An experimental investigation has been conducted with regard to the natural convection of water near the density extremum in rectangular enclosures with the vertical walls held al different temperatures. The water in the enclosure is initially set at a uniform temperature and then the temperatures of the cold and hot walls are suddenly changed to 0 and 8 degrees C, respectively. The flow patterns of the steady state are presented for the initial temperature range of 2-6 degrees C at high Rayleigh numbers, not considered previously. Al an initial temperature of 4 degrees C, the convective flow field consists of a symmetric double-cell circulation Forming a downward flow, called a sinking jet in the interior of the enclosure. For Ra >9 x 10(6), the interior sinking jet shows an oscillatory behavior due to free-shear instability, which enhances the heat transfer rate between vertical walls. However, al an initial temperature other than 4 degrees C, an asymmetric double-cell circulation is revealed. Oscillatory flow is suppressed near the vertical walls. These phenomena are satisfactorily explained by a numerical analysis.