Interaction of the subtropical and eddy-driven jets in idealized models

The atmosphere supports two dynamically distinct mechanisms for the maintenance of zonal mean zonal winds, one related to conservation of angular momentum in the poleward branch of the Hadley circulation and the other related to the meridional convergence of eddy momentum flux from baroclinic turbulence. These lead respectively to a baroclinic jet at the edge of the Hadley Cell and a more barotropic midlatitude eddy-driven jet in the region of the storm tracks. However, despite a meridional separation between these mechanisms, these two jets are not often distinct in the atmosphere, especially in a zonal mean. Rather, they merge to form one jet, although sometimes and in some geographical locations, they split to form two distinct structures. To better understand this we investigate the interaction of an eddy-driven jet with an idealization of the subtropical jet using idealized barotropic and shallow water models. A key factor determining whether the jets split or merge is the structure of mean wind and mean vorticity as these properties set the propagation characteristics of the eddies that support the midlatitude jet and determine the location of critical latitudes where these waves may break and deposit momentum.

We find that in our idealized setting the two jets have a tendency to merge if the forcings of the subtropical and eddy-driven jets are not too far apart. As the eddy-stirring is moved further polewards, away from the latitude of the subtropical jet, we find that the zonal mean zonal wind bifurcates abruptly into two jets at a separation distance far greater than if the two jets were independently formed. The relationship of the behavior and the transition between one and two jets states in the real atmosphere (for example in the NAO and in blocking events) will be discussed.