Isentropic meridional mass transport by baroclinic waves and the annular mode

Abstract

Baroclinic life cycles are simulated by a 25 layer primitive equation model with the aim to investigate the isentropic meridional mass flux (MMF). The development of perturbed fronts with its associated jet is studied for different model parameters and initial conditions. For equal model parameters, the developing situation differs for a different initial wave number on the perturbed front. Wave number 5 develops a wave train with no growth and decay, whereas wave number 6 sustains growing and decaying baroclinic life cycles. The different solutions can be compared to different phases of the Northern Annular Mode (NAM) index in the real atmosphere by looking at the isentropic meridional mass fluxes (MMF) and isentropic meridional potential vorticity substance fluxes (MPF). A perfect translation of the NAM is not found in the model. However, the different solutions of the model have some similarities with the positive or negative NAM phase.

An increase in the Newtonian cooling coefficient, a parametrisation of radiation, and an decrease in the static stability both lead to stronger northward MMF in the midlatitudinal upper troposphere. An increase in the strength of the jet leads to an increased MMF. A wider and weaker initial jet leads to a smaller MMF than a smaller and stronger initial jet.

A specific region with a higher Newtonian cooling coefficient leads to the formation of baroclinic life cycles downstream of the region. Even with a Newtonian cooling coefficient equal to zero and an initial zonal jet, baroclinic waves can grow due to downstream development.