Spatial Pattern and Zonal Shift of the North Atlantic Oscillation. Part I: A Dynamical Interpretation

Abstract

This paper presents a possible dynamical explanation for why the North Atlantic Oscillation (NAO) pattern exhibits an eastward shift from the period 1958?77 (P1) to the period 1978?97 (P2) or 1998?2007 (P3). First, the empirical orthogonal function analysis of winter mean geopotential heights during P1, P2, and P3 reveals that the NAO dipole anomaly exhibits a northwest?southeast (NW?SE) tilting during P1 but a northeast?southwest (NE?SW) tilting during P2 and P3. The NAO pattern, especially its northern center, undergoes a more pronounced eastward shift from P1 to P2. The composite calculation of NAO events during P1 and P2 also indicates that the negative (positive) NAO phase dipole anomaly can indeed exhibit such a NW?SE (NE?SW) tilting. Second, a linear Rossby wave formula derived in a slowly varying basic flow with a meridional shear is used to qualitatively show that the zonal phase speed of the NAO dipole anomaly is larger (smaller) in higher latitudes and smaller (larger) in lower latitudes during the life cycle of the positive (negative) NAO phases because the core of the Atlantic jet is shifted to the north (south). Such a phase speed distribution tends to cause the different movement speeds of the NAO dipole anomaly at different latitudes, thus resulting in the different spatial tilting of the NAO dipole anomaly depending on the phase of the NAO. The zonal displacement of the northern center of the NAO pattern appears to be more pronounced because the change of the mean flow between two phases of the NAO is more distinct in higher latitudes than in lower latitudes. In addition, a weakly nonlinear analytical solution, based on the assumption of the scale separation between the NAO anomaly and transient synoptic-scale waves, is used to demonstrate that an eastward shift of the Atlantic storm-track eddy activity that is associated with the eastward extension of the Atlantic jet stream is a possible cause of the whole eastward shift of the center of action of the NAO pattern during P2/P3.