The Future of Malawi's Growing Season

High resolution regional climate model projections are used to evaluate future mid-century (2041 – 2060) and late-century (2081 – 2100) changes in Malawi's growing season due to global warming under the Intergovernmental Panel on Climate Change Representative Concentration Pathway 8.5 emissions forcing scenario. Three different methods for estimating growing season characteristics, namely, a precipitation/potential evapotranspiration water-balance model approach and two root zone soil moisture approaches, are applied and evaluated. All three methods yield reasonable growing season length, onset, and demise date estimates over Malawi.

The projections indicate the high likelihood for a shorter growing season over Malawi south of 13.5°S. At mid-century the growing season length is predicted to be 20 – 40% (20 – 55 days) shorter over the southernmost districts of Malawi and 5 – 20% (5 – 30 days) shorter over the central districts. By late-century the growing season length is predicted to be 25 – 55% (20 – 70 days) shorter with significant differences extending into northern Malawi. The shorter growing season is primarily associated with an earlier demise date, with no significant change in the onset date predicted by any of the methods.

Analysis of the modeled regional circulation and horizontal moisture flux fields indicates that the future shortening of the growing season and earlier demise date are associated with a strengthening of the continental thermal low over southwestern Africa due to increased surface heating over the Kalahari Desert. It is likely that this amplified warming is due to the increased levels of greenhouse gases in the atmosphere, similar to the amplified warming detected over the Sahara Desert. The thermal low, which is a shallow circulation, is centered to the south and west of Malawi over Angola, Namibia, and Botswana and it is capped by the Kalahari anticyclone at 700 hPa. As the thermal low strengthens in the future simulation, low-level moisture flux convergence and rainfall increase over Angola, Namibia, and Botswana drawing moisture from the east, while the Kalahari anticyclone becomes larger in size and increases the mid-tropospheric moisture flux divergence over Malawi. Together these factors act to suppress convective activity over southern Malawi in the future during the demise period.