The Budget of Local Available Potential Energy of Low-Frequency Eddies in Northern Hemispheric Winter

Low-frequency (LF) transient eddies (intraseasonal eddies with time scales longer than 10 days) are increasingly found to be important in large-scale atmospheric circulation, high-impact climate events, and subseasonal-to-seasonal forecasts. In this study, the features and maintenance of available potential energy of LF eddies (LF EAPE), which denote LF temperature fluctuations, have been investigated. Our study shows that wintertime LF EAPE, with greater amplitude than that of the extensively studied high-frequency (HF) eddies, exhibits distinct horizontal and vertical structures. Different from HF eddies, whose action centers are over midlatitude oceans, the LF EAPE is most active in the continents in the midlatitudes, as well as the subpolar region with shallower vertical structure. By diagnosing the derived energy budget of LF EAPE, we find that, with the strong background temperature gradient in mid- and high-latitude continents (e.g., coast regions along the Greenland, Barents, and Kara Seas), baroclinic generation is the major source of LF EAPE. The generated LF EAPE in the subpolar region is transported downstream and southward to midlatitude continents via background flow. The generated LF EAPE is also dissipated by HF eddies, damped by diabatic effects, and converted to LF EKE via vertical motions. The above energy budget, together with the barotropic dynamics revealed by previous works, suggests multiple energy sources and thus complicated dynamics of LF variabilities.