The Effect of CO2 Concentration on the Frequency of Blocking in a General Circulation Model Coupled to a Simple Mixed Layer Ocean Model

Abstract

The effect of a doubling of atmospheric CO2 on the characteristics of the 500 mb height field and persistent height anomalies associated with blocking phenomena are investigated in two experiments with the NCAR Community Climate Model (CCM) coupled to a simple ocean mixed layer. This version of the CCM with a seasonal cycle, computed hydrology and the simple mixed layer ocean produces a somewhat improved simulation compared with earlier model versions in spite of a lack of ocean heat transport and overextensive sea ice. In a control experiment with present amounts of CO2, 500 mb height statistics compared best with observations during winter while summer is not simulated as well. In a second experiment, where CO2 is doubled, the troposphere experiences warming most everywhere and 500 mb heights increase, especially near areas where sea ice has retreated and surface air temperature warming is greatest. In most regions of significant blocking activity, standard deviations of 500 mb height and blocking activity are decreased in all seasons. In the Northern Hemisphere, there are also increases in 500 mb standard deviations and blocking activity in the North Pacific during winter, and an increase in standard deviations at high latitudes over Asia and Alaska during summer. There also is a coincident increase of blocking over Asia at those latitudes, but a decrease of blocking over Alaska in summer, partially due to increased variability on shorter time scales there. Thus, in this hemisphere the incidence of blocking does not seem to change significantly with increased CO2, but the centers of action move geographically. On the other hand, in the Southern Hemisphere, blocking activity is generally reduced.