The Forcing of the Preferred Positions of the Atlantic Eddy-Driven Jet: Importance of Orography and SSTs

It has previously been observed that the North Atlantic winter-time eddy-driven jet exhibits preferred positions, or ‘regimes’ in latitude. Using a jet latitude index to study the latitudinal distribution of the maximum in daily zonal mean zonal wind in the Atlantic region, three preferred positions are found. Previous research has associated the southernmost of these ‘regimes’ to Greenland blocking; however, a robust explanation for the separation between the central and northern peaks has not yet been determined. Here we study the response of the North Atlantic jet regimes, or preferred positions, to various forcings, and offer an explanation for the presence of the northern regime. Using the WACCM model, we explore the impacts of orography on the Atlantic jet regimes, showing that the presence of Asian orography significantly reduces the frequency of the northern peak; however, the latitude of the peaks remains relatively fixed. Without Greenland orography, the distinct separation of the central and northern peaks is removed. Further analysis with the WACCM model and ERA-interim data shows that the existence of the northern peak in jet latitude index may be due to the orographic forcing of Greenland, including the Greenland Tip Jet. Using this new understanding we explore whether the inability of many models to accurately reproduce the distinct three peaks in the observed jet latitude index can be explained simply through biases in the mean climatological jet position and strength. Further simulations using the WACCM model forced with coupled model SSTs shows that SST biases strongly impact a model’s ability to reproduce the observed jet latitude index, consistent with biases in the climatological jet. We show that the distribution of the jet latitude index is affected by sudden stratospheric warmings, finding a strong shift in the jet latitude index distribution following SSWs, with the northern regime significantly less likely, and the central and southern regimes significantly more likely; however, the latitude of the northern and southern ‘regimes’ are relatively unaltered. We explore possible future changes in the jet latitude index, with the RCP8.5 scenario and a 4xCO2 forcing. In light of the importance of Greenland orography, we also consider the impacts of the reduction in the height of the Greenland Ice Sheet with warming.