<P> When the Atlantic Multidecadal Oscillation's mode is favorable to tropical cyclone development (1995--present), it amplifies the subtropical ridge across the central and eastern Atlantic . </P> <P> When the subtropical ridge in the northwest Pacific is stronger than normal, it leads to a wet monsoon season for Asia . The subtropical ridge position is linked to how far northward monsoon moisture and thunderstorms extend into the United States . The subtropical ridge across North America typically migrates far enough northward to begin monsoon conditions across the Desert Southwest from July to September . When the subtropical ridge is farther north than normal towards the Four Corners, monsoon thunderstorms can spread northward into Arizona . When the high pressure moves south, its circulation cuts off the moisture and the atmosphere dries out across the Desert Southwest, causing a break in the monsoon regime . </P> <P> On the subtropical ridges western edge (eastern coast of continents), the high pressure cell creates a southerly flow of tropical air toward the lower east sides of continents in the summer months . In the United States the subtropical ridge Bermuda High helps create the hot, sultry summers with daily thunderstorms typical of the Gulf and Atlantic states . This flow pattern also occurs on the eastern coasts of continents in other subtropical climates such as South China, southern Japan, central - eastern South America, southern Queensland and KwaZulu Natal province in South Africa . </P> <P> When surface winds become light, the subsidence produced directly under the subtropical ridge can lead to a buildup of particulates in urban areas under the ridge, leading to widespread haze . If the low level relative humidity rises towards 100 percent overnight, fog can form . </P>

In the southern hemisphere spring the subtropical highs shift