Diagnosis of Arctic Cyclone Structure and Comparison with Midlatitude Cyclones

Using the ERA5 reanalysis and an Arctic cyclone (AC) track dataset with more than 10,000 AC tracks, we examined the similarities and differences between Arctic cyclones and midlatitude cyclones, regarding how the cyclone characteristics change as they propagate into the Arctic, and how Arctic cyclones developing in the Arctic differ from those originating from the midlatitudes. Three groups of cyclones are examined, i) Arctic cyclones forming in the polar region (≥70N); ii) Arctic cyclones originating from the midlatitudes (<70N); and iii) midlatitude cyclones that later propagate into the Arctic.

More ACs form in the polar region than in midlatitudes in all seasons. Regarding seasonality, ACs of the polar origin occur most frequently in boreal fall and winter, mainly due to frequent cyclogenesis over the Nordic Seas. ACs of the midlatitude origin occur most frequently in boreal summer. As cyclones propagate from midlatitudes into the Arctic, their vertical tilt is reduced, and they become more symmetric. Arctic cyclones are, on average, weaker than midlatitude cyclones and undergo strong intensification less frequently. In addition, ACs of the midlatitude origin are generally weaker than those of the polar origin. Positive potential vorticity anomalies extend into the stratosphere for ACs of the midlatitude origin and of the polar origin, although the former are associated with weaker stratospheric PV anomalies than the latter.

ACs are characterized by a warm-core structure in the lower stratosphere, consistent with previous studies. However, the warm-core structure cannot be fully explained by adiabatic warming as suggested by previous studies. Instead, it is found that diabatic processes play an important role.