Zonally averaged zonal wind (U) responses to Ap-index (first row), sunspot number (SSN, second row), Nino 3.4 (third row), aerosol optical depth (AOD, fourth row) and Quasi–Biennial Oscillation (QBO, fifth row) in December–March (columns 1–4). Responses are obtained using MLR with Ap, SSN, Nino3.4, AOD and QBO as explanatory variables. Yellow and red correspond to positive responses, blue to negative. Black contours correspond to the 95% significance level and grey contours to the 90% level.