Response of the Madden–Julian Oscillation and Tropical Atmosphere to Changes in Oceanic Mixed Layer Depth over the Indian Ocean

Abstract

Abstract The Madden–Julian oscillation (MJO) is a component of tropical variability that influences high-impact events such as hurricane activity and Asian monsoons on intraseasonal (2–8 weeks) time scales. However, the atmosphere–ocean dynamics responsible for the MJO are highly debated. To gain insight into MJO–Indian Ocean dynamics, we conduct climate model experiments where the ocean is replaced by a motionless slab whose thickness, called the mixed layer depth (MLD), varies in space but not in time. Changes in the MLD and ocean heat convergence over the Indian Ocean have no discernible impact on MJO propagation, predictability, or variability within the Community Earth System Model (CESM) version 1.2.1. This suggests that ocean dynamics may not be critical to the MJO over the Indian Ocean in this dynamical model (CAM5 coupled to motionless slab). To diagnose changes in intraseasonal variability beyond the MJO, a discriminant analysis technique is used to optimize differences in variability between experiments. This analysis reveals that differences caused by changing Indian Ocean MLD were restricted mostly to local surface fluxes and could be explained by simple energy balance physics. Despite modeling adjustments intended to preserve the climate, the control slab has a warmer climate than the fully coupled model. The resulting changes in the mean climate are consistent with changes theoretically expected from global warming, particularly the “wet-gets-wetter” mechanism. Significance Statement The Madden–Julian oscillation (MJO) is a tropical eastward-moving pulse of convection that can influence a wide variety of global phenomena. Currently, models do not capture many key aspects of the MJO such as speed and spatial structure. The uppermost ocean layer that communicates with the atmosphere is widely believed to play a large role in the evolution of the MJO. To test the importance of ocean dynamics on simulated MJO, a slab-model configuration is used to allow the atmosphere and ocean to communicate through surface fluxes of heat and moisture while suppressing interactive ocean currents. Changes to the upper ocean did not impact the MJO in our model setup.