Principal Interaction Patterns in Baroclinic Wave Life Cycles

Abstract

A principal interaction pattern (PIP) analysis aims at finding a limited number of structures in seemingly very complicated physical scenarios that are time independent up to their amplitudes and phases. These vary according to nonlinear equations determining the interaction between the different structures. By minimizing a suitably chosen error function, calculated by comparing a PIP model with observed or synthetic datasets, both the structures and their interaction coefficients are determined simultaneously. This might therefore be a useful tool for identifying basic structures and processes underlying baroclinic wave life cycles. As a first step in this direction, an accordingly devised PIP model has been applied to a synthetic dataset obtained by numerically integrating the tendency equations of a very simple spherical and quasigeostrophic two-layer model incorporating surface drag and thermal damping. For fairly typical dissipative parameters, a PIP analysis identifies three basic structures that give a good description of the complete dynamics. The shape of these patterns and their interaction coefficients seem to be controlled by the diabatic parameters of the two-layer model. The initial conditions of an examined time series have virtually no influence. The role of the three PIPS in the baroclinic life cycle is discussed. An analysis of their interplay with each other and the zonal wind indicates that dissipation and forcing of the eddies themselves is an important factor in the maintenance of multiple baroclinic wave life cycles. Comparative analyses of cases with stronger and weaker dissipation indicate that the number of dynamically relevant patterns decreases with increasing dissipation, so that PIPS appear to he a valuable tool for the analysis of sufficiently dissipative systems.