Preferred modes of variability and their relationship with climate change

Abstract

Spatial structure of annular modes shows a remarkable resemblance to that of the recent trend in the observed circulation (Thompson et al.). This study performs a series of multilevel primitive equation model simulations to examine the extent to which the annular mode is capable of predicting changes in the zonal-mean flow response to external heat perturbations. Each of these simulations represents a statistically steady state and differs from each other in the values of the imposed tropical heating (H) and high-latitude cooling (C). Defining the annula r mode as the first empirical orthogonal function (EOF1) of zonal-mean tropospheric zonal wind, it is found that the "climate predictability" is generally high in the small C-large H region of the parameter space, but is markedly low in the large C-small H region. In the former region, EOF1 represents meridional meandering of the midlatitude jet, while in the latter region, EOF1 and EOF2 combine to represent coherent poleward propagation of zonal-mean flow anomalies. It is also found that the climate predictability tends to be higher with respect to changes in C than to changes in H. The implications of these findings for the Southern Hemisphere climate predictability are also presented. © 2006 American Meteorological Society.