Numerical experiments are conducted to investigate the interannual variability of water mass formation processes in the Mediterranean Sea. The experiments are conducted with a general circulation model which includes a set of calibrated air-sea heat flux bulk formulas to account for the correct surface heat budget of the Mediterranean Basin. An analysis of the model response to different atmospheric forcing frequencies (monthly versus 12-hour) for the period 1980--1988 is conducted in the areas of different water mass formation processes: the Wester Mediterranean Deep Water (WMDW), the Eastern Mediterranean Deep Water (EMDW) (only the Adriatic component), the Levantine Intermediate Water (LIW), and the Levantine Deep Water (LDW). All the experiments are able to simulate well the LIW and LDW; however, the 12-hour forced experiments (D experiments) tend to produce more LDW than LIW compared to the monthly forced experiments (M experiments). On the other hand, only the D experiments, with a salinity enhancement, are able in some years to model the WMDW deep convection. The WMDW and EMDW are formed approximately every 3 years depending on the model parameterizations; however, LIW is formed each year but with large-amplitude fluctuations in the volume of water formed. LDW has a formation rate similar to that of deep waters. ¿ 2000 American Geophysical Union