5.4 Why is the Atlantic storm track stronger than its Pacific counterpart?

Wednesday, 10 June 2009: 9:00 AM
Pinnacle BC (Stoweflake Resort and Confernce Center)
Edmund K. M. Chang, Stony Brook University, Stony Brook, NY; and W. Lin

One puzzle confronting atmospheric dynamicists is why during the Northern Hemisphere (NH) mid-winter, the Atlantic storm track is stronger than its Pacific counterpart. This is puzzling because the upper level jet and low level baroclinicity are much stronger over the Pacific storm track entrance region.

In this study, a dry global circulation model is used to examine the contributions made by orographic and diabatic forcings in shaping the zonal asymmetries of the NH winter storm tracks. By design, the model mean flow is forced to bear a close resemblance to the observed zonal mean and stationary waves. The model also provides a decent simulation of the storm tracks. In particular, the maxima over the Pacific and Atlantic, and minima over Asia and North America, are fairly well simulated. The model also successfully simulates the observation that the Atlantic storm track is stronger than the Pacific storm track. Sensitivity experiments are performed by imposing and removing various parts of the total forcings.

Results of the dry model study suggest that NH extratropical heating is the most important forcing. Zonal asymmetries in NH extratropical heating acts to force the Pacific storm track to shift equatorward, and the Atlantic storm track to shift poleward, attain a southwest-northeast tilt, and intensify. It appears to be the main forcing responsible for explaining why the Atlantic storm track is stronger than the Pacific storm track. Tibet and the Rockies are also important, mainly in suppressing the storm tracks over the continents, forcing a clearer separation between the two storm tracks. In contrast, asymmetries in tropical heating appear to play only a minor role in forcing the model storm track distribution.

To assess whether the dry model results are robust, several experiments have also been conducted using the full physics CAM, comparing the simulated circulations either in the presence or absence of mountains, forced either with fixed SST distributions or coupled with a slab ocean model. These results are being diagnosed and will be presented at the meeting.

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