A powerful resonance in the O1 and K1 diurnal tides occurs over the Kashevarov Bank in the Okhotsk Sea. Model studies show that this resonance creates tidal mixing and an associated vertical heat flux, both with diurnal and fortnightly components. In midwinter this flux controls the behavior of the resultant polynya; in early winter it affects the polynya formation. For the midwinter case, examination of numerical results and of imagery derived from the Special Sensor Microwave Imager (SSM/I), the advanced very high resolution radiometer (AVHRR), and the RADARSAT ScanSAR shows that the polynya area varies diurnally and fortnightly. Specifically, for the 2000 and 2001 winters, daily averaged passive microwave images clearly show a fortnightly cycle in the polynya area. At the diurnal period the model shows that the location of the maximum heat flux oscillates between the northwestern and southwestern edge of the bank and that the polynya oscillates between the western edge of the bank and being elongated in the east-west direction over the southern edge. Passive microwave swath and AVHRR imagery show a similar polynya behavior. For the early winter case and during a period of northerly winds, synthetic aperture radar imagery shows that the polynya consists of two parts, an upwelling polynya over the bank and a secondary polynya downwind of the bank with properties similar to coastal polynyas. In the case of the secondary polynya the region of upwelled warm water over the bank serves as its northern boundary.