EAPS

A synoptic-scale perspective to climate


Currently, there are two complementary approaches to study midlatitude atmospheric variability. In an Eulerian approach, the eddies are defined as deviations from the background mean flow, and the atmospheric variability is measured using statistics of the time-mean steady-state. Alternatively, in a Lagrangian approach, the eddies are identified with the synoptic-scale weather systems, and are studied using a Lagrangian feature-tracking method. The Eulerian approach is the most widely used for the study of midlatitude dynamics. However, as opposed to the Lagrangian approach, it is often limited in the insights it gives on the temporal evolution and the underlying dynamics. In this work we apply a Lagrangian synoptic-scale approach to study mechanisms that control the midlatitude climate and its response to global warming, and show how it can be useful for deciphering the large-scale structure of atmospheric variability.
   Two applications of the synoptic-scale perspective are presented. The first concerns the poleward deflection of the northern hemisphere midlatitude storm tracks. Two dominant mechanisms responsible for the poleward propagation of storms are identified: advection by upper level winds, and diabatic forcing associated with latent heat release. Moreover, we suggest that the projected poleward shift of the midlatitude storm tracks (defined in an Eulerian sense as regions of enhanced eddy kinetic energy), is a result of enhanced poleward propagation of storms. We demonstrate that this occurs due to stronger upper level winds and increased atmospheric water vapor, and show that enhanced propagation of storms occurs also in an ensemble of CMIP5 models.
   The second applications concerns the atmospheric temperature variability. The storm-tracking algorithm is modified, so that temperature anomalies can be tracked instead of cyclones and anti-cyclones. We study what controls the dynamics of warm and cold temperature anomalies, and what gives rise to the observed spatial structure of the temperature distribution in reanalysis data and CMIP5 climate change simulations. We develop a simple theory to explain how temperature variance and skewness changes are generated dynamically from mean temperature gradient changes, and demonstrate the crucial role of regional warming patterns in shaping the distinct response of cold and warm anomalies.
   Potential future applications of the synoptic-scale perspective to study the midlatitude atmospheric variability, climate extremes, cloud feedbacks, stratosphere-troposphere coupling, and seasonal forecasting are discussed.