A Minimal Model for the Midwinter Minimum

Overview

The North Pacific storminess in mid-winter is less intense than should be according to classical theories. We use idealized models of simplified physics and no continents to investigate this phenomenon in a more general setting. Using these simulations helps us pick out important processes that determine the storminess. We quantify the importance of these processes using a scaling which may be useful for indicating the onset of these midwinter suppressions, and thus help predict wind storm intensity on the western coasts of America and Europe.

Abstract

A midwinter suppression in storm track activity in the North Pacific was first observed several decades ago, but its cause is still not clearly determined. Our Eulerian and Langrangian analysis of the North Pacific storm track indicates that the first month of the suppression has barotropic characteristics. This is corroborated by a hierarchy of idealized simulations. These show that a midwinter suppression is a general feature of storm tracks that can be triggered in a zonally symmetric and barotropic atmosphere, given a suitable mean flow configuration. We develop a scaling to quantify the relationship between the eddy kinetic energy and the mean flow configuration. Coupling of this barotropic scaling to an idealized baroclinic seasonal cycle of the storm track yields a minimal model for the midwinter suppression. While many other neglected processes can shape the exact properties of the observed midwinter suppression, this model describes the minimal ingredients to trigger it. The minimal model is tested using the long-term and interannual variability of the observed storm tracks.