The time evolution of the ocean mixed layer in the seasonal sea ice zone of the Okhotsk Sea has been observed with profiling floats. The heat storage rate in the mixed layer estimated from the float data well coincides with the surface heat flux. Mixed layer deepening and the onset of ice formation can be well reproduced by a bulk mixed layer model, suggesting that onset of ice formation can be predicted by local atmospheric conditions. This explains the remarkably high correlation of the onset day of sea ice formation with the surface heat loss in the preceding fall. The anomalously large heat loss in the fall of 2000 led to the anomalously early ice formation. Strong stratification due to the Amur River fresh water flux is indispensable for ice formation through suppression of deep convection.