Notes on Jet Streams

by Joe Archive on May 29, 2013

Based on the Meted unit “Jet Streams”

Jet Streams

 

Near the tropopause, corridors of very strong winds.

 

Fastest: Polar front jet and Subtropical jet, flow from west to east.

There is also a Tropical Easterly Jet in the upper troposphere.

 

Jets defined where wind speeds exceeds 50 knots (25m/s)

 

Also lower tropospheric jets. e.g. Somali Jet and African Easterly Jet

 

Upper Level Jets

General Characteristics.

 

Polar and Sub-tropical jets meander wave-like around the globe. Climatological means are in midlatitudes, but they experience excursions into the tropics and the polar regions.

 

Direction of flow?

On a global scale, the tropical heating (from solar radiation) creates a meridional gradient in temperature. This creates a pressure gradient in the upper troposphere (high near the equator), which causes upper air to flow towards the poles.

This flow curves eastwards due to the Coriolis effect, forming the upper tropospheric westerlies in the mid latitudes.

 

The Polar Jet

Found at latitudes between 30 and 70 degrees, and between the 300 and 200 hPa pressure surfaces (7.5–11 km altitude).

Strongest during winter, may migrate tot he tropics and merge with the subtropical jet.

 

The Polar front jet forms in the region of strong temperature gradients between cold polar air and warmer sub-tropical air. The gradients become sharper with altitude, because of the greater vertical expansion of the warmer columns of air – leading to a stronger pressure gradient at altitude and strong winds (through geostrophy?).

 

Why near the tropopause? Because of the change in vertical temperature gradient at the tropopause, it is the location of the strongest horizontal gradients.

 

Strong winds may exist wherever there are strong thermal gradients – so jet streams are commonly associated with active frontal systems in the midlatitudes.

 

Jet Streaks

Local maxima, called jet streaks, occur along the jet. They are especially common when the polar and subtropical jets merge, or in regions with strong temperature gradients.

 

Usually identified as oval-shaped maxima in 300–200hPa wind speeds.

 

When the jet stream is mostly zonal, short wavelength disturbances and jet streaks move quickly along the track. However, when the jet stream is more meridional in direction, upper level troughs and ridges are common -> warm air flows poleward and cool air flows equator ward, and the disturbances generally move slowly. These disturbance can generate strong temperature gradients over short longitude ranges and create cyclones or anticyclones that cause persistent wet and dry conditions.

 

The Subtropcial Jet

 

Unusally strongest near 30 degrees on at 200hPa, has strong vertical wind shear in the upper troposphere. The jet is only weakly recognizable below about 500hPa.

 

During the winter, the STJ is nearly continuous and can attain speeds up to 100m/s. In the NH, the jet exhibits a quasi-stationary 3-wave pattern, with ridges and high wind speeds occurring over the SE USA, the Mediterranean and the NW pacific. The troughs are usually located over the central pacific, the central atlantic and between the arabian sea and india.

 

The STJ exists all year in the SH, but it is intermittent int he NH summer.

The STJ pattern is more zonal and stronger in the SH because of the smaller land masses there (and the lack of orography?)

 

STJ usually identified in satellite images as a plume of high-level clouds.

 

Results from air flowing upwards and polewards int he hadley cell. As the air parcels move to smaller latitude circles, their velocity increases through conservation of angular momentum. Large-scale eddies transport some of the momentum from the Hadley cell to mid-latitudes and small-scale turbulence slows air parcels. The transfer of zonal momentum and KE by the eddies helps to maintain the STJ.

 

Tropical Easterly Jet. A unique and dominant feature of the NH summer over southern asia and northern africa. Found between 5 and 20 N, with maximum speeds around 40–50 m/s, at ~150hPa.

 

During the south asian summer monsoon, the TEJ induces secondary circulations that enhance convection over south india, the maritime continent and nearby, while suppressing upward motion in its exit region over Africa and the Atlantic.

 

Not fully understood, may be caused by the strong solar heating at altitude occurring on the tibetan plateau.

 

 

Low level Jets

Somali Jet. Occurs during summer (northern) over north madagascar and off the coast of Somalia. Most intense from June–August, with average speeds ~18m/s (daily speeds can reach 50m/s).

 

*Maximum wind speed occurs near the northern tip of madagascar and off the somali coast.

*A major cross-equatorial flow from the southern IO to the central Arabian sea.

*A relative minimum in speed along the axis of strong winds near the equator.

*A split in the axis of the jet over the arabian sea.

 

The AEJ is a low-level wind maximum that is one of the major feature of the atmosphere over northern tropical africa and the eastern atlantic during northern summer.

 

Has a maximum between 700 and 600 hPa and 13–17N, with wind speeds 10-25m/s

Strongest during West african monsoon June–Sept.

 

Existence of the jet is associated with the strong reversal of the typical meridional temperature gradient during the summer. Rather than being warmer near the equator, the sahara desert creates a dry and dusty SAL and an elevated warm anomaly that is north of the cooler air above the equator. This creates a strong easterly geotropic flow.

The mountains of north-central africa can create perturbations in the low-level easterlies, accelerating the winds on the lee side.

 

Synoptic weather.

Mid latitude cyclones.

 

When waves form along the jet streams, strong temperature gradients develop over limited longitude ranges and result in cyclones and anti-cyclones.

 

Where low and high pressure regions develop is influenced by the location of jet streaks – rising (sinking) motion is induced by divergence (convergence) at the entrance and exit regions of the streaks.

 

e.g. Divergence n the right entrance region induces low level convergence, accelerating the low-level winds and creating rising motions along the sloped isentropes towards the divergent region. This lowers surface pressure, leading to cyclogenesis.

 

Turbulence

Jetstreams cause strong shear, both horizontally and vertically –> turbulence. Generally this occurs in clear air.

 

AEJ and Easterly waves.

 

AEWs are the primary synoptic systems over tropical north africa and the atlantic during the warm season. They are often precursors tropical cyclones.

 

Variability

 

The positions of the STJ and the Polar Jet shift dramatically during the ENSO cycle, in response to the SST anomalies and the atmospheric circulation changes over the tropical pacific.

 

 

Previous post:

Next post: