A case study of temperature tendency mechanisms operating over northern Africa during and following midlatitude winter troughs

We investigate temperature tendency mechanisms operating over northern Africa following midlatitude upper-level winter troughs. A case study sequence reveals how Iberia and Central Mediterranean troughs perturb the prevailing cold advection of the Harmattan winds, leading to a several-day warming event over northeastern Africa, especially marked over the eastern Sahel (ESL). Data used includes a reanalysis product that attributes three-hour temperature tendency to specific mechanisms. Focused over Days 3–5 (Day 0 is the strong Iberia trough), ESL experiences strong anomalous low-level warming due to dynamics, attributable to anomalous meridional advection, with partial damping by turbulence, leaving a net low-level tendency of about +1°C/day. However, these tendency features are confined to the lowest layers; by 700 hPa, they are absent. Net tendency behaves differently, and remains positive up to near 650 hPa, where the boundary layer anomalously rises to in the afternoon, allowing turbulence to share some of the anomalous warm advection to these levels. In this winter case study, there is no evidence of any other substantial forcing or feedback (e.g., from atmospheric moisture radiation, surface latent heating, surface solar radiation), therefore we propose that over ESL, the land-surface—boundary-layer system adjusts towards a new equilibrium, driven by the several-day reduced cold advection from near-surface to about 850 hPa. The trough sequence induces a continental-scale cold front, that approaches ESL with distinct advection properties; however, on average, extended warming in ESL events is attributed to advection by the climatological meridional wind operating on the anomalous temperature gradient. During build-up (e.g. Day 0), anomalous descent over ESL contributes a modest adiabatic compression warming tendency at low levels. In contrast, averaged over Days 3–5, there is modest anomalous ascent, one aspect of the boundary-layer characteristics during the reduced cold advection that are explored. Generalization of highlighted processes now requires more cases.