The MJO Across Future Time Periods and Scenarios in CESM1: When are Changes Detectable?

Tuesday, December 11, 2018 - 15:25
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The Madden Julian Oscillation (MJO) is the most globally impactful phenomena on sub-seasonal timescales, not only influencing the simulation characteristics in climate models throughout the Indo-Pacific region, but also influencing local and non-local forecast skill due to its remote teleconnections. In a warmer world, the impact of changes in the MJO characteristics on the climate system and sub-seasonal forecast capabilities could be significant. Key questions regarding a future MJO center not only on how the characteristics change (e.g., strength, frequency, speed, structure, geographical extent), but also how they evolve in time in response to varying 21st warming scenarios.

In order to answer these questions, output is analyzed from the NCAR Large Ensemble project, which uses the Community Earth System Model, version 1 (CESM) subjected to a suite of CMIP5 forcing scenarios. Analysis focuses on ensembles for pre-industrial, historical and future emissions CMIP5 emission scenarios ranging between RCP2.6 and RCP8.5. These ensemble experiment sets not only allow us to examine the evolution of the MJO in increasingly warmer worlds, but also to determine the robustness of any changes for each scenario and to contrast the time periods of mid and end of 21st century.

Previous studies show that the MJO is moderately well simulated in the coupled CESM, but with significant weak propagation in the West Pacific. By the end of the 21st century in the highest warming scenario, RCP8.5, the amplitude of the equatorial wave variability, including the MJO increases significantly. By certain measures of equatorial wave coupling mode structures, the MJO of the future matches the characteristics of the observed MJO more accurately than the present day CESM1 simulation. Our investigation will show why the characteristics of the warmer modeled coupled climate are more conducive to a better simulation of the MJO than in present day climates. Furthermore, we also present the combinations of time period and emission scenarios when, and for which measures, the MJO becomes statistically different from that of the present day climate.

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