A quantitative assessment of the sensitivity of the downstream midlatitude flow response to extratropical transition of tropical cyclones

During extratropical transition (ET) of tropical cyclones (TC) reduced predictability for the midlatitude flow is often observed. In this study we assess the sensitivity of the midlatitude flow response to ET. To this end, a simple novel metric, the “equivalent forecast hour difference,” is introduced and applied to experimental simulations for eight recent ET events, in which the TC has been removed or relocated. Early during ET, the midlatitude flow response is sensitive to ridgebuilding directly downstream of the transitioning TC. In cases with strong ridgebuilding and Rossby wave triggering, the sensitivity remains high. The midlatitude flow response is even more sensitive to error in the initial position of the TC. This study newly quantifies the high degree of sensitivity of the downstream midlatitude flow response to ET. It corroborates the need for correct tropical cyclogenesis and TC track forecasts for improving midlatitude numerical weather prediction during ET. Key Points Midlatitude flow response highly sensitive to the presence of ET and to TC track error Sensitivity is due to direct TC‐midlatitude flow interaction and later due to downstream development Poorly represented ET in NWP has high potential to degrade midlatitude forecast skill