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

Abstract

n the basis of the total energy balance within an atmosphere?surface column, an attribution analysis is conducted for the Northern Hemisphere (NH) atmospheric and surface temperature response to the northern annular mode (NAM) in boreal winter. The local temperature anomaly in the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) is decomposed into partial temperature anomalies because of changes in atmospheric dynamics, water vapor, clouds, ozone, surface albedo, and surface dynamics with the coupled atmosphere?surface climate feedback?response analysis method (CFRAM). Large-scale ascent/descent as part of the NAM-related mean meridional circulation anomaly adiabatically drives the main portion of the observed zonally averaged atmospheric temperature response, particularly the tropospheric cooling/warming 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 tropical upper troposphere and stratosphere, both cloud and ozone forcings are critical in leading to the observed NAM-related temperature anomalies. Radiative forcing due to changes in water vapor acts as the main driver of the surface warming of southern North America during a positive phase of NAM, with atmospheric dynamics providing additional warming. In the negative phase of NAM, surface albedo change drives the surface cooling of southern North America, with atmospheric dynamics providing additional cooling. Over the subpolar North Atlantic and northern Eurasia, atmospheric dynamical processes again become the largest contributor to the NAM-related surface temperature anomalies, although changes in water vapor and clouds also contribute positively to the observed surface temperature anomalies while change in surface dynamics contributes negatively to the observed temperature anomalies.