The Impact of an Envelope Orography on Low-Frequency Variability and Blocking in a Low-Resolution General Circulation Model

Abstract

Sensitivity experiments with a perpetual January version of a low-revolution general circulation model (GCM) are conducted to investigate the influence of different, nonzero specifications of orography on low-frequency variability (LFV) and blocking in the Northern Hemisphere. Two 1200-day integrations are compared to examine the impact of an enhanced ?envelope? orography. An ensemble of eight independent 90-day realizations is extracted from each simulation. Distributions of ensemble-mean statistics for the two simulations are presented, along with estimates of the statistical significance of the differences. The use of an envelope orography leads to significant changes in the distribution of the LFV (periods 10?90 days) over the Northern Hemisphere. When the LFV is partitioned into contributions from intramonthly ( 10?30-day periods) and intermonthly (30?90-day periods) fluctuations, it is found that the envelope orography significantly alters the distributions of intramonthly scale variability over the North Atlantic Ocean. The impact of envelope orography on blocking, as measured by an objective criterion, is then examined. Significant changes in its spatial distribution are found over the North Atlantic but not in the total number of blocking days occurring anywhere within the Atlantic basin. No significant changes are found over the North Pacific. The changes in blocking distribution over the North Atlantic make the model's climatology more consistent with observations. The GCM results are interpreted in light of results from simple modeling studies. Based on this comparison, it is hypothesized that the changes in LFV and blocking over the North Atlantic are a response to differences in the orographic forcing downstream of the Rocky Mountains. It is concluded that a modest change in the representation of orography can significantly affect local distributions of intramonthly variability and blocking in a low-resolution GCM.