What Drives the North Atlantic Oscillation's Surface Air Temperature Anomaly Pattern?

In this study, we investigate the surface air temperature (SAT; approximately 2 meters above the ground) anomaly pattern that is associated with the North Atlantic Oscillation (NAO), a recurring atmospheric teleconnection that explains much of the interannual and intraseasonal variability over the North Atlantic sector of the Northern Hemisphere. Many studies have surmised that this SAT anomaly pattern is driven by horizontal temperature advection. This viewpoint is qualitatively supported by sea level pressure maps, from which the implied direction of anomalous winds relative to the climatological temperature gradient suggests the observed SAT anomaly pattern. However, based on the thermodynamic energy equation, it is possible that other processes contribute to the temperature changes observed during the NAO, such as advection of the anomalous temperature by the climatological wind, advection of the anomalous temperature by the anomalous wind, longwave radiative warming/cooling, anomalous latent heating, adiabatic warming/cooling and vertical mixing. Therefore, in this study, we investigate not only the role of temperature advection, but also the roles of each of the other terms in the thermodynamic energy equation using data provided by ERA-interim at the lowest model level (approximately 10 meters above ground). Since the temperature anomalies at this level closely match those at 2 meters, i.e., the SAT anomalies, we assume that the same processes drive the temperature anomalies at both levels. Specifically, we analyze composites of each term in the thermodynamic energy equation using the formulation that the reanalysis model implements at the lowest model level.

We also investigate the surface energy budget, focusing on those processes that drive the skin temperature anomalies associated with the NAO. Our analysis of the surface energy budget during the NAO is relevant for studies of Arctic amplification because out of the four temperature anomalies associated with the NAO, two overlie high-latitude regions. By analyzing both the thermodynamic energy equation on the lowest model level and the surface energy budget, we are able to determine how skin temperature and SAT changes are related when the NAO is active.

Prior to addressing the ultimate question of what drives the SAT anomaly pattern, we show that the autocorrelation function of the composite SAT field suggests that SAT anomalies grow and decay at different rates. We find that the two anomalies that overlie high-latitudes grow and decay over a longer time period than their mid-latitude counterparts. We then address the thermodynamic energy budget and show that the SAT anomaly pattern is driven primarily by anomalous advection of climatological temperature by anomalous winds, as presumed by many previous studies, and that this advection is strongly opposed by longwave radiative heating/cooling that ultimately causes the SAT anomalies to decay after the NAO reaches its peak. All the other terms in the thermodynamic energy equation were found to make a much smaller contribution to the NAO anomaly evolution. In contrast, the surface energy budget indicates that the skin temperature anomaly pattern, although similar to the SAT anomaly pattern, is driven by downward longwave radiation at the surface. Therefore, the SAT and skin temperature anomalies associated with the NAO are driven by different processes. We propose that the changes in surface downward longwave radiation which (based on the Stefan-Boltzmann law) are caused by changes in air temperature and moisture throughout much of the depth of the troposphere, ultimately arise from the advection of warm moist air by the anomalous wind field of the NAO.