On the Causes of Synoptic-Scale Eddy Heat Flux Decline

The poleward heat flux by atmospheric waves plays a pivotal role in maintaining the meridional temperature gradient. A recent study found that in the Northern Hemisphere the heat flux by transient eddies has been weakening, and the study attributed this weakening to the smaller equator-to-pole temperature gradient caused by Arctic warming. During the period of 1979–2019 examined here, for the annual mean, both the synoptic-scale eddy heat flux and the temperature gradient had indeed declined. However, from October to April, the synoptic-scale eddy flux trend is more closely tied to the planetary-scale eddy heat flux trend, than to the temperature gradient trend. From June to August, the synoptic-scale eddy flux decline can be attributed to a warming of the high-latitude land areas. Therefore, a more comprehensive interpretation of the synoptic-scale eddy heat flux trend needs to include the dynamics of the planetary-scale waves and summer land warming.