AN INVESTIGATION OF THE MOVEMENT OF PACIFIC HIGH PRESSURE IN SUMMER

In the summer half year, the average; intensity of the Pacific. high pressure changes more or less abruptly. It strongly increases from June to July and decreases gradually from April to June. In.middle September it decreasees suddenly. In.the summer season (from June to August),the advance and retreat of the western Pacific pressure ridge (moving from east to west or reversed) is intimately correlated with the upper westerlies. At the same time, the Pacific High itself is an important factor of the atmospheric circulation:Where the zonal circulation is strong, the intensity of the Pacific High is also relatively strong and the position of the western Pacific pressure ridge changes little. When the small waves in the westerlies move out from Asia into the Pacific Ocean, the position of the western Pacific pressure ridge only vibrates slightly. But, when the waves in the westernlies are strong, the western Pacific pressure ridge advances westward and then retreats eastward apparently. The period of advance and retreat of the ridge is about five or six days. During the period of low zonal circulation index, the western Pacific pressure ridge moves eastward. When the circulation index grows from low to high, the ridge advances westward. The range oaf longitudes of oscillation of the ridge in this type is largest, about 30-40 degrees, and the period of oscillation is longest, about half month in general.The longest period reaches one month.Besides, the movement of the Pacific High ridge is also studied in relation to the tropical systems, such as typhoons.