Jets in the South Atlantic

By Jochen Kerkmann (EUMETSAT)

In general, the Southern Hemisphere has a more zonal flow than the Northern Hemisphere and winds tend to be very strong around a latitude of 40 to 60 degrees South (the roaring fourties). The main reasons for this are a stronger north-south temperature contrast (because of the Antarctic) and the absense of disturbing land masses with mountain barriers.

A good example of a zonal flow in the South Atlantic is given in the Meteosat-8 image below. This image is presented as an Airmass RGB composite, which combines four infrared channels into one RGB image (see recipe of this RGB combination. In this RGB composite, high clouds appear as white, mid-level clouds as light ochre and cloud-free areas as green (warm air mass with high tropopause) to blue colours (cold air mass with low tropopause).

The dominant feature on these images is the long frontal cloud band that stretches from South America right across the Southern Atlantic. It indicated the presence of the southern polar front and the related jet stream. The jet axis itself is located right on the southern edge of this cloud band, between the cloud band and the red stripe that indicates dry, descending air (see Colour Interpretation. The polar front separates the polar airmass (in blue colours) from the sub-tropical airmass (in green colours). The distinct colour separation is due to the use of the IR9.7–IR10.8 brightness temperature difference (BTD) on the green colour beam of the RGB composite. Polar airmasses with a lower tropopause have a much larger negative BTD (around -40K) than tropical and sub-tropical airmasses with a high tropopause (around -25K).

Another interesting feature on this image is the orange stripe with some fibrous, high-level clouds attached to it some 15 degrees further to the north of the polar jet. This stripe represents the sub-tropical jet, at the poleward edge of the Hadley cell, which is usually located at a latitude of around 30 degrees south. In this particular case, the sub-tropical jet is nearly parallel to the polar jet.

As for the polar jet, due to the cross-circulation around the sub-tropical jet there is rising motion on the northern side of the jet axis (indicated by the fibrous high clouds) and sinking motion on the southern side (indicated by the orange colour that indicates dry, descending air).