Retrograding Wintertime Low-Frequency Disturbances over the North Pacific Ocean

Abstract

Retrograding (westward-moving) features over the middle and high latitudes of the Northern Hemisphere during winter are examined in observations and data from a GCM simulation, using complex principal component analysis. The first eigenvector of the low-frequency middle troposphere geopotential height over the North Pacific has a zonal scale of 120?160 deg longitude and a dipole-like meridional structure, with maximum amplitude over the Gulf of Alaska and the Bering Sea. These disturbances propagate westward and go through a life cycle of growth and decay over a period of about three weeks. The temporal evolution of the energy and its conversions during the life cycle of the simulated Pacific disturbances are composited, using the time-dependent coefficient of the principal component analysis as an index for their occurrence. The composite energy cycle indicates that the kinetic and available potential energy of these disturbances grow and decay together. Both baroclinic and barotropic conversions contribute to the growth of disturbance energy.