The divergence fields associated with time-dependent jet streams

This study examines the effect of temporal variations in either location or structure of both straight and curved jets on their associated divergence patterns. For each of these jets the time-dependent geopotential height field is prescribed analytically, from which the geostrophic and ageostrophic wind fields are extracted. The divergence fields are calculated separately for the steady state (which was commonly assumed in studies on this subject) and for the following time-dependent cases: progressing, retrograding, intensifying, and weakening jets. In this study, the 'intensification' of a straight jet implies an increase in the wind speed while in the case of a curved jet the intensification refers to an increase in the meandering aspect, that is, amplitude divided by wavelength. The dominant divergence pattern of a straight jet consists of a quadrupole structure of four divergence/convergence centers, and in the case of a curved jet the pattern consists of a periodic chain of centers (of alternating signs) located along the jet axis at the inflection points. In progressing and retrograding straight and curved jets, the divergence patterns remain unaltered compared with those of the steady state, but the amplitude decreases in the former and increases in the latter. For an intensifying straight jet, the divergence patterns increase at the entrance region and weaken at the exit, while at the same time being slightly shifted downstream. The reverse holds for the weakening straight jet. The changes in the meandering aspect of a curved jet are accompanied by an intensification of the divergence patterns in both the intensifying and weakening jets. These are accompanied by a shift in their location relative to the jet: downstream for the intensifying jet and upstream for the weakening one. These findings point to the effect of time dependence on the cyclogenesis associated with jet streams and on feedback mechanisms there. For each of the cases considered here, the authors demonstrate that the divergence patterns indicate a negative feedback, which tends to suppress the prescribed temporal change.