Ageostrophic relative vorticity forcing in the upper troposphere associated with the onset of blocking in the Southern Pacific

The increase in upper tropospheric divergence preceding the onset of blocking in 14 analyzed Southern Hemisphere cases is primarily forced by horizontal advection and ageostrophic relative vorticity. Ageostrophic vorticity locally generates divergence that, in turn, can force anticyclonic vorticity that is associated with blocking. Advection is the primary mechanism by which divergence-induced vorticities are driven towards the developing block.

The relationship between divergence and vorticity is explored in this study under the hypothesis that divergence, either locally generated or horizontally advected upstream of an incipient block, can force a local increase in anticyclonic vorticity, which can be advected towards the developing block. A tropical region of enhanced convection can be the initial and remote forcing mechanism prior to blocking. This anomalous forcing, associated with cyclonic relative vorticity tendencies in the upper tropospheric subtropical regions, can produce a local increase in horizontal divergence through the ageostrophic portion of this vorticity. The response to enhanced convection in the tropics and enhanced convergence in the downward branch of a Hadley-type circulation generated, is enhanced divergence at the same upper tropospheric level in mid-latitudes and, therefore, anticyclonic relative vorticity tendencies, with the ageostrophic portion again contributing to the anomalies. Consequently, cyclonic relative vorticity in the tropical/subtropical regions can lead to anticyclonic relative vorticity tendencies in mid-latitudes. This can be further amplified if the above features interact with a region of climatological divergence directly equatorward from the region of highest blocking frequency in the Southern Pacific.