Insignificant QBO-MJO skill relationship in the subseasonal reforecasts

The impact of the stratospheric quasi-biennial oscillation (QBO) on the prediction of the tropospheric Madden-Julian oscillation (MJO) is evaluated in reforecasts from models participating in subseasonal prediction projects, including the Subseasonal Experiment (SubX) and Subseasonal to Seasonal (S2S). When MJO prediction skill is analyzed for DJF, MJO prediction skill is higher in the easterly phase of the QBO (EQBO) than the westerly phase (WQBO), consistent with previous studies. However, when MJO prediction is compared over extended boreal winter (ONDJFM) in order to increase the number of MJO events, models show lower MJO prediction skill in EQBO than WQBO. This QBO-MJO skill relation change between two winter periods (ONDJFM vs. DJF) is partly related to the change in the ratio of MJO events in EQBO versus WQBO: there are comparably fewer MJO events in EQBO (relative to WQBO) in extended winter compared to DJF. Thisdecrease of the EQBO/WQBO MJO ratio contributes to both an MJO skill decrease in EQBO and a skill increase in WQBO during ONDJFM relative to DJF. Perhaps of more importance, the QBO-MJO skill relation is mostly insignificant in models for both DJF and ONDJFM. This insignificant QBO-MJO skill relation is further confirmed by comparing two subseasonal reforecast experiments with the Community Earth System Model, version 1 (CESM1) using both a high-top (L46) and low-top (L30) version of the Community Atmosphere Model, v5 (CAM5). While there are clear differences in the forecasted QBO between the two models, a negligible change is shown in the MJO prediction, indicating that the QBO in this model may not directly control the MJO prediction, or the deficiencies of the low-top model are not relevant on S2S time-scales, and supporting the insignificant QBO-MJO skill relation shown in SubX and S2S models.