Southern Annular Mode

Southern Annular Mode
Name	Southern Annular Mode
Abbreviation	SAM
Other names	Antarctic Oscillation
Region	Southern Hemisphere
Timescale	Interannual to decadal
Primary impacts	Mid-latitude circulation, storm tracks, precipitation, Antarctic climate

Southern Annular Mode The Southern Annular Mode is a dominant mode of atmospheric variability in the Southern Hemisphere that describes north–south shifts of the mid-latitude jet and storm tracks. It links atmospheric pressure anomalies between the Antarctic region and the mid-latitudes, influencing circulation patterns that affect Antarctica, Australia, New Zealand, Argentina, and South Africa. The index captures zonal pressure differences and is central to understanding Southern Hemisphere climate variability, seasonal weather anomalies, and connections to oceanic and cryospheric systems.

Definition and Indexing

The Southern Annular Mode is defined as the leading empirical orthogonal function of austral extratropical sea level pressure or geopotential height, representing an annular pattern of pressure anomalies centered on Antarctica and extending toward the mid-latitudes. Index formulations include the zonal mean sea level pressure difference between 40°S and 65°S, principal component time series from reanalysis datasets such as ERA-Interim, NCEP/NCAR, and JRA-55, and projections of geopotential height at 500 hPa used in studies by institutions like the National Center for Atmospheric Research, CSIRO, and the Met Office. Alternate indices sometimes reference the Antarctic Oscillation terminology used in paleoclimate reconstructions involving ice cores from Vostok and Dome C.

Physical Mechanisms and Variability

SAM represents variability in the strength and latitude of the Southern Hemisphere subtropical and polar jets and the associated storm tracks influenced by baroclinic instability, eddy fluxes, and planetary-scale wave interactions. Mechanistic drivers include stratosphere–troposphere coupling, annular mode dynamics described in theories by researchers at Princeton University and MIT, and teleconnections with tropical forcing such as the El Niño–Southern Oscillation and the Madden–Julian Oscillation. Interactions with the Southern Ocean circulation, sea ice extent around the Weddell Sea and Ross Sea, and ozone-related radiative changes in the stratosphere modulate SAM's amplitude and phase on seasonal to decadal timescales.

Observed Impacts on Climate and Weather

Phases of the Southern Annular Mode alter mid-latitude storm frequency, track position, and intensity, producing measurable consequences for precipitation, temperature, and sea ice distribution across Southern Hemisphere regions. Positive SAM phases are linked to poleward-shifted storm tracks, enhanced westerlies off Tasmania and the Patagonia coast, and warmer conditions on parts of Antarctic Peninsula; negative phases often yield equatorward storm tracks and increased precipitation over parts of southern Australia and southern Chile. Impacts extend to oceanic processes such as upwelling on the Peru–Chile Trench and mixed layer changes in the South Atlantic Gyre, with consequential effects on fisheries, coastal erosion, and cryospheric mass balance in glaciers like those in the Falkland Islands and Prince Edward Islands.

Trends, Drivers, and Climate Change Connections

Instrumental and reanalysis records reveal trends toward a more positive Southern Annular Mode during the late 20th century, particularly in summer, attributed in part to stratospheric ozone depletion identified by studies involving World Meteorological Organization assessments and Intergovernmental Panel on Climate Change reports. Recovery of the ozone hole following the Montreal Protocol and increasing greenhouse gas concentrations from industrial sources both influence SAM trends, with climate model ensembles from the Coupled Model Intercomparison Project exploring future projections. Natural variability associated with modes like Indian Ocean Dipole and Pacific decadal oscillations modulates these trends, complicating attribution in regional climate impacts.

Regional Effects (Antarctic, Australia, South America, Southern Africa, New Zealand)

Antarctic: SAM phases alter sea ice distribution around the Ross Sea and Amundsen Sea, modulating katabatic winds and surface mass balance on the Antarctic ice sheet, with implications for ice-shelf stability near Pine Island Glacier and Thwaites Glacier. Australia: Positive SAM typically brings drier winters and springs to southeast Australia and impacts rainfall in the Murray–Darling Basin and western Tasmania, affecting agriculture and water resources managed by agencies such as the Bureau of Meteorology. South America: In southern Chile and Argentina, SAM variations shift frontal passage frequency across the Patagonian Andes, influencing snowpack, glacier mass balance in the Southern Patagonian Ice Field, and river runoff to the Atlantic Ocean. Southern Africa: SAM modulates moisture transport to the Cape Province and affects rainfall variability over the Western Cape and KwaZulu-Natal, interacting with synoptic systems like cut-off lows tracked by regional centers including South African Weather Service. New Zealand: Changes in the westerly wind belt associated with SAM influence storminess, precipitation distribution across the South Island and North Island, and alpine snow accumulation affecting hydropower managed by entities like Mercury NZ.

Measurement, Modeling, and Predictability

Measurement of SAM relies on instrumental networks including surface pressure stations, radiosonde records, satellite retrievals from platforms like NOAA satellites and reanalysis products produced by ECMWF and NCEP. Climate models of varying complexity, from idealized atmospheric general circulation models at Geophysical Fluid Dynamics Laboratory to coupled Earth system models in CMIP6, simulate SAM behavior, sensitivity to forcings, and predictability. Seasonal forecast systems and data assimilation techniques improve projection skill by incorporating stratospheric ozone chemistry, sea surface temperature boundary conditions, and initialization approaches used at centers such as BoM, Met Office Hadley Centre, and NOAA Geophysical Fluid Dynamics Laboratory.

Category:Climate patterns