Inertial air flows over the North Pacific at mid-latitudes in spring

Analyses of the bi-daily (two-day) variation of meteorological properties from ship observations along 35°N across the entire Pacific in the spring of 1976 are explained by a series of inertial flows that drift eastward with the mean flow (about 20 kts) as they oscillate in strength from one day to the next. Each inertial flow is a nearly circular cell with a diameter of about 3000 km. Within a cell the horizontal flow is clockwise as viewed from above. High pressure occurs at the center of the cell, low pressure along the perimeter. Two or three contiguous cells occur over the North Pacific simultaneously, lined up in a row in the east-west direction. The vertical scale of a cell is assumed to be the scale-height of the atmosphere (about 8 km). Satellite images of cloud patterns over the northeast Pacific, taken simultaneously with the ship measurements, provide the basis for the interpretation of size, arrangement and eastward speed of the cells. The inertial flows are caused by the gravitationally unstable southward pressure gradient that is associated with the large-scale mean temperature gradient at sea level. Cold, dry air accelerates south into the warm humid environment. As it does so the Coriolis force continually bends the flow to the right, forming a cell. At the edges of the cell some warm humid air is forced upward to a higher elevation by the penetrative convection. The vertical motion at the cellʹs perimeter is made visible by the formation of a narrow and solid ring of clouds, which makes the cell itself visible by the relative lack of clouds within it. Solar heating, which is most intense around local noon, can cause a cell to form and can increase the circulation of an existing cell at that time (noon). Cell circulations decrease in strength when not being forced due to bottom friction with the sea surface and side friction with neighboring cells and with mountain ranges. Forcing and dissipation lead to circulations that oscillate in strength from one day to the next, which explains the two-day periodicity found at a fixed (or slowly moving) point on the sea surface. Each inertial flow provides for a net poleward transport of heat and moisture but not eastward angular momentum.