Just in January 2021, southern Africa (SAF) was hit by 2 major weather systems, namely the tropical storm (TS) #TS_Chalane and the tropical cyclone (TC) #TC_Eloise.
#TS_Chalane (Fig.1) formed on the 19th of December 2020, in a region approximately 1000 km north-east of Mauritius. It made landfall over Madagascar on the 26th of December and therefore it weakened, nevertheless, as it progressed westwards and passed over the Mozambique Channel it was revived and eventually made landfall over Mozambique as a TS on the 30th of December near the Beira region, having wind speeds approximately 110 km/h.
Figure 1: Chalane’s track.
The formation of Chalane was favored by the co-occurrence of a Kelvin wave and of an equatorial Rossby wave. Kelvin waves have been found to exert a strong impact on the formation of cyclones by modulating potential vorticity, total column water vapor and vertical wind shear (Carl and Schreck, 2015). Cyclonic vorticity is also greatly enhanced by the passage of an equatorial Rossby wave (Gall et. Al., 2010). In addition, the warm sea surface temperatures (SSTs) over the region favored convection. According to News Central Africa, 7 people lost their lives, while more than 11.000 people in Mozambique were severely affected.
#TC_Eloise (Fig.2) formed on the 16th of January as a tropical depression and on the 17th of January it was upgraded to a moderate tropical storm. On the 19th it was categorized as a severe tropical storm and later on the same day it made landfall over Madagascar as a moderate tropical storm. After its passage over the Mozambique Channel it was strengthened and became a Category-1 cyclone. The high SSTs and the low vertical wind shear helped Eloise to become a Category-2 cyclone and eventually on the 23rd it made landfall over Mozambique with wind speeds that reached 160 km/h.
Figure 2: Eloise’s track.
Eloise caused the death of 21 people and further affected 270.000 people across SAF. Severe flooding events destroyed houses, settlements and farmlands.
During December-January-February (DJF) the southern hemisphere experiences its summer season. Sea surface temperatures (SSTs) over the Indian Ocean rise (Fig. 3), favoring convection and the formation of depressions that can evolve to tropical storms and/or to tropical cyclones.
Figure 3: Mean monthly sea surface temperature (Celsious) during 1982-2020 over south-western Indian Ocean
The westerly winds blowing over the Indian Ocean towards SAF, help to drift TCs along. If SSTs, vertical wind shear and vorticity fields are conducive, TC survive and they make landfall over Madagascar. Usually, after landfall their strength is reduced, but as they transverse Madagascar and pass over the Mozambique Channel they are strengthened again, making landfall mainly over Mozambique. However, several landlocked countries are also affected, since landfalling TCs navigate through Zimbabwe, Botswana and even Namibia. Once TCs make landfall their intensity is severely reduced, nonetheless they still cause severe flooding events and excess economic damage to local populations and economy. January 2021 can be listed among the wettest Januaries over SAF (Fig. 4).
Figure 4: Monthly precipitation anomaly (mm/month) for January over southern Africa. Anomalies are calculated using the 1983-2012 climatology.
Data Source: https://www.tamsat.org.uk/
It must be highlighted that the largest burden of impacts caused by tropical cyclone activity over SAF is placed on Madagascar and Mozambique. Both countries experience landfalling storms in their greatest severity and Mozambique in particular (Fig. 5), functions as a natural “barrier” to the intense tropical storms for the rest of SAF. Consequently, both countries experience great losses, both in human lives but also in the economic sector. Also, both Madagascar and Mozambique are listed among the poorest countries in the world. An interesting point in the literature is that under climate change, the frequency of landfalling tropical cyclones over SAF is expected to decrease (Muthige et al., 2018).
Figure 5: Monthly precipitation anomaly (mm/month) for January over Mozambique. Anomalies are calculated using the 1983-2012 climatology.
Data Source: https://www.tamsat.org.uk/