Nonlinear North Atlantic atmospheric response to meridional shift of the Gulf Stream path

A remarkably strong nonlinear behavior of the atmospheric circulation response to North Atlantic SST anomalies (SSTA) is revealed from a set of large-ensemble, high-resolution, and hemispheric-scale Weather Research and Forecasting (WRF) model simulations. The model is forced with the SSTA associated with meridional shifts of the Gulf Stream (GS) path, constructed from a lag regression of the winter SST on a GS Index from observation. Analysis of the systematic set of experiments with SSTAs of varied size and sign representing various GS-shift scenarios provides unique insights into the potential mechanism for emergence and evolution of atmospheric circulation response to GS shift in the North Atlantic. Results show that, independent of sign and amplitude of the SSTA, the time-mean response is characterized by anomalous troughs over the western North Atlantic and the Western Europe concurrent with enhanced storm track, increased rainfall, and reduced blocking days. To the north of the anomalous lows, an anomalous ridge emerges over the Greenland, Iceland, and Norwegian Seas accompanied by weakened storm track, reduced rainfall and increased blocking days. This equilibrium response patterns strongly resemble the negative polarity of the NAO, the leading pattern of the internal variability in the model. The nonlinear component of the total response dominates the weak and oppositely signed linear response that is directly forced by the SSTA, which yields an anomalous ridge (trough) downstream of the warm (cold) SSTA. The amplitude of the linear response is proportional to that of the SSTA, but the nonlinear response shows no particular correspondence to the size and sign of SSTA. The nonlinear response patterns tend to emerge in 3-4 weeks after the initialization in November and reach its first significant peak by week 6-7 in December. Composite evolution of the circulation anomalies in association with the formation of a blocking near the Greenland reveals that both the high-frequency transient eddies, through vorticity flux convergence, and an incoming low-frequency Rossby wave train, through the convergence of wave activity density, appear to contribute equally to the formation of the barotropic ridge near the Greenland. Analysis of NCEP/NCAR reanalysis dataset also supports the existence of nonlinearity in circulation response to the observed GS shift, and points to the importance of southward shift in the North Atlantic eddy-driven jet for the observed nonlinear response.