Tropical cyclone Irina over the Mozambique Channel

By Thandiwe Gumede (SAWS)

Tropical cyclone Irina started off as a low pressure system east of Madagascar, on 25 February, and moved towards the west over Madagascar as a high cloud and underlying convective low cloud band that contained ice, as seen on the Natural Colour RGB (Figure 1).

From 26–28 February the centre of the system was over the warm Mozambique Channel, where the strongest storms were confined to the east of the system.

The system resulted in heavy rainfall and flooding over the northern parts of Madagascar and very rough seas between the eastern parts of Mozambique and western parts of Madagascar.

According to the South African Weather Service (SAWS) media release the system was categorised as tropical storm Irina on 29 February with winds reaching up to 65km/h (35kts) and 101km/h (55kts) in places.

Mature storms with large ice particles were located to the south and east of Madagascar on 1 March. Immature storms with strong updrafts and small ice particles were to the north and west of Madagascar, stretching to the northern parts of Mozambique — as shown on the Convection RGB (Figure 2).

Irina intensified into a tropical cyclone on 1 March, with gale force winds — maximum wind speeds up to 101km/h (55kts). However, the system did not have a distinct eye. Later that evening the system weakened to a tropical storm as a result of the storm interacting with the western parts of Madagascar.

From the 2–4 March the tropical storm moved towards the south and south west towards Mozambique and intensified due to the presence of an upper jet on the western edge of the cloud band, that enhanced upper air divergence, in turn enhancing surface convergence and uplift — seen on the Airmass RGB (Figure 3).

Between 5 and 6 March Irina was over the Indian Ocean east of South Africa, with wind speeds of up to 93km/h (50kts). The strongest storms were situated south of the system.

The system moved towards the east, as it was being pushed by a ridging surface high pressure system to the south, as shown on the surface synoptic circulation chart from 6 March (Figure 4).

Between 7 and 8 March the system was still situated over the Mozambique Channel, but on 7 March it started moving towards the west and north west, and on 8 March it moved towards the southwest.

The system further intensified over the warm Mozambique Channel due to the high sea surface temperatures (SST), the latitudinal position which was away from the equator, an unstable atmosphere, low vertical wind shear, moist mid troposphere, upper level high pressure and low surface pressures.

The progression of the system from 29 February 12:00 UTC–5 March 12:00 UTC is shown in the infrared animation (Figure 5). Irina had a cloud band with cold cloud top temperatures of about -77°C.

Irina caused destructive waves along coastal areas (wave heights ranging from 4–6m), as well as heavy rainfall and flooding east of Mozambique, east of Swaziland and east of South Africa.

Figure 6 shows the accumulated rainfall for 5 and 6 March. The KwaZulu Natal province on the east coast of South Africa had up to 30mm in 24 hours on the 5th. Irina then moved towards the east towards the open ocean where the sea surface temperatures were lower and resulting in the system dissipating.