Attempts are made to link observed variations of the length-of-day with a period near 50 days to the atmospheric 40- to 50-day oscillation in the tropics. Because the earth-atmosphere system maintains constant angular momentum a longer length of day (slower rotation rate of the earth) goes with higher momentum of the atmosphere. It is shown that the longest length of day and greatest atmospheric momentum occurs when tropical convection associated with the 40- to 50-day oscillation is beginning to weaken near or east of the date line. We present some evidence suggesting that, over a broad period range bridging 40--50 days, a strengthening of the easterlies east of the convection increases wind stress and frictional torques, and serves an important mechanism for exchange of momentum from earth to atmosphere. ¿ American Geophysical Union 1987