The MJO Cycle Forcing of the North Atlantic Circulation: Intervention Experiments with the Community Earth System Model

In order to understand how the northern extratropics in general (and the North Atlantic circulation in particular) respond to the entire cycle of heating and cooling associated with the Madden-Julian Oscillation (MJO), we have performed experiments in which a particular evolution of MJO diabatic heaing is added to the temperature tendencies of each member of an ensemble of 48 (01 Oct - 31 Mar) simulations with the Community Earth System Model. [This three-dimensional heating evolution is constructed from satellite data, and contains three MJO cycles over the season.] Since each member of the ensemble feels the same added heating evolution, predictable component analysis is highly appropriate, and was carried out on three fields (separately): the 200 hPa height field (Z200), the height tendency due to vorticity flux convergence (DZ300), and the Rossby wave source (RWS200). The leading two modes of each analysis form an oscillation; cross-correlations between modes of the different fields give evidence for a remarkably coherent hemispheric-wide oscillation as an MJO response. This cycle shows a North Atlantic Oscillation (NAO+) –like response 15-25 days after the convection crosses the 90E meridian. To verify the nature of the response over the North Atlantic, we carried out a cluster analysis on daily unfiltered 500 hPa heights. The NAO+ cluster is 10 percent more likely than the NAO- cluster in the model runs with heating, similar to reanalyses, but different from the control simulations (in which the two clusters are equally likely). Thus the addition of the MJO heating has corrected a bias in the North Atlantic Circulation.