Why is the mid-winter Pacific storm track weaker during years when the jet is strong?
Abstract
This study seeks to explain the observation that the midwinter Pacific storm track is weaker during years when the Pacific jet is strong. A common suggestion has been that perturbations move faster through the unstable storm track region when the jet is strong. A simple calculation based on the Eady model, however, suggests that the increase in growth rate will compensate for the smaller residence time, resulting in no change with shear. We study how this picture varies when many of the assumptions made in the Eady model are relaxed. We also study the growth of an initial upper level perturbation in a series of simple models with varying shear, which is motivated by the observation that the Pacific storm track is mostly seeded by upper-level tropopause perturbations. We find that most models, in particular when β !=q 0, predict an increase of the storm track with shear. A negative correlation is only found when we take into account the observation that the Pacific midwinter jet narrows when it strengthens. By decreasing the meridional wavelength of the waves as the jet strengthens, we get a notable decrease in growth rate, and a weaker storm track. Furthermore, we find that when the basic state is Eady-like (small or zero meridional PV gradients in the troposphere) the short wave cutoff for instability moves to larger waves as shear is increased, due to the accompanying increase in meridional wavenumber. This results, for realistic parameters, in a transition of the storm track from a regime where upper level perturbations spin up a surface circulation very rapidly, and normal mode growth ensues, to a regime where the initial perturbations take a very long time to excite growth. Since the waves slow down when a surface perturbation develops, this can explain the observations that the storm track perturbations are more ``upper level'' during strong jet years and their group velocities increase faster than linearly with shear. Finally, we show that our results hold when surface damping is added.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2002
- Bibcode:
- 2002AGUSM.A52E..07H
- Keywords:
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- 3300 METEOROLOGY AND ATMOSPHERIC DYNAMICS;
- 3319 General circulation;
- 3364 Synoptic-scale meteorology;
- 3367 Theoretical modeling;
- 3384 Waves and tides