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| Southern Hemisphere jet stream | |
|---|---|
| Name | Southern Hemisphere jet stream |
| Type | Atmospheric jet stream |
| Location | Southern Hemisphere |
| Altitude | Upper troposphere and lower stratosphere |
| Typical speed | 30–150 km/h |
| Orientation | West-to-east |
| Primary influences | Antarctic Circumpolar Current; El Niño–Southern Oscillation; Southern Annular Mode |
Southern Hemisphere jet stream is a dominant high-altitude westerly airflow encircling the mid to high latitudes of the Southern Hemisphere. It strongly influences the climates of Antarctica, Australia, New Zealand, Chile, and South Africa and interacts with major oceanic and atmospheric features such as the Southern Ocean, Antarctic Circumpolar Current, and El Niño–Southern Oscillation. The jet stream modulates storm tracks, precipitation regimes, and aviation routes and is a key component in understanding Southern Hemisphere variability across seasonal to decadal timescales.
The jet is a narrow, fast-flowing ribbon of air located near the tropopause between the troposphere and stratosphere that encircles southern midlatitudes downstream of the Antarctic Peninsula and across the Tasman Sea and South Pacific Ocean. Its existence follows from strong meridional temperature gradients between polar and subtropical air masses near features like Patagonia and the Roaring Forties. The jet is often described in association with pressure systems such as the Southern Annular Mode and the circumpolar troughs surrounding Antarctica, and it is monitored by agencies including the Bureau of Meteorology (Australia), MetService (New Zealand), and NOAA.
The jet consists of core maxima in wind speed often situated over the midlatitudes where the thermal wind balance arises from horizontal temperature contrasts near convergent zones like the Subtropical Convergence and frontal zones off Cape Horn. Baroclinic instability, aided by planetary waves linked to the Antarctic Oscillation, supports cyclogenesis along the jet and shapes eddy momentum fluxes that feed back on the mean flow. Interactions with orography—most prominently the Andes Mountains and the Great Dividing Range—produce downstream wave trains and localized jet splitting, while transient synoptic-scale disturbances tied to the South Pacific Convergence Zone modulate jet streaks and ageostrophic circulations.
Seasonal migration and intensity changes occur as the jet shifts poleward in austral summer and equatorward in austral winter, influenced by the seasonal heating contrast between Antarctica and lower latitudes and by tropical-extratropical exchanges near the South Atlantic Convergence Zone. Regional modulation arises from oceanic boundary conditions: sea surface temperature anomalies associated with El Niño–Southern Oscillation events induce systematic displacement and amplification of the jet over the Tasman Sea and Southeast Pacific. Interannual modes such as the Indian Ocean Dipole and the Pacific Decadal Oscillation also correlate with regional jet anomalies that affect rainfall over Victoria (Australia), Auckland, and Buenos Aires.
The jet is central to teleconnection patterns linking polar processes and remote climate responses. Positive phases of the Southern Annular Mode are associated with a poleward-shifted, stronger jet, altering storm tracks that impact Tasmania, Falkland Islands, and South Africa coastlines. Coupling with the El Niño–Southern Oscillation modifies atmospheric bridges between the tropical Pacific and southern midlatitudes, producing downstream anomalies over Mediterranean Basin-like climates in Chile and rainfall changes in New Zealand's South Island. Interactions with modes such as the Antarctic Circumpolar Wave and stratospheric events like Sudden stratospheric warming episodes can propagate influence through the troposphere, producing multi-seasonal climate signals.
The jet governs the genesis and trajectory of extratropical cyclones that deliver frontal precipitation, strong winds, and temperature shifts to populated regions including Perth, Canberra, Santiago, Chile, and Cape Town. Its jet streaks enhance severe weather and heavy rainfall when phasing with upper-level troughs near coastal headlands like Cape Agulhas. For aviation, the jet presents both hazards and opportunities: transoceanic flight planning for carriers such as Qantas, LATAM Airlines, and Air New Zealand exploit tailwinds for fuel savings while avoiding jet-induced clear-air turbulence that can threaten aircraft on routes between Buenos Aires and Adelaide or over the South Pacific Ocean.
Monitoring relies on radiosonde launches from stations including Davis Station (Antarctica), Casey Station, and Macquarie Island, supplemented by satellite-borne instruments on platforms like NOAA-19 and MetOp, and by reanalysis products from ECMWF and NCEP. Research campaigns using dropsondes from aircraft operated by agencies such as NASA and the Australian Antarctic Division provide high-resolution cross-sections, while scatterometer data and GPS radio occultation help map wind and temperature structure. Long-term measurement continuity is maintained via international programs including the World Meteorological Organization upper-air network and the Global Climate Observing System.
Observational and model-based studies link anthropogenic forcing from greenhouse gas increases and ozone depletion to changes in the jet's position and strength, with large-scale ozone recovery and rising greenhouse concentrations exerting competing influences. Trends over recent decades have included a poleward shift and intensification in certain seasons, affecting precipitation patterns in South America and Southern Africa and altering sea-ice extent around East Antarctica. Coupled climate models from intercomparison projects coordinated by IPCC simulate continued changes under higher-emission pathways that imply further jet modulation, with consequential impacts on midlatitude storm tracks, coastal flooding risk in regions like New Zealand and Chile, and global atmospheric circulation.
Category:Atmospheric dynamics