The Seasonality and Nonlinearity of the Atmospheric Response to SST Anomalies Associated with Shifts in the Oyashio Front

We conducted atmospheric GCM experiments to examine how SST anomalies due to meridional displacement of the Oyashio Extension front impact the atmosphere. Experiments were conducted using a high (0.25°) or standard resolution (1°) version of the NCAR Community Atmospheric Model (CAM5), with the front either in a northward position (“warm”), its climatological position (“control”) or a southward position (“cold”). Based on the difference between the warm and cold cases, the high and standard resolution versions of the model exhibited very different atmospheric anomalies. The high-resolution simulations obtained a robust local atmospheric response dominated by changes in the eddy heat and moisture transports with a high pressure downstream over the Gulf of Alaska, while in the standard resolution simulations the local atmospheric response exhibited a strong heating by surface fluxes that was balanced by the mean equatorward advection of cold air, resulting in a weak large-scale response. Here we further examine these simulations with a focus on the two aspects of the atmospheric response: (1) degree of nonlinearity by comparing the anomalies from the warm-minus-control and cold-minus-control cases, and (2) the impact of background seasonal cycle as shown by the evolution of the anomalies over the winter months. In the high-resolution cases, the responses from the warm and cold experiments become asymmetric (i.e. the sign of the response is the same in the two cases) starting in December. Furthermore, in early winter (Dec-Jan) the significant large-scale high pressure anomalies appear only in the warm experiment, while in late winter (Feb-Mar) the opposite signed anomalies are found only in the cold case. The degree of nonlinearity is found to be even greater in the standard-resolution experiments. Detail analyses will be presented to explain the nonlinearity and seasonality of the response.