subtropical jet stream

subtropical jet stream
Name	Subtropical jet stream
Altitude	Upper troposphere to lower stratosphere
Latitude	~20°–35°
Associated with	Hadley cell, Ferrel cell
Typical wind speed	30–120 m/s
Seasonality	Stronger in winter hemisphere
Discovery	Early 20th century balloon and aircraft observations

subtropical jet stream

The subtropical jet stream is a narrow, fast-flowing ribbon of air in the upper troposphere near the boundary of the tropics and midlatitudes. It links large-scale circulations such as the Hadley cell with midlatitude phenomena like the polar jet and midlatitude cyclones, and it modulates weather across regions from the Sahara Desert to the Great Plains (United States), the Mediterranean Sea, and the East Asian Monsoon domain. Variations in its position and strength influence precipitation, storm tracks, and the transport of aerosols and tracers between the tropics and extratropics.

Overview

The subtropical jet is typically centered near 20°–35° latitude in both hemispheres and occurs at pressures between 200–100 hPa, near the tropopause. It coexists with the polar jet at higher latitudes and with tropical easterlies closer to the equator, forming part of the global zonal circulation that includes the Hadley cell, Ferrel cell, and Walker circulation. Historically, understanding of the subtropical jet expanded with upper-air observations from pioneers such as Richard Assmann and later advances by institutions like the Royal Meteorological Society and the US Weather Bureau.

Formation and Dynamics

The subtropical jet arises primarily from conservation of angular momentum in poleward-moving air within the Hadley cell and from the thermal wind response to meridional temperature gradients. Strong subtropical heating over regions like the Sahara Desert, Tibetan Plateau, and Sonoran Desert modifies the vertical temperature profile, enhancing the jet through the thermal wind balance. Baroclinic adjustment processes studied by researchers at places such as the Lamont–Doherty Earth Observatory and Scripps Institution of Oceanography further refine jet intensity. Rossby wave dynamics associated with features like the Rocky Mountains and Andes create localized jet streaks and influence jet exit and entrance regions.

Structure and Variability

Vertically, the subtropical jet often shows a core near the tropopause with secondary maxima in the lower stratosphere. Zonal asymmetries arise from land–sea contrasts exemplified by the Mediterranean Sea, Gulf of California, and the Bay of Bengal, and from orographic forcing by mountain ranges such as the Himalayas and Atlas Mountains. Seasonal migration occurs: in boreal winter the jet shifts equatorward and intensifies under the influence of the Siberian High and the Aleutian Low, while in boreal summer it weakens and moves poleward, interacting with the Monsoon trough and the Intertropical Convergence Zone. Interannual variability links to climate modes like the El Niño–Southern Oscillation, the North Atlantic Oscillation, the Pacific Decadal Oscillation, and the Indian Ocean Dipole.

Interactions with Other Atmospheric Circulation

The subtropical jet interfaces dynamically with the polar jet, facilitating energy and momentum exchange through Rossby wave breaking and jet–jet interactions documented in work by the National Center for Atmospheric Research and European Centre for Medium-Range Weather Forecasts. It modulates the propagation of tropical plumes tied to Hurricane Katrina-type extratropical transition events and affects the routing of atmospheric rivers connected to regions such as California and the Iberian Peninsula. Stratosphere–troposphere exchanges, including sudden stratospheric warming events studied at NCAR and NOAA, can feed back onto subtropical jet strength and latitude, altering midlatitude weather regimes.

Climatic and Weather Impacts

Shifts in the subtropical jet govern precipitation patterns across subtropical and midlatitude regions, influencing droughts in the Southwestern United States and flooding over the Indian subcontinent and Southeast China. The jet’s position determines storm-track pathways that affect the United Kingdom, the Mediterranean Basin, and the Gulf Stream corridor. Long-term changes in jet behavior are monitored for links to anthropogenic forcing examined by groups like the Intergovernmental Panel on Climate Change and studies on stratospheric ozone recovery overseen by the World Meteorological Organization. Extreme weather outcomes, from prolonged heatwaves over the European Union to cold snaps impacting the Russian Federation, have been associated with persistent subtropical jet anomalies.

Observations and Measurement

Observational records combine radiosonde networks operated by agencies such as the Japan Meteorological Agency, the UK Met Office, and NOAA with satellite remote sensing from instruments on NOAA polar-orbiting satellites, METEOSAT, and the GOES series. Aircraft reconnaissance, including research flights by NOAA Aircraft Operations Center and field campaigns by the National Science Foundation, provide in situ profiling. Reanalysis datasets assembled by ECMWF and NASA integrate these data to produce continuous global depictions of subtropical jet variability. Ground-based lidar and microwave radiometers supplement upper-air observations in key regions like the Tibetan Plateau.

Representation in Weather and Climate Models

Global climate models developed at centers such as GFDL, CESM (NCAR), Hadley Centre (UK Met Office), and MPI-M aim to resolve the subtropical jet but face challenges in accurately simulating jet latitude, strength, and seasonal migration. Model biases arise from parameterizations of convection (investigated by NOAA Geophysical Fluid Dynamics Laboratory), land–atmosphere coupling over the Amazon Basin, and stratospheric processes. High-resolution regional models and coupled atmosphere–ocean ensembles improve fidelity for regional impacts studied in projects like the Coupled Model Intercomparison Project and national assessments by agencies including EPA and USGS.

Category:Atmospheric circulation