Assessing Observed Changes of Poleward Energy Transport for Northern Hemisphere Winter Extratropical Cyclones: Eulerian and Lagrangian Approaches

Transporting excess energy from the equator to polar regions is one of the most critical roles of Earth’s climate system. In addition to the total transport of energy, the transport can be further broken down into stationary and transient terms, with the transient component indicative of synoptic scale Extratropical Cyclone (ETC) activity. For the total transport, the two dominant terms are the dry static energy (DSE) and latent energy (LE). In past studies, annually and zonally averaged, vertically integrated total transports of DSE exhibit a double peak on the order of 3 Petawatts in the Northern Hemisphere, with local maxima south of 20°N and at 40°N. Conversely, LE in the Northern Hemisphere shows a single maximum at 40°N on the order of 2 Petawatts. It is thought that with global warming, fewer or weaker ETCs in the midlatitudes may be necessary to achieve the same poleward transport of energy, primarily due to a weakened low-level temperature gradient and more atmospheric moisture availability. However, more water vapor in the atmosphere can also allow ETCs to intensify through latent heat release (LHR), offsetting the reduced baroclinicity. With the future of ETCs unsettled, this work aims to quantify the contribution of the transient term (ETCs) to the total poleward transport of energy in the Northern Hemisphere winter over the most recent 42-year period (1980-2022), and test if this contribution has changed significantly, and if so, where and by how much it has changed. The most intense ETCs will then be isolated to see if a different pattern or sign change of the transport will result compared to the contribution of all ETCs. Since it is thought that LHR from the most extreme precipitation can further intensify ETCs, LHR will be analyzed as a possible factor discriminating the most intense ETCs from the entire distribution. Results from this Eulerian perspective will then be compared to a cyclone-following (Lagrangian) analysis which can more effectively isolate the ETC life-cycle and the role of LHR, which remains unclear.