Interannual length-of-day variations and ENSO indices such as the Southern Oscillation Index (SOI) and Nino 3.4 SST are well correlated as a consequence of angular momentum conservation. During an El Nino event, the westerly winds increase, which raises the atmospheric angular momentum (AAM); as a result, the solid Earth must slow down, which increases the duration of the day (length-of-day: LOD). However, a lag has been observed with the SOI and Nino 3.4 SST leading the LOD and AAM series by one–two months; to date no dynamical explanation has been offered. The dominant excitation mechanism of interannual LOD is the wind term, driven largely by thermal winds arising from the poleward gradient of tropical temperature (TT). We show that the TT gradient (TTG), which peaks 1–2 months after the Nino 3.4 SST anomaly, is the source of the thermal winds that drive the LOD anomaly and account for this well-known ENSO-Earth rotation lag.