Free and forced atmospheric responses to sea surface temperatures using a GCM with idealized topography

Previous studies have shown that there is a strong connection between the variation in the sea surface temperature (SST) in the North Atlantic Ocean and the global atmospheric circulation. However, the extent of this connection is still unclear. More fundamental questions rising from this connection include how the North Atlantic SST anomalies are affecting the atmospheric circulation and how this original effect is modulated by the land mass and the orography of the continents. Answers to these questions are essential for understanding the atmospheric circulation response to the North Atlantic SST variation and would allow us to focus on what may be the major factors to examine in order to improve predictions of the circulation changes in response to the SST anomalies. While there have been separate studies investigating the atmospheric response to SST forcing in the various contexts of ocean, land and orography, few has looked at their effects in a comprehensive manner.

This study reports results from an investigation of the individual and modified responses of the atmospheric circulation to SST anomalies in the North Atlantic in a series of experiments using the CESM 1.0.5 suite of models. In these experiments, three different configurations are used. The first is an all ocean (i.e., the “aquaplanet”) and is used to determine the free response of the atmosphere to the SST anomaly. The second configuration includes simple land and is used to determine how land-ocean configuration impacts the original response of the atmospheric circulation to the SST anomaly. The third configuration includes idealized Rocky Mountains, Alps and the Tibetan Plateau. This configuration allows for determining modulation on the atmospheric response by additional influence from the major orography of the continents. In all the three configurations, three SST experiments are done; a control (i.e., zonally symmetric) and cold and warm zonally asymmetric experiments. The zonally asymmetric experiments use a monopole anomaly of 4°C centered in the latitude and longitude of the realistic North Atlantic Ocean. These experiments allow for breakdown of the free and modulated atmospheric responses to the SST anomaly and land as well as orography configurations. In addition, these results can aid our understanding of why the atmosphere responds stronger in certain regions than others.