Projected Changes in Malawi's Growing Season: Connections with the Large-scale Circulation

Rainfall and altitude play dominant roles in determining crop cycles and growing seasons in the three major agro-climatic zones defined for Malawi. These zones are characterized by elevation, and include semi-arid low altitudes (<600 m), semi-arid/sub-humid mid altitudes (600–1,300 m), and sub-humid high altitudes (>1,300 m). More than 90 % of Malawi's total annual precipitation occurs between November and April. The rainy season begins in the south in late October and progresses northward in association with the development and movement of the South Indian convergence zone. The demise of the rains also occurs first in the south in late March/April, and in late April/early May in the north. Rainfall and, as a result, growing seasons are highly variable, including on the decadal scale, and known to be sensitive to the large-scale circulation.

Regional model simulations with 30-km resolution are used to better resolve this narrow, mountainous country. Simulations of the late-20th century are compared with precipitation observations and reanalyses, and show that the high-resolution simulation is able to capture the regional climate dynamics well. Simulations of the mid- and late-21st c. under the RCP8.5 emissions scenario project precipitation reductions that are accompanied by significantly shorter growing seasons south of 13.5°S and an earlier demise of the rainy season. The changes in precipitation are related to amplified warming of the land surface compared with the ocean, resulting in the intensification of the Kalahari thermal low/ anticyclone system and an increase in the moisture flux divergence over Malawi that suppresses convective activity.