change project a continued positive trend under future scenarios, with implications for Antarctic ice sheet stability and hemispheric climate.

Measurement and indices

It is quantified using indices derived from atmospheric pressure data. The most common index is based on the difference in normalized pressure anomalies between 40°S and 65°S latitudes, using reanalysis products from centers like the National Centers for Environmental Prediction and the European Centre for Medium-Range Weather Forecasts. Daily, monthly, and seasonal indices are calculated, with long-term records extending back to the late 19th century using station data from locations like Orcadas Base and Hobart. Other methods use zonal mean sea level pressure or geopotential height at 700 hPa. These indices are maintained and disseminated by research institutions including the British Antarctic Survey and NOAA. Category:Climate patterns Category:Atmospheric science Category:Antarctica