Southern Annular Mode (SAM)
The Southern Annular Mode (SAM), also known as the Antarctic Oscillation (AAO), is the leading mode of atmospheric variability in the Southern Hemisphere. It refers to the north–south movement of the westerly wind belt that circles Antarctica and influences climate patterns across the Southern Hemisphere. SAM plays a crucial role in regulating weather, precipitation, and temperature distributions over mid-latitude regions, as well as in shaping Antarctic climate conditions.
Definition and Mechanism
SAM is defined as the difference in atmospheric pressure between mid-latitudes (around 40°S) and high latitudes (around 65°S). It describes the zonally symmetric pattern of variability in atmospheric circulation, manifested primarily through shifts in the strength and position of the circumpolar westerly winds.
There are two phases of SAM:
- Positive Phase: The belt of westerly winds contracts towards Antarctica, leading to stronger winds at higher latitudes. This typically results in drier conditions in southern Australia, New Zealand, South America, and southern Africa, while bringing warmer conditions to the Antarctic Peninsula.
- Negative Phase: The westerlies shift towards the equator, weakening over Antarctica but strengthening in the mid-latitudes, often associated with increased rainfall over Australia and South America and colder conditions over Antarctica.
Drivers and Influences
The behaviour of SAM is influenced by several natural and anthropogenic factors:
- Stratospheric Ozone Depletion: The thinning of the ozone layer above Antarctica during the late 20th century has pushed SAM towards its positive phase, especially in summer.
- Greenhouse Gas Emissions: Increasing atmospheric carbon dioxide strengthens the positive phase of SAM by enhancing the poleward shift of westerly winds.
- Natural Variability: Ocean–atmosphere interactions and internal variability also play a role in modulating SAM across different seasons.
Climatic Impacts
SAM exerts wide-ranging effects on climate and weather in the Southern Hemisphere:
- Antarctica: In its positive phase, SAM contributes to warming on the Antarctic Peninsula and cooling over East Antarctica.
- Australia: A positive SAM phase tends to bring reduced winter rainfall in southern Australia but can enhance summer rainfall in eastern regions by drawing in moist air.
- South America and Africa: The mode influences rainfall distribution, with impacts on agriculture and water resources.
- Southern Ocean: Changes in westerly winds driven by SAM affect ocean currents, upwelling, and the exchange of carbon dioxide between the ocean and atmosphere.
Ecological and Environmental Significance
The influence of SAM extends beyond weather to ecological and environmental systems. Stronger circumpolar winds associated with a positive SAM phase increase upwelling of nutrient-rich waters in the Southern Ocean, supporting marine ecosystems. Conversely, altered rainfall and temperature patterns on land influence vegetation, agricultural productivity, and water availability in southern latitudes.
SAM also has implications for the global carbon cycle, as variations in Southern Ocean circulation can affect carbon sequestration. This makes it a critical component in understanding feedback mechanisms between the atmosphere, ocean, and climate.
Monitoring and Prediction
SAM is monitored through atmospheric pressure measurements, reanalysis data, and climate models. Forecasting its phases is essential for seasonal climate prediction, particularly in regions dependent on rainfall variability. Improved understanding of SAM dynamics is crucial for projecting future climate under scenarios of continued greenhouse gas emissions and ozone layer recovery.
The Southern Annular Mode therefore stands as a central atmospheric circulation pattern of the Southern Hemisphere, linking Antarctic climate, mid-latitude weather, and global environmental processes in an interconnected system.