Differences in Extratropical Cyclone Structures between Northwestern Pacific and Northwestern Atlantic Oceans

In the Northern Hemisphere,two major storm tracks exist over the Northwestern Pacific (NWP) and Northwestern Atlantic(NWA) Oceans. Although a number of literatures showed zonal asymmetry of storm tracks that result from activities of extratropical cyclones (ECs) (e.g., Nakamura 1992), differences in the structures of extratropical cyclones between NWP and NWA have not been studied in detail. In order to understand the differences, we have performed a composite analysis of ECs that develop in NWP and NWA between 1979 and 2016 using JRA-55 reanalysis data set.

Firstly, ECs are automatically detected with the tracking method developed by Hodges (1994, 1995, 1999). Secondly, ECs that develop in NWP (PEC) and NWA (AEC) were selected based on the locations where the ECs underwent the maximum deepening rate (in Bergeron; Sanders and Gyakum 1980). In the present study, only ECs with Bergeron larger than 0.5 are used to examine the structures of fully-developing ECs. In the composite analysis, physical variables are superposed with respect to the cyclone center. Furthermore, to examine differences in frontal characteristics between PECs and AECs, atmospheric fronts are objectively detected with a method using thermal front parameter (e.g., Schemm et al. 2015).

The composite analysis shows clear differences in structures between PECs and AECs, which are most notable in warm fronts: Warm fronts for AECs tend to extend eastward or east-northeastward from the EC center, while those for PECs southeastward. An analysis using a frontogenesis function shows that this difference in warm front is due to the effect of shear deformation: the shear deformation in the southeast quadrant of PECs is larger than that of AECs, resulting in development of the warm fronts in the southeast quadrant of the EC center for PECs.

It is further suggested that the difference in the warm frontal structure between PECs and AECs is closely related to the difference in the jet structure between NWP and NWA: The jet in NWP is characterized by stronger westerly and associated meridional shear than those in NWA. Since PECs propagate north of the jet core, where cyclonic shear is stronger, the warm fronts of PECs are affected by the deformation associated with cyclonic shear of the jet, and tend to extend more southeastward from the EC center than those of AECs.