The Euro-Atlantic Circulation Response to the MJO Cycle of Tropical Heating using Coupled GCM Intervention Experiments.

In a recent observational analysis, Yadav and Straus (2017, hereafter YS) used cluster analysis in the Euro-Atlantic region to characterize the response to both slow and (separately) fast episodes of the MJO. They found a very large increase in the likelihood of the occurrence of the negative North Atlantic Oscillation (NAO-) regime within 10 days of the MJO heating reaching the western and central Pacific (MJO phases 6 - 8). This response was particularly strong for the slow episodes.

The primary goal of this paper is to determine the robustness of the observational results using ensemble experiments in which specified tropical diabatic heating evolutions (a slow MJO repeated cycle, a slow single MJO cycle, a fast MJO repeated cycle, and a fast single cycle MJO) are added to a fully coupled ocean-atmosphere-land model.

For the repeated cycle experiments, the leading predictable modes of 300 hPa geopotential height, 200 hPa Rossby wave source, and tropical diabatic heating all come in pairs, which describe a periodic cycle or oscillation. For the repeated cycle experiments, the leading mode of height is very similar to the NAO- circulation regime, as verified by comparing the evolution of this mode with the frequency of occurrence of the NAO- regime calculated from unfiltered data. The leading mode of tropical heating represents a dipole between the Maritime continent and central Pacific, while the second mode of heating represents a dipole between the Indian Ocean and western Pacific. The leading mode of heating is highly correlated with the leading mode of height, with the heating leading the height mode by 5 days. This verifies the MJO phase 6-8 forcing of the NAO- regime seen in YS.

The lags between heating and response are shorter for the fast cycle, as would be expected. The leading height mode evolution is again very well correlated with the frequency of occurrence of the NAO- regime, as with the slow cycle. The role of synoptic eddy forcing of the height field is captured in the leading predictable components, which form an oscillation. The eddy forcing is seen to lead the development of the NAO- regime, with lags of 5 days and 1 day for slow and fast cycles, respectively.

Reference: Yadav & Straus (2017), Circulation response to Fast and Slow MJO episodes, MWR. DOI: 10.1175/MWR-D-16-0352.1