A Cyclone/Anticyclone Couplet over North America: An Example of Anticyclone Evolution

Abstract

A detailed case study has been made of a cyclone/anticyclone couplet over North America during early winter, 11?18 November 1969. The anticyclone was the dominant member of the couplet in this case. Objective analyses of the wind and mass fields were carried out in both the isobaric and isentropic coordinate systems. Calculated quantities which are discussed include quasi-geostrophic vertical velocity and height tendency, potential vorticity, and quasi-Lagrangian kinetic energy budgets. The movement of the cold, polar anticyclone out of its source region in Alaska southeastward to the Gulf of Mexico is seen to be thermally steered. During this stage the anticyclone moves toward the region of descent forced by the low level cold advection. As the anticyclone recurves and begins to move northeastward along the cut coast of the United States, it becomes a warm, dynamic system. The forcing directing the movement is that of differential vorticity advection. The static stability structure of the anticyclone changes in a manner consistent with this changeover in forcing in that the lower troposphere becomes more stable and the upper troposphere becomes somewhat less stable. Calculations show that the strong cyclogenetic diabatic forcing created by cold air flowing over warm water along the east coast is overwhelmed by the quasi-geostrophically driven height rises. The mechanism by which the cold air dome associated with the anticyclone moves southward intact is explained from two complementary perspectives. One viewpoint is that of asymmetries in the jet stream about the long wave trough with a jet streak to the west of the cold dome. The other is that a maximum in potential vorticity located above the coldest air will prevent the dome from subsiding. The migration of the jet streak to the eastern side of the cold dome and the movement of the potential vorticity maximum from its position over the cold air lead to collapse of the cold air and induce cyclogenesis on the eastern side of the cold air. Furthermore, the calculations show that the potential vorticity is not at all conserved in the region of the deepening cyclone. The kinetic energy budgets indicate that the anticyclone region serves as a source of upper level energy for the downstream cyclone. The collapse of the cold air converts potential to kinetic energy and this energy is fed into the jet streak on the eastern side of the long wave trough.