Southern Hemisphere westerlies

Southern Hemisphere westerlies
Name	Southern Hemisphere westerlies
Type	Atmospheric circulation
Location	Southern Hemisphere
Dominant periods	Annual, seasonal, interannual
Related	Roaring Forties, Furious Fifties, Polar Front, Westerlies

Southern Hemisphere westerlies The Southern Hemisphere westerlies are a circumpolar belt of prevailing winds that flow from west to east across the midlatitudes of the Southern Hemisphere, influencing weather, climate, and ocean circulation. These winds interact with major features such as the Antarctic Circumpolar Current, the Southern Annular Mode, and the Falkland Islands region, and they affect regions from the Patagonia coastline to the Tasman Sea and the Southern Ocean. Their behavior links atmospheric centers such as the Amundsen Sea Low, the South Pacific Convergence Zone, and the Ross Sea with variability modes including the El Niño–Southern Oscillation, the Indian Ocean Dipole, and the Pacific Decadal Oscillation.

Overview

The westerly wind belt in the Southern Hemisphere extends broadly between the latitudes of the Roaring Forties and the Furious Fifties, wrapping around continents and ocean basins and influencing islands like the South Georgia and the South Sandwich Islands, the Prince Edward Islands, and the Kerguelen Islands. Historically noted by explorers such as James Cook and navigators associated with the Age of Discovery, these winds aided maritime routes from the Cape of Good Hope to the Cape Horn and played roles in voyages like those of HMS Beagle and expeditions linked to Sir Ernest Shackleton. The westerlies are a key component of the hemispheric circulation connecting the Hadley cell and the Polar cell boundaries while interacting with the Jet Stream near the Antarctic Polar Front.

Atmospheric Dynamics and Formation

The formation of the westerlies arises from latitudinal temperature gradients between regions such as Patagonia, New Zealand, and the Antarctic Peninsula and from conservation of angular momentum around the Earth. Baroclinic instability, cyclogenesis near the South Atlantic Convergence Zone, and eddy-driven momentum transfer—processes studied by researchers at institutions like the National Center for Atmospheric Research and the Commonwealth Scientific and Industrial Research Organisation—generate synoptic systems including lows near the Drake Passage and highs near the South Indian Ocean. Interactions with jets linked to the Southern Hemisphere jet stream and with modes observed in datasets from the ERA-Interim and JRA-55 reanalyses determine the latitude and intensity of the flow. Influential theoretical work by scientists associated with the Wegener Institute and the Scripps Institution of Oceanography has clarified roles of eddy momentum fluxes and stratosphere–troposphere coupling involving the Antarctic ozone hole.

Seasonal and Interannual Variability

Seasonal shifts move the westerly belt poleward in austral summer and equatorward in austral winter, affecting regions such as Chile, Australia, and the South Island (New Zealand). Interannual variability is modulated by the El Niño–Southern Oscillation, the Southern Annular Mode, and teleconnections with the Madden–Julian Oscillation and the Indian Ocean Dipole. Extreme phases influence storm tracks linked to events recorded at observatories like Dunedin Observatory and research platforms operated by the British Antarctic Survey and the Australian Antarctic Division. Paleoceanographic records from the Drake Passage, the Agulhas Current region, and the South Tasman Rise show longer-term shifts tied to epochs studied by teams from the Lamont–Doherty Earth Observatory and the Alfred Wegener Institute.

Climate Impacts and Teleconnections

Changes in the westerlies alter precipitation and temperature patterns across river basins like the Rio de la Plata, the Murray–Darling basin, and the Rio Negro, impacting cities including Buenos Aires, Santiago, and Wellington. They modulate sea ice extent around East Antarctica and the Ross Sea and affect storminess that influences infrastructure in ports such as Port Stanley and Hobart. Teleconnections link westerly shifts to variability in the Southern Annular Mode and to remote impacts on the Amazon Basin and Southern Africa through pathways investigated by groups at the WMO, the IPCC, and universities such as University of Cape Town and University of Tasmania.

Oceanic Interactions and Southern Ocean Dynamics

The westerlies drive the Antarctic Circumpolar Current and influence upwelling zones near the Antarctic Convergence and the Subantarctic Front, affecting carbon uptake and biological productivity examined by expeditions aboard vessels like RV Polarstern and RV Investigator. Wind stress associated with the westerlies modulates the Meridional Overturning Circulation and interacts with western boundary currents including the West Wind Drift and the Agulhas leakage, with implications for heat transport to regions like the South Atlantic Gyre and the Indian Ocean Gyre. Biogeochemical consequences are studied in collaboration with programs such as the Global Ocean Ship-based Hydrographic Investigations Program and the Southern Ocean Observing System.

Observational Evidence and Measurement Methods

Observations use satellite platforms such as ERS-1, ERS-2, ENVISAT, and Sentinel-3, buoy networks like the Argo array, and reanalysis products from NCEP and ECMWF. In situ measurements derive from research stations on Antarctica (e.g., Casey Station, Mawson Station', McMurdo Station) and oceanographic cruises deployed by agencies including the NOAA and CSIRO. Paleoclimate proxies from ice cores at Dome C and Vostok, marine sediment cores from the Drake Passage and the Weddell Sea, and tree-ring chronologies from Patagonia and New Zealand provide multi-century reconstructions used by investigators at institutions like NOAA Paleoclimatology and the British Antarctic Survey.

Human and Ecological Consequences

Shifts in the westerlies influence fisheries off Patagonia, South Georgia, and the Kerguelen Plateau, affecting stakeholders including the Commission for the Conservation of Antarctic Marine Living Resources and national authorities such as the Falkland Islands Government and the New Zealand Ministry for Primary Industries. Changes in storm tracks affect aviation routes over the Southern Ocean and maritime safety for vessels in passages like the Drake Passage and the Roaring Forties trade routes, with economic repercussions for ports like Valparaíso, Port of Melbourne, and Cape Town. Ecological impacts include altered breeding success for species researched by the Antarctic Treaty System signatories, such as Adélie penguin colonies, krill populations, and Southern Ocean seabirds studied by conservation groups including the BirdLife International network.

Category:Atmospheric dynamics Category:Southern Ocean Category:Climate