Abstract
This paper illustrates how the ocean can affect and be affected by the overlying atmosphere over time periods of months and seasons and how these effects are transmitted by the atmosphere to North America. The period from September 1972 through August 1973 shows the remarkable continuity of a monthly-mean mid-tropospheric trough which can be traced from 155E in September to 90W in May. The development and motion of the trough over the ocean is associated with and perhaps explained by sea-surface-temperature (SST) contrasts which supply a baroclinic source of energy to the overlying atmosphere. Estimates of ocean-surface flow around the North Pacific gyre suggest that the zone of SST contrast was advected the proper distance to account for the observed displacement. The SST contrast was enhanced, meanwhile, by the cyclonic activity.
After reaching the west coast in March, the trough was supplied with a new source of baroclinic energy by the contrast between a cold polar-Pacific air mass, kept cold by persistent extensive snow cover, and a warm tropical Gulf of Mexico air mass, kept warm by the seasonal heating of the southern Plains. The expanding wave length between this eastward-migrating trough and the seasonally forced retrogression of the Asiatic monsoon trough, combined with weakening zonal westerlies, led to the development in the central Pacific of a new trough and readjustment of the SST pattern.
