On the relationship between subtropical and eddy-driven jets

A series of numerical model experiments challenges the notion that baroclinic wave growth is most favored along the subtropical jet. While this notion is true when the subtropical jet is sufficiently strong, for a subtropical jet of modest strength, the most favorable region for baroclinic wave growth often lies 20 to 30 degrees poleward of the subtropical jet, establishing an eddy-driven jet that is latitudinally well separated from the subtropical jet.

During the boreal winter, there are two zonal jets over the North Atlantic ocean, one at 25N and the other at 45N. The former is a subtropical jet, while the latter is entirely eddy-driven jet. In light of the model results, we attribute this two jet structure to the sufficiently weak subtropical jet over the Atlantic ocean. On the other hand, there is a strong, single jet over the North Pacific. Again, this is consistent with the above model results that a sufficiently strong subtropical jet does not allow a latitudinally separate, eddy-driven jet to develop.

There are important implications of the above results for midlatitude storm track structure, the North Atlantic Oscillation, and the sensitivity of the general circulation to tropical heating.