An Equivalent Latitude Formulation of the Stratospheric Northern Annular Mode

The Northern Annular Mode (NAM) index is often used as a diagnostic for stratosphere-troposphere coupling, and in the stratosphere it is commonly used as an index of polar vortex strength. The NAM index generally represents meridional shifts of atmospheric mass into or out of the polar cap. However, the northern hemisphere polar vortex exhibits large variability in geometrical characteristics such as location and shape that often only reflect reversible influences of wave propagation along its edge. As a result, the NAM index in the stratosphere is often biased negative in a way that can exaggerate vortex disturbances, including sudden stratospheric warmings (SSWs). Herein, a formulation of the NAM index for the stratosphere is presented that uses equivalent latitude (EqL) coordinates to help remove much of the geometrical influence on the NAM index. Comparisons of the EqL and regular NAM indices based on reanalysis data show that the regular NAM gives a higher “false alarm rate” of weak vortex events that can help to explain why some weak vortex events (including SSWs) do or do not end up influencing the tropospheric circulation.