Zonal mean momentum in northern summer: is budget and some possible implications

The quasi-stationary subtropical anticyclone centered over the eastern North Atlantic shifts westward in midsummer. This displacement can be seen as a near surface decrease in vorticity (anticyclonic tendency) and increase in geopotential height in July in the Intra-Americas Seas (IAS) region. The displacement of the North Atlantic surface anticyclone occurs in concert with a western displacement of the cyclone aloft, whose upper-level convergence acts to maintain the anticyclone. It is hypothesized that the weakening of the zonal mean zonal wind ([u]) in the subtropics causes the strengthening of the subtropical anticyclone in the IAS due to the sign change of zonal vorticity advection.

The observed changes of [u] appear to be forced by meridional eddy momentum flux divergence as positively tilted summer stationary waves advect westerly momentum northwards. The primary source of the zonal mean of this eddy momentum flux appears to be largely confined vertically to 200mb, and geographically to the Asian Monsoon region. The upper-level monsoon anticyclone (“Tibetan High”) interacts strongly with the background flow on its northwest corner, causing momentum divergence (westward acceleration) south of 30°N in July.

The negative tendency in the zonal wind field in July causes a positive (cyclonic) tendency of zonal vorticity advection aloft in the IAS domain, implying a strengthening of the surface anticyclone. There is a fairly good negative correlation between the observed weakening of [u] aloft and observed increases in the Z850 field (proportional to a decrease in vorticity) from June to July in interannual data.