The Inter-tropical Convergence Zone (ITCZ)

By Jochen Kerkmann (EUMETSAT)

The Inter-tropical Convergence Zone (ITCZ) is the region near the Equator where the north-easterly and south-easterly tradewinds converge, forming an often continuous band of clouds or thunderstorms. The location of the ITCZ zone varies seasonally, following the Sun's zenith point from the Northern to the Southern Hemisphere.

This annual variation is strongest over the continents and less pronounced over the oceans (over the Atlantic Ocean, the ITCZ usually stays north of the Equator). Sometimes, a double ITCZ forms, with a ridge of high pressure forming between the two convergence zones, one of which is usually stronger than the other.

In satellite images the ITCZ can be easily recognised as an elongated band of clouds several degrees in latitude across (see upper image below). Thus, satellite images play a crucial role in monitoring and understanding the ITCZ. However, from visible and infrared imagery alone it is difficult to find the exact position of the surface convergence line (where the tradewinds converge), which is an important piece of information for shipping in general.

For this, microwave measurements of the ocean surface are needed as provided by scatterometer instruments. EUMETSAT's Metop-A polar-orbiting satellite carries the Advanced Scatterometer (ASCAT) instrument, which provides ocean surface wind measuremenst at 25 and 50 km resolution.

The lower image shows the ASCAT scatterometer winds (in arrows), superimposed upon an infrared satellite image (from Meteosat-9), corresponding to the RGB composite image shown above (see wind product overlaid on RGB image). The exact data acquisition time is plotted in red next to the satellite swath. A yellow dot means that the KNMI quality control flag is set (most likely strong rain influenced the measurements).

According to this scatterometer wind product, the position of the main ITCZ (in terms of surface convergence line) is at about 4-5 degrees north, at the northern edge of the band of thunderstorm clouds visible in the RGB composite image. A second, more subtle, convergence line is visible further south, close to the Equator.