Baroclinic developments in jet entrances and exits. I: Linear normal modes

This study examines the effect of the ageostrophic circulations associated with jet entrance and exit regions on the structure and growth rates of bar oclinic normal modes. A linearized GCM is used to find the most unstable no rmal-mode disturbances for a zonally symmetric basic state consisting of a zonal jet and meridional circulation. Normal-mode growth rates are reduced by up to 20% for realistic strengths of meridional overturning, and normal- mode structures are significantly affected. For example, the normal-mode po leward tilt with height varies, affecting the conversion of basic-state ava ilable energy. A mechanism for the change in poleward tilt is proposed. Age ostrophic meridional circulations may play a significant role in determinin g the magnitude and length of mid-latitude storm tracks.