Asian Pollution Fuelling North American Storms

Satellite images show that the north Pacific Ocean has recently become much cloudier, particularly during winter. Over a large portion of the ocean, the area covered by deep convective clouds increased by 20 to 50% in a decade. These tall, thick clouds head for North America's west coast where they forcefully release large dumps of precipitation.

The changing cloud patterns observed over the Pacific between 1984 and 2005 do not correspond to natural patterns generated by sea surface temperatures, El Niño or East Asian winter monsoons. Nor are the same changes appearing with cloud cover over the north Atlantic. Instead, scientists have traced the cause to human activity in Asia.

Besides documenting this dramatic change in Pacific cloud cover between latitudes 30 north and 50 north, satellites have also measured soot and sulphur dioxide levels in the atmosphere over Asia. As a consequence of expanding industrialization discharge of these air pollutants has risen sharply, particularly in the last ten years. For instance, sulphur dioxide emissions over eastern China have climbed each decade by 35%. The pollutants peak during winter when most coal burning occurs.

Scientists link the increase in airborne particles from Asian countries with the recent intensification of the winter storm track that travels west across the northern Pacific. The particles released over Asia and carried westward on prevailing winds cause clouds to form differently. When aerosols are present, clouds produce smaller water droplets that they retain longer. The storm clouds end up holding more precipitation and developing stronger convection.

The strengthened Pacific storm track not only means more vigorous winter storms pounding the west coast of North America. Scientists note that the implications of this finding are even broader-reaching. The magnitude of the changes to clouds is so vast that it alters general global circulation patterns. Additionally, these more energetic storms can carry heat and air pollutants farther north, thereby accelerating warming in the Arctic.