Radiative and Dynamical Forcing of the Surface and Atmospheric Temperature Anomalies Associated with the Northern Annular Mode

Based upon the total energy balance within an atmosphere-surface column, an attribution analysis is conducted for the Northern Hemisphere (NH) atmospheric and surface temperature anomalies associated with the Northern Annular Mode (NAM) in boreal winter. Local temperature anomaly is decomposed into partial temperature anomalies due to changes in atmospheric dynamics, water vapor, clouds, ozone, surface albedo and surface dynamics. Large-scale ascent/descent as part of the NAM-related mean meridional circulation anomaly drives adiabatically a large portion of the observed zonally-averaged atmospheric temperature response, particularly the tropospheric warming/cooling over northern extratropics. Contributions from diabatic processes are generally small but could be locally important especially at lower latitudes where radiatively-active substances such as clouds and water vapor are more abundant. For example, in the upper-troposphere-lower-stratosphere (UTLS) of the tropics, both cloud and ozone forcing are critical in leading to the observed warming (cooling) during a positive (negative) phase of NAM. Radiative forcing due to changes in water vapor (surface albedo) acts as the main driver of the surface warming (cooling) of southern North America during a positive (negative) phase of NAM with atmospheric dynamics providing additional warming (cooling). Over the sup-polar North Atlantic and northern Eurasia, atmospheric dynamical processes again become the largest contributor to the NAM-related surface temperature anomalies, however diabatic heating anomalies due to changes in water vapor and clouds (surface dynamics) serve as significant positive (negative) feedbacks to the total temperature response at the surface.